485BPOS

Securities Act of 1933 Registration No. 002-74808

Investment Company Act of 1940 Registration No. 811-03320

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM N-1A

REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933 x

o Pre-Effective Amendment No.  ______

x Post-Effective Amendment No.  86

and

REGISTRATION STATEMENT UNDER THE INVESTMENT COMPANY ACT OF 1940 x

x Amendment No.  86

Fidelity Colchester Street Trust

 (Exact Name of Registrant as Specified in Charter)

245 Summer Street, Boston, Massachusetts 02210

(Address of Principal Executive Offices)(Zip Code)

Registrant’s Telephone Number: 617-563-7000

William C. Coffey, Secretary

245 Summer Street

Boston, Massachusetts 02210

(Name and Address of Agent for Service)

It is proposed that this filing will become effective immediately upon filing pursuant to paragraph (b) of Rule 485.

EXHIBIT INDEX

Exhibit	Exhibit No.
XBRL Instance Document	EX-101.INS
XBRL Taxonomy Extension Schema Document	EX-101.SCH
XBRL Taxonomy Extension Calculation Linkbase Document	EX-101.CAL
XBRL Taxonomy Extension Definition Linkbase Document	EX-101.DEF
XBRL Taxonomy Extension Labels Linkbase Document	EX-101.LAB
XBRL Taxonomy Extension Presentation Linkbase Document	EX-101.PRE

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933 and the Investment Company Act of 1940, the Registrant certifies that it meets all of the requirements for the effectiveness of this Registration Statement pursuant to Rule 485(b) under the Securities Act of 1933 and has duly caused this Post-Effective Amendment No.86to the Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Boston, and Commonwealth of Massachusetts, on the 17th day of June 2019.

	Fidelity Colchester Street Trust
	By	/s/Laura M. Del Prato
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||		Laura M. Del Prato, President

Pursuant to the requirements of the Securities Act of 1933, this Registration Statement has been signed below by the following persons in the capacities and on the dates indicated.

(Signature)		(Title)	(Date)
/s/Laura M. Del Prato		President and Treasurer	June 17, 2019
Laura M. Del Prato		(Principal Executive Officer)
/s/ John J. Burke III		Chief Financial Officer	June 17, 2019
John J. Burke III		(Principal Financial Officer)
/s/Abigail P. Johnson	†	Trustee	June 17, 2019
Abigail P. Johnson
/s/Elizabeth S. Acton	*	Trustee	June 17, 2019
Elizabeth S. Acton
/s/Ann E. Dunwoody	*	Trustee	June 17, 2019
Ann E. Dunwoody
/s/John Engler	*	Trustee	June 17, 2019
John Engler
/s/Robert F. Gartland	*	Trustee	June 17, 2019
Robert F. Gartland
/s/Arthur E. Johnson	*	Trustee	June 17, 2019
Arthur E. Johnson
/s/Michael E. Kenneally	*	Trustee	June 17, 2019
Michael E. Kenneally
/s/Marie L. Knowles	*	Trustee	June 17, 2019
Marie L. Knowles
/s/Mark A. Murray	*	Trustee	June 17, 2019
Mark A. Murray
/s/Jennifer Toolin McAuliffe	*	Trustee	June 17, 2019
Jennifer Toolin McAuliffe

†	By:	/s/ Kevin M. Meagher
		Kevin M. Meagher, pursuant to a power of attorney dated September 30, 2018 and filed herewith.
*	By:	/s/Megan C. Johnson
		Megan C. Johnson, pursuant to a power of attorney dated January 1, 2019 and filed herewith.

POWER OF ATTORNEY

We, the undersigned Directors or Trustees, as the case may be, of the following investment companies:

Fidelity Aberdeen Street Trust Fidelity Advisor Series II Fidelity Advisor Series IV Fidelity Boylston Street Trust Fidelity California Municipal Trust Fidelity California Municipal Trust II Fidelity Central Investment Portfolios II LLC Fidelity Charles Street Trust Fidelity Colchester Street Trust Fidelity Court Street Trust Fidelity Court Street Trust II Fidelity Garrison Street Trust Fidelity Hereford Street Trust Fidelity Income Fund Fidelity Massachusetts Municipal Trust Fidelity Merrimack Street Trust Fidelity Money Market Trust Fidelity Municipal Trust Fidelity Municipal Trust II Fidelity Newbury Street Trust Fidelity New York Municipal Trust Fidelity New York Municipal Trust II Fidelity Oxford Street Trust Fidelity Oxford Street Trust II Fidelity Phillips Street Trust Fidelity Revere Street Trust Fidelity Salem Street Trust Fidelity School Street Trust Fidelity Union Street Trust Fidelity Union Street Trust II Variable Insurance Products Fund V

in addition to any other Fidelity Fund for which the undersigned individuals serve as Directors or Trustees (collectively, the “Funds”), hereby revoke all previous powers of attorney we have given to sign and otherwise act in our names and behalf in matters involving any investment company for which FMR or an affiliate acts as investment adviser and hereby constitute and appoint Thomas C. Bogle, John V. O’Hanlon, Robert W. Helm, Megan C. Johnson, and Anthony H. Zacharski, each of them singly, our true and lawful attorneys-in-fact, with full power of substitution, and with full power to each of them, to sign for us and in our names in the appropriate capacities, all Registration Statements of the Funds on Form N-1A, or any successors thereto, any and all subsequent Amendments, Pre-Effective Amendments, or Post-Effective Amendments to said Registration Statements or any successors thereto, and any supplements or other instruments in connection therewith, and generally to do all such things in our names and behalf in connection therewith as said attorneys-in-fact deem necessary or appropriate, to comply with the provisions of the Securities Act of 1933 and the Investment Company Act of 1940, and all related requirements of the Securities and Exchange Commission.  We hereby ratify and confirm all that said attorneys-in-fact or their substitutes may do or cause to be done by virtue hereof.  This power of attorney is effective for all documents filed on or after January 1, 2019.

WITNESS our hands on this first day of January 2019.

/s/Elizabeth S. Acton	/s/Michael E. Kenneally
Elizabeth S. Acton	Michael E. Kenneally
/s/Ann E. Dunwoody	/s/Marie L. Knowles
Ann E. Dunwoody	Marie L. Knowles
/s/John Engler	/s/Jennifer Toolin McAuliffe
John Engler	Jennifer Toolin McAuliffe
/s/Robert F. Gartland	/s/Mark A. Murray
Robert F. Gartland	Mark A. Murray
/s/Arthur E. Johnson
Arthur E. Johnson

POWER OF ATTORNEY

I, the undersigned Trustee of the following investment companies:

Fidelity Aberdeen Street Trust Fidelity Advisor Series II Fidelity Advisor Series IV Fidelity Boylston Street Trust Fidelity California Municipal Trust Fidelity California Municipal Trust II Fidelity Central Investment Portfolios II LLC Fidelity Charles Street Trust Fidelity Colchester Street Trust Fidelity Court Street Trust Fidelity Court Street Trust II Fidelity Garrison Street Trust Fidelity Hereford Street Trust Fidelity Income Fund Fidelity Massachusetts Municipal Trust
Fidelity Merrimack Street Trust Fidelity Money Market Trust Fidelity Municipal Trust Fidelity Municipal Trust II Fidelity Newbury Street Trust Fidelity New York Municipal Trust Fidelity New York Municipal Trust II Fidelity Oxford Street Trust Fidelity Oxford Street Trust II Fidelity Phillips Street Trust Fidelity Revere Street Trust Fidelity Salem Street Trust Fidelity School Street Trust Fidelity Union Street Trust Fidelity Union Street Trust II Variable Insurance Products Fund V

in addition to any other Fidelity Fund for which the undersigned individual serves as Trustee (collectively, the “Funds”), hereby constitute and appoint Kevin M. Meagher, my true and lawful attorney-in-fact, with full power of substitution, and with full power to sign for me and in my name in the appropriate capacity, all Registration Statements of the Funds on Form N-1A, or any successors thereto, any and all subsequent Amendments, Pre-Effective Amendments, or Post-Effective Amendments to said Registration Statements or any successors thereto, and any supplements or other instruments in connection therewith, and generally to do all such things in my name and behalf in connection therewith as said attorney-in-fact deems necessary or appropriate, to comply with the provisions of the Securities Act of 1933 and the Investment Company Act of 1940, and all related requirements of the Securities and Exchange Commission.  I hereby ratify and confirm all that said attorney-in-fact or his substitutes may do or cause to be done by virtue hereof.  

This Power of Attorney shall remain in full force and effect only for such time as Kevin M. Meagher shall continue to be an officer of Fidelity Management & Research Company, provided that, notwithstanding the foregoing, this Power of Attorney may be revoked at any time by the undersigned in writing.

This Power of Attorney has been executed as of September 30, 2018.

/s/Abigail P. Johnson

Abigail P. Johnson

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Document Type	dei_DocumentType	485BPOS
Document Period End Date	dei_DocumentPeriodEndDate	Mar. 31, 2019
Registrant Name	dei_EntityRegistrantName	Fidelity Colchester Street Trust
Central Index Key	dei_EntityCentralIndexKey	0000356173
Investment Company Type	dei_EntityInvCompanyType	N-1A
Amendment Flag	dei_AmendmentFlag	false
Document Creation Date	dei_DocumentCreationDate	May 28, 2019
Document Effective Date	dei_DocumentEffectiveDate	May 30, 2019
Prospectus Date	rr_ProspectusDate	May 30, 2019

XML

03.31 FIMM Funds PRO - Institutional Class-06 | Government Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Government Portfolio</b>/Institutional</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Government Portfolio}	03.31 FIMM Funds PRO - Institutional Class-06 Government Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Government Portfolio}	03.31 FIMM Funds PRO - Institutional Class-06 Government Portfolio Government Portfolio-Institutional Class FIMM
Management fee	0.14%
Distribution and/or Service (12b-1) fees	none
Other expenses	0.04%
Total annual operating expenses	0.18%
Fee waiver and/or expense reimbursement	0.04%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.14%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Government Portfolio}	03.31 FIMM Funds PRO - Institutional Class-06 Government Portfolio Government Portfolio-Institutional Class FIMM USD ($)
1 year	$ 14
3 years	53
5 years	96
10 years	$ 225

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Normally investing at least 99.5% of total assets in cash, U.S. Government securities and/or repurchase agreements that are collateralized fully (i.e., collateralized by cash or government securities).

• Investing in U.S. Government securities issued by entities that are chartered or sponsored by Congress but whose securities are neither issued nor guaranteed by the U.S. Treasury.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Government securities and repurchase agreements for those securities.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.54%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.57%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Government Portfolio} - 03.31 FIMM Funds PRO - Institutional Class-06 - Government Portfolio - Government Portfolio-Institutional Class FIMM	Past 1 year	Since Inception	[1]	Inception Date
Before Taxes				May 14, 2014
Return Before Taxes	1.76%	0.62%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Institutional Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.14% (the Expense Cap). If at any time during the current fiscal year expenses for Institutional Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

[1] From May 14, 2014

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds PRO - Institutional Class-06 | Government Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Government Portfolio</b>/Institutional</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Normally investing at least 99.5% of total assets in cash, U.S. Government securities and/or repurchase agreements that are collateralized fully (i.e., collateralized by cash or government securities).

• Investing in U.S. Government securities issued by entities that are chartered or sponsored by Congress but whose securities are neither issued nor guaranteed by the U.S. Treasury.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Government securities and repurchase agreements for those securities.

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Money Market Fund May Not Preserve Dollar [Text]

rr_RiskMoneyMarketFundMayNotPreserveDollar

<font style="FONT-FAMILY: Times New Roman" size="3">Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">www.fidelity.com or institutional.fidelity.com</font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.54%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.57%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds PRO - Institutional Class-06 | Government Portfolio | Government Portfolio-Institutional Class FIMM

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.04%

Total annual operating expenses

rr_ExpensesOverAssets

0.18%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.04%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.14%

1 year

rr_ExpenseExampleYear01

$ 14

3 years

rr_ExpenseExampleYear03

53

5 years

rr_ExpenseExampleYear05

96

10 years

rr_ExpenseExampleYear10

$ 225

2015

rr_AnnualReturn2015

0.02%

2016

rr_AnnualReturn2016

0.29%

2017

rr_AnnualReturn2017

0.79%

2018

rr_AnnualReturn2018

1.76%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.57%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.54%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

Inception Date

rr_AverageAnnualReturnInceptionDate

May 14, 2014

03.31 FIMM Funds PRO - Institutional Class-06 | Government Portfolio | Return Before Taxes | Government Portfolio-Institutional Class FIMM

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Government Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Institutional Class</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.76%

Since Inception

rr_AverageAnnualReturnSinceInception

0.62%

^[2]

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Institutional Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.14% (the Expense Cap). If at any time during the current fiscal year expenses for Institutional Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

[2] From May 14, 2014

XML

03.31 FIMM Funds PRO - Institutional Class-06 | Money Market Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Money Market Portfolio</b>/Institutional</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Money Market Portfolio}	03.31 FIMM Funds PRO - Institutional Class-06 Money Market Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Money Market Portfolio}	03.31 FIMM Funds PRO - Institutional Class-06 Money Market Portfolio Money Market Portfolio-Institutional Class FIMM
Management fee	0.14%
Distribution and/or Service (12b-1) fees	none
Other expenses	0.04%
Total annual operating expenses	0.18%
Fee waiver and/or expense reimbursement	0.04%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.14%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Money Market Portfolio}	03.31 FIMM Funds PRO - Institutional Class-06 Money Market Portfolio Money Market Portfolio-Institutional Class FIMM USD ($)
1 year	$ 14
3 years	53
5 years	96
10 years	$ 225

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.59%	December 31, 2018
Lowest Quarter Return	0.02%	March 31, 2014
Year-to-Date Return	0.63%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Money Market Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds PRO - Institutional Class-06 | Money Market Portfolio | Money Market Portfolio-Institutional Class FIMM | Return Before Taxes	2.04%	0.79%	0.56%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Institutional Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.14% (the Expense Cap). If at any time during the current fiscal year expenses for Institutional Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds PRO - Institutional Class-06 | Money Market Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Money Market Portfolio</b>/Institutional</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Strategy Portfolio Concentration [Text]

rr_StrategyPortfolioConcentration

<font style="FONT-FAMILY: Times New Roman" size="3">Investing more than 25% of total assets in the financial services industries.</font>

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund May Impose Fees or Suspend Sales [Text]

rr_RiskMoneyMarketFundMayImposeFeesOrSuspendSales

<font style="FONT-FAMILY: Times New Roman" size="3">The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Money Market Fund May Not Preserve Dollar [Text]

rr_RiskMoneyMarketFundMayNotPreserveDollar

<font style="FONT-FAMILY: Times New Roman" size="3">Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3"> www.fidelity.com or institutional.fidelity.com</font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.59%	December 31, 2018
Lowest Quarter Return	0.02%	March 31, 2014
Year-to-Date Return	0.63%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds PRO - Institutional Class-06 | Money Market Portfolio | Money Market Portfolio-Institutional Class FIMM

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.04%

Total annual operating expenses

rr_ExpensesOverAssets

0.18%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.04%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.14%

1 year

rr_ExpenseExampleYear01

$ 14

3 years

rr_ExpenseExampleYear03

53

5 years

rr_ExpenseExampleYear05

96

10 years

rr_ExpenseExampleYear10

$ 225

2009

rr_AnnualReturn2009

0.77%

2010

rr_AnnualReturn2010

0.28%

2011

rr_AnnualReturn2011

0.20%

2012

rr_AnnualReturn2012

0.22%

2013

rr_AnnualReturn2013

0.12%

2014

rr_AnnualReturn2014

0.09%

2015

rr_AnnualReturn2015

0.15%

2016

rr_AnnualReturn2016

0.52%

2017

rr_AnnualReturn2017

1.17%

2018

rr_AnnualReturn2018

2.04%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.63%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.59%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2014

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

0.02%

03.31 FIMM Funds PRO - Institutional Class-06 | Money Market Portfolio | Return Before Taxes | Money Market Portfolio-Institutional Class FIMM

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Money Market Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Institutional Class</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

2.04%

Past 5 years

rr_AverageAnnualReturnYear05

0.79%

Past 10 years

rr_AverageAnnualReturnYear10

0.56%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Institutional Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.14% (the Expense Cap). If at any time during the current fiscal year expenses for Institutional Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds PRO - Institutional Class-06 | Prime Money Market Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Prime Money Market Portfolio</b>/Institutional</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Prime Money Market Portfolio}	03.31 FIMM Funds PRO - Institutional Class-06 Prime Money Market Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Prime Money Market Portfolio}	03.31 FIMM Funds PRO - Institutional Class-06 Prime Money Market Portfolio Prime Money Market Portfolio-Institutional Class FIMM
Management fee	0.14%
Distribution and/or Service (12b-1) fees	none
Other expenses	0.04%
Total annual operating expenses	0.18%
Fee waiver and/or expense reimbursement	0.04%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.14%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Prime Money Market Portfolio}	03.31 FIMM Funds PRO - Institutional Class-06 Prime Money Market Portfolio Prime Money Market Portfolio-Institutional Class FIMM USD ($)
1 year	$ 14
3 years	53
5 years	96
10 years	$ 225

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Although the fund is a money market fund, it will price and transact at a "floating" net asset value (NAV) that will fluctuate based on changes in the value of the fund's investments.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Floating NAV. The fund does not maintain a constant NAV per share of $1.00. The value of the fund's shares will be calculated to four decimal places and will vary as a result of changes in the value of the securities in which the fund invests.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.58%	December 31, 2018
Lowest Quarter Return	0.01%	March 31, 2014
Year-to-Date Return	0.63%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Prime Money Market Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds PRO - Institutional Class-06 | Prime Money Market Portfolio | Prime Money Market Portfolio-Institutional Class FIMM | Return Before Taxes	1.98%	0.76%	0.51%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Institutional Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.14% (the Expense Cap). If at any time during the current fiscal year expenses for Institutional Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds PRO - Institutional Class-06 | Prime Money Market Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Prime Money Market Portfolio</b>/Institutional</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Although the fund is a money market fund, it will price and transact at a "floating" net asset value (NAV) that will fluctuate based on changes in the value of the fund's investments.

Strategy Portfolio Concentration [Text]

rr_StrategyPortfolioConcentration

<font style="FONT-FAMILY: Times New Roman" size="3">Investing more than 25% of total assets in the financial services industries.</font>

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Floating NAV. The fund does not maintain a constant NAV per share of $1.00. The value of the fund's shares will be calculated to four decimal places and will vary as a result of changes in the value of the securities in which the fund invests.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Price Fluctuates [Text]

rr_RiskMoneyMarketFundPriceFluctuates

<font style="FONT-FAMILY: Times New Roman" size="3">Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them.</font>

Risk Money Market Fund May Impose Fees or Suspend Sales [Text]

rr_RiskMoneyMarketFundMayImposeFeesOrSuspendSales

<font style="FONT-FAMILY: Times New Roman" size="3">The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">www.fidelity.com or institutional.fidelity.com</font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.58%	December 31, 2018
Lowest Quarter Return	0.01%	March 31, 2014
Year-to-Date Return	0.63%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds PRO - Institutional Class-06 | Prime Money Market Portfolio | Prime Money Market Portfolio-Institutional Class FIMM

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.04%

Total annual operating expenses

rr_ExpensesOverAssets

0.18%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.04%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.14%

1 year

rr_ExpenseExampleYear01

$ 14

3 years

rr_ExpenseExampleYear03

53

5 years

rr_ExpenseExampleYear05

96

10 years

rr_ExpenseExampleYear10

$ 225

2009

rr_AnnualReturn2009

0.64%

2010

rr_AnnualReturn2010

0.25%

2011

rr_AnnualReturn2011

0.17%

2012

rr_AnnualReturn2012

0.17%

2013

rr_AnnualReturn2013

0.08%

2014

rr_AnnualReturn2014

0.06%

2015

rr_AnnualReturn2015

0.13%

2016

rr_AnnualReturn2016

0.54%

2017

rr_AnnualReturn2017

1.10%

2018

rr_AnnualReturn2018

1.98%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.63%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.58%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2014

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

0.01%

03.31 FIMM Funds PRO - Institutional Class-06 | Prime Money Market Portfolio | Return Before Taxes | Prime Money Market Portfolio-Institutional Class FIMM

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Prime Money Market Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Institutional Class</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.98%

Past 5 years

rr_AverageAnnualReturnYear05

0.76%

Past 10 years

rr_AverageAnnualReturnYear10

0.51%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Institutional Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.14% (the Expense Cap). If at any time during the current fiscal year expenses for Institutional Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds PRO - Institutional Class-06 | Prime Reserves Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Prime Reserves Portfolio</b>/Institutional</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Prime Reserves Portfolio}	03.31 FIMM Funds PRO - Institutional Class-06 Prime Reserves Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Prime Reserves Portfolio}	03.31 FIMM Funds PRO - Institutional Class-06 Prime Reserves Portfolio Prime Reserves Portfolio-Institutional Class FIMM
Management fee	0.14%
Distribution and/or Service (12b-1) fees	none
Other expenses	0.05%
Total annual operating expenses	0.19%
Fee waiver and/or expense reimbursement	0.05%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.14%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Prime Reserves Portfolio}	03.31 FIMM Funds PRO - Institutional Class-06 Prime Reserves Portfolio Prime Reserves Portfolio-Institutional Class FIMM USD ($)
1 year	$ 14
3 years	54
5 years	100
10 years	$ 236

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Although the fund is a money market fund, it will price and transact at a "floating" net asset value (NAV) that will fluctuate based on changes in the value of the fund's investments.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Floating NAV. The fund does not maintain a constant NAV per share of $1.00. The value of the fund's shares will be calculated to four decimal places and will vary as a result of changes in the value of the securities in which the fund invests.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.58%	December 31, 2018
Lowest Quarter Return	0.23%	March 31, 2017
Year-to-Date Return	0.63%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Prime Reserves Portfolio} - 03.31 FIMM Funds PRO - Institutional Class-06 - Prime Reserves Portfolio - Prime Reserves Portfolio-Institutional Class FIMM	Past 1 year	Since Inception	[1]	Inception Date
Before Taxes				Jun. 02, 2016
Return Before Taxes	2.00%	1.32%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Institutional Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.14% (the Expense Cap). If at any time during the current fiscal year expenses for Institutional Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

[1] From June 2, 2016

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds PRO - Institutional Class-06 | Prime Reserves Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Prime Reserves Portfolio</b>/Institutional</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Although the fund is a money market fund, it will price and transact at a "floating" net asset value (NAV) that will fluctuate based on changes in the value of the fund's investments.

Strategy Portfolio Concentration [Text]

rr_StrategyPortfolioConcentration

<font style="FONT-FAMILY: Times New Roman" size="3">Investing more than 25% of total assets in the financial services industries.</font>

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Floating NAV. The fund does not maintain a constant NAV per share of $1.00. The value of the fund's shares will be calculated to four decimal places and will vary as a result of changes in the value of the securities in which the fund invests.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Price Fluctuates [Text]

rr_RiskMoneyMarketFundPriceFluctuates

<font style="FONT-FAMILY: Times New Roman" size="3">Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them.</font>

Risk Money Market Fund May Impose Fees or Suspend Sales [Text]

rr_RiskMoneyMarketFundMayImposeFeesOrSuspendSales

<font style="FONT-FAMILY: Times New Roman" size="3">The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">www.fidelity.com or institutional.fidelity.com</font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.58%	December 31, 2018
Lowest Quarter Return	0.23%	March 31, 2017
Year-to-Date Return	0.63%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds PRO - Institutional Class-06 | Prime Reserves Portfolio | Prime Reserves Portfolio-Institutional Class FIMM

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.05%

Total annual operating expenses

rr_ExpensesOverAssets

0.19%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.05%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.14%

1 year

rr_ExpenseExampleYear01

$ 14

3 years

rr_ExpenseExampleYear03

54

5 years

rr_ExpenseExampleYear05

100

10 years

rr_ExpenseExampleYear10

$ 236

2017

rr_AnnualReturn2017

1.12%

2018

rr_AnnualReturn2018

2.00%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.63%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.58%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2017

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

0.23%

Inception Date

rr_AverageAnnualReturnInceptionDate

Jun. 02, 2016

03.31 FIMM Funds PRO - Institutional Class-06 | Prime Reserves Portfolio | Return Before Taxes | Prime Reserves Portfolio-Institutional Class FIMM

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Prime Reserves Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Institutional Class</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

2.00%

Since Inception

rr_AverageAnnualReturnSinceInception

1.32%

^[2]

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Institutional Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.14% (the Expense Cap). If at any time during the current fiscal year expenses for Institutional Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

[2] From June 2, 2016

XML

03.31 FIMM Funds PRO - Institutional Class-06 | Treasury Only Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Treasury Only Portfolio</b>/Institutional</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks as high a level of current income as is consistent with the security of principal and liquidity.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Treasury Only Portfolio}	03.31 FIMM Funds PRO - Institutional Class-06 Treasury Only Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Treasury Only Portfolio}	03.31 FIMM Funds PRO - Institutional Class-06 Treasury Only Portfolio Treasury Only Portfolio-Institutional Class FIMM
Management fee	0.14%
Distribution and/or Service (12b-1) fees	none
Other expenses	0.04%
Total annual operating expenses	0.18%
Fee waiver and/or expense reimbursement	0.04%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.14%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Treasury Only Portfolio}	03.31 FIMM Funds PRO - Institutional Class-06 Treasury Only Portfolio Treasury Only Portfolio-Institutional Class FIMM USD ($)
1 year	$ 14
3 years	53
5 years	96
10 years	$ 225

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Normally investing at least 99.5% of total assets in cash and U.S. Treasury securities.

• Normally investing in securities whose interest is exempt from state and local income taxes.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.54%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.57%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Treasury Only Portfolio} - 03.31 FIMM Funds PRO - Institutional Class-06 - Treasury Only Portfolio - Treasury Only Portfolio-Institutional Class FIMM	Past 1 year	Since Inception	[1]	Inception Date
Before Taxes				May 14, 2014
Return Before Taxes	1.75%	0.59%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Institutional Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.14% (the Expense Cap). If at any time during the current fiscal year expenses for Institutional Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

[1] From May 14, 2014

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds PRO - Institutional Class-06 | Treasury Only Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Treasury Only Portfolio</b>/Institutional</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks as high a level of current income as is consistent with the security of principal and liquidity.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Normally investing at least 99.5% of total assets in cash and U.S. Treasury securities.

• Normally investing in securities whose interest is exempt from state and local income taxes.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities.

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Money Market Fund May Not Preserve Dollar [Text]

rr_RiskMoneyMarketFundMayNotPreserveDollar

<font style="FONT-FAMILY: Times New Roman" size="3">Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">www.fidelity.com or institutional.fidelity.com</font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.54%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.57%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds PRO - Institutional Class-06 | Treasury Only Portfolio | Treasury Only Portfolio-Institutional Class FIMM

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.04%

Total annual operating expenses

rr_ExpensesOverAssets

0.18%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.04%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.14%

1 year

rr_ExpenseExampleYear01

$ 14

3 years

rr_ExpenseExampleYear03

53

5 years

rr_ExpenseExampleYear05

96

10 years

rr_ExpenseExampleYear10

$ 225

2015

rr_AnnualReturn2015

0.01%

2016

rr_AnnualReturn2016

0.21%

2017

rr_AnnualReturn2017

0.75%

2018

rr_AnnualReturn2018

1.75%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.57%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.54%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

Inception Date

rr_AverageAnnualReturnInceptionDate

May 14, 2014

03.31 FIMM Funds PRO - Institutional Class-06 | Treasury Only Portfolio | Return Before Taxes | Treasury Only Portfolio-Institutional Class FIMM

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Treasury Only Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Institutional Class</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.75%

Since Inception

rr_AverageAnnualReturnSinceInception

0.59%

^[2]

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Institutional Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.14% (the Expense Cap). If at any time during the current fiscal year expenses for Institutional Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

[2] From May 14, 2014

XML

03.31 FIMM Funds PRO - Institutional Class-06 | Treasury Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Treasury Portfolio</b>/Institutional</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Treasury Portfolio}	03.31 FIMM Funds PRO - Institutional Class-06 Treasury Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Treasury Portfolio}	03.31 FIMM Funds PRO - Institutional Class-06 Treasury Portfolio Treasury Portfolio-Institutional Class FIMM
Management fee	0.14%
Distribution and/or Service (12b-1) fees	none
Other expenses	0.04%
Total annual operating expenses	0.18%
Fee waiver and/or expense reimbursement	0.04%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.14%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Treasury Portfolio}	03.31 FIMM Funds PRO - Institutional Class-06 Treasury Portfolio Treasury Portfolio-Institutional Class FIMM USD ($)
1 year	$ 14
3 years	53
5 years	96
10 years	$ 225

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Normally investing at least 99.5% of total assets in cash, U.S. Treasury securities and/or repurchase agreements for those securities.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities and repurchase agreements for those securities.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.54%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.57%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Treasury Portfolio} - 03.31 FIMM Funds PRO - Institutional Class-06 - Treasury Portfolio - Treasury Portfolio-Institutional Class FIMM	Past 1 year	Since Inception	[1]	Inception Date
Before Taxes				May 14, 2014
Return Before Taxes	1.76%	0.60%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Institutional Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.14% (the Expense Cap). If at any time during the current fiscal year expenses for Institutional Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

[1] From May 14, 2014

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds PRO - Institutional Class-06 | Treasury Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Treasury Portfolio</b>/Institutional</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Normally investing at least 99.5% of total assets in cash, U.S. Treasury securities and/or repurchase agreements for those securities.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities and repurchase agreements for those securities.

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Money Market Fund May Not Preserve Dollar [Text]

rr_RiskMoneyMarketFundMayNotPreserveDollar

<font style="FONT-FAMILY: Times New Roman" size="3">Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">www.fidelity.com or institutional.fidelity.com</font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.54%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.57%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds PRO - Institutional Class-06 | Treasury Portfolio | Treasury Portfolio-Institutional Class FIMM

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.04%

Total annual operating expenses

rr_ExpensesOverAssets

0.18%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.04%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.14%

1 year

rr_ExpenseExampleYear01

$ 14

3 years

rr_ExpenseExampleYear03

53

5 years

rr_ExpenseExampleYear05

96

10 years

rr_ExpenseExampleYear10

$ 225

2015

rr_AnnualReturn2015

0.02%

2016

rr_AnnualReturn2016

0.24%

2017

rr_AnnualReturn2017

0.78%

2018

rr_AnnualReturn2018

1.76%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.57%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.54%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

Inception Date

rr_AverageAnnualReturnInceptionDate

May 14, 2014

03.31 FIMM Funds PRO - Institutional Class-06 | Treasury Portfolio | Return Before Taxes | Treasury Portfolio-Institutional Class FIMM

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Treasury Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Institutional Class</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.76%

Since Inception

rr_AverageAnnualReturnSinceInception

0.60%

^[2]

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Institutional Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.14% (the Expense Cap). If at any time during the current fiscal year expenses for Institutional Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

[2] From May 14, 2014

XML

03.31 FIMM Funds Class 3 Combo PRO-06 | Government Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Government Portfolio</b>/III</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Government Portfolio}	03.31 FIMM Funds Class 3 Combo PRO-06 Government Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Government Portfolio}	03.31 FIMM Funds Class 3 Combo PRO-06 Government Portfolio Government Portfolio-Class III
Management fee	0.14%
Distribution and/or Service (12b-1) fees	0.25%
Other expenses	0.07%
Total annual operating expenses	0.46%
Fee waiver and/or expense reimbursement	0.03%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.43%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Government Portfolio}	03.31 FIMM Funds Class 3 Combo PRO-06 Government Portfolio Government Portfolio-Class III USD ($)
1 year	$ 44
3 years	144
5 years	254
10 years	$ 575

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Normally investing at least 99.5% of total assets in cash, U.S. Government securities and/or repurchase agreements that are collateralized fully (i.e., collateralized by cash or government securities).

• Investing in U.S. Government securities issued by entities that are chartered or sponsored by Congress but whose securities are neither issued nor guaranteed by the U.S. Treasury.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Government securities and repurchase agreements for those securities.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.47%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.50%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Government Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds Class 3 Combo PRO-06 | Government Portfolio | Government Portfolio-Class III | Return Before Taxes	1.47%	0.40%	0.22%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class III of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.43% (the Expense Cap). If at any time during the current fiscal year expenses for Class III of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Class 3 Combo PRO-06 | Government Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Government Portfolio</b>/III</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Normally investing at least 99.5% of total assets in cash, U.S. Government securities and/or repurchase agreements that are collateralized fully (i.e., collateralized by cash or government securities).

• Investing in U.S. Government securities issued by entities that are chartered or sponsored by Congress but whose securities are neither issued nor guaranteed by the U.S. Treasury.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Government securities and repurchase agreements for those securities.

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Money Market Fund May Not Preserve Dollar [Text]

rr_RiskMoneyMarketFundMayNotPreserveDollar

<font style="FONT-FAMILY: Times New Roman" size="3">Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">institutional.fidelity.com </font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.47%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.50%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Class 3 Combo PRO-06 | Government Portfolio | Government Portfolio-Class III

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.25%

Other expenses

rr_OtherExpensesOverAssets

0.07%

Total annual operating expenses

rr_ExpensesOverAssets

0.46%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.03%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.43%

1 year

rr_ExpenseExampleYear01

$ 44

3 years

rr_ExpenseExampleYear03

144

5 years

rr_ExpenseExampleYear05

254

10 years

rr_ExpenseExampleYear10

$ 575

2009

rr_AnnualReturn2009

0.15%

2010

rr_AnnualReturn2010

0.01%

2011

rr_AnnualReturn2011

0.01%

2012

rr_AnnualReturn2012

0.01%

2013

rr_AnnualReturn2013

0.01%

2014

rr_AnnualReturn2014

0.01%

2015

rr_AnnualReturn2015

0.01%

2016

rr_AnnualReturn2016

0.02%

2017

rr_AnnualReturn2017

0.50%

2018

rr_AnnualReturn2018

1.47%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.50%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.47%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

03.31 FIMM Funds Class 3 Combo PRO-06 | Government Portfolio | Return Before Taxes | Government Portfolio-Class III

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Government Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Class III</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.47%

Past 5 years

rr_AverageAnnualReturnYear05

0.40%

Past 10 years

rr_AverageAnnualReturnYear10

0.22%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class III of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.43% (the Expense Cap). If at any time during the current fiscal year expenses for Class III of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds Class 3 Combo PRO-06 | Money Market Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Money Market Portfolio</b>/III</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Money Market Portfolio}	03.31 FIMM Funds Class 3 Combo PRO-06 Money Market Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Money Market Portfolio}	03.31 FIMM Funds Class 3 Combo PRO-06 Money Market Portfolio Money Market Portfolio-Class III
Management fee	0.14%
Distribution and/or Service (12b-1) fees	0.25%
Other expenses	0.07%
Total annual operating expenses	0.46%
Fee waiver and/or expense reimbursement	0.03%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.43%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Money Market Portfolio}	03.31 FIMM Funds Class 3 Combo PRO-06 Money Market Portfolio Money Market Portfolio-Class III USD ($)
1 year	$ 44
3 years	144
5 years	254
10 years	$ 575

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.52%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.56%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Money Market Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds Class 3 Combo PRO-06 | Money Market Portfolio | Money Market Portfolio-Class III | Return Before Taxes	1.75%	0.57%	0.34%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class III of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.43% (the Expense Cap). If at any time during the current fiscal year expenses for Class III of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Class 3 Combo PRO-06 | Money Market Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Money Market Portfolio</b>/III</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Strategy Portfolio Concentration [Text]

rr_StrategyPortfolioConcentration

<font style="FONT-FAMILY: Times New Roman" size="3">Investing more than 25% of total assets in the financial services industries.</font>

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund May Impose Fees or Suspend Sales [Text]

rr_RiskMoneyMarketFundMayImposeFeesOrSuspendSales

<font style="FONT-FAMILY: Times New Roman" size="3">The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Money Market Fund May Not Preserve Dollar [Text]

rr_RiskMoneyMarketFundMayNotPreserveDollar

<font style="FONT-FAMILY: Times New Roman" size="3">Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">institutional.fidelity.com </font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.52%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.56%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Class 3 Combo PRO-06 | Money Market Portfolio | Money Market Portfolio-Class III

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.25%

Other expenses

rr_OtherExpensesOverAssets

0.07%

Total annual operating expenses

rr_ExpensesOverAssets

0.46%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.03%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.43%

1 year

rr_ExpenseExampleYear01

$ 44

3 years

rr_ExpenseExampleYear03

144

5 years

rr_ExpenseExampleYear05

254

10 years

rr_ExpenseExampleYear10

$ 575

2009

rr_AnnualReturn2009

0.48%

2010

rr_AnnualReturn2010

0.01%

2011

rr_AnnualReturn2011

0.01%

2012

rr_AnnualReturn2012

0.01%

2013

rr_AnnualReturn2013

0.01%

2014

rr_AnnualReturn2014

0.01%

2015

rr_AnnualReturn2015

0.01%

2016

rr_AnnualReturn2016

0.23%

2017

rr_AnnualReturn2017

0.88%

2018

rr_AnnualReturn2018

1.75%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.56%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.52%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

03.31 FIMM Funds Class 3 Combo PRO-06 | Money Market Portfolio | Return Before Taxes | Money Market Portfolio-Class III

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Money Market Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Class III</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.75%

Past 5 years

rr_AverageAnnualReturnYear05

0.57%

Past 10 years

rr_AverageAnnualReturnYear10

0.34%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class III of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.43% (the Expense Cap). If at any time during the current fiscal year expenses for Class III of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds Class 3 Combo PRO-06 | Prime Money Market Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Prime Money Market Portfolio</b>/III</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Prime Money Market Portfolio}	03.31 FIMM Funds Class 3 Combo PRO-06 Prime Money Market Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Prime Money Market Portfolio}	03.31 FIMM Funds Class 3 Combo PRO-06 Prime Money Market Portfolio Prime Money Market Portfolio-Class III
Management fee	0.14%
Distribution and/or Service (12b-1) fees	0.25%
Other expenses	0.07%
Total annual operating expenses	0.46%
Fee waiver and/or expense reimbursement	0.03%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.43%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Prime Money Market Portfolio}	03.31 FIMM Funds Class 3 Combo PRO-06 Prime Money Market Portfolio Prime Money Market Portfolio-Class III USD ($)
1 year	$ 44
3 years	144
5 years	254
10 years	$ 575

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Although the fund is a money market fund, it will price and transact at a "floating" net asset value (NAV) that will fluctuate based on changes in the value of the fund's investments.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Floating NAV. The fund does not maintain a constant NAV per share of $1.00. The value of the fund's shares will be calculated to four decimal places and will vary as a result of changes in the value of the securities in which the fund invests.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.51%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2014
Year-to-Date Return	0.55%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Prime Money Market Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds Class 3 Combo PRO-06 | Prime Money Market Portfolio | Prime Money Market Portfolio-Class III | Return Before Taxes	1.68%	0.55%	0.31%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class III of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.43% (the Expense Cap). If at any time during the current fiscal year expenses for Class III of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Class 3 Combo PRO-06 | Prime Money Market Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Prime Money Market Portfolio</b>/III</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Although the fund is a money market fund, it will price and transact at a "floating" net asset value (NAV) that will fluctuate based on changes in the value of the fund's investments.

Strategy Portfolio Concentration [Text]

rr_StrategyPortfolioConcentration

<font style="FONT-FAMILY: Times New Roman" size="3">Investing more than 25% of total assets in the financial services industries.</font>

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Floating NAV. The fund does not maintain a constant NAV per share of $1.00. The value of the fund's shares will be calculated to four decimal places and will vary as a result of changes in the value of the securities in which the fund invests.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Price Fluctuates [Text]

rr_RiskMoneyMarketFundPriceFluctuates

<font style="FONT-FAMILY: Times New Roman" size="3">Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them.</font>

Risk Money Market Fund May Impose Fees or Suspend Sales [Text]

rr_RiskMoneyMarketFundMayImposeFeesOrSuspendSales

<font style="FONT-FAMILY: Times New Roman" size="3">The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">institutional.fidelity.com</font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.51%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2014
Year-to-Date Return	0.55%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Class 3 Combo PRO-06 | Prime Money Market Portfolio | Prime Money Market Portfolio-Class III

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.25%

Other expenses

rr_OtherExpensesOverAssets

0.07%

Total annual operating expenses

rr_ExpensesOverAssets

0.46%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.03%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.43%

1 year

rr_ExpenseExampleYear01

$ 44

3 years

rr_ExpenseExampleYear03

144

5 years

rr_ExpenseExampleYear05

254

10 years

rr_ExpenseExampleYear10

$ 575

2009

rr_AnnualReturn2009

0.34%

2010

rr_AnnualReturn2010

0.01%

2011

rr_AnnualReturn2011

0.01%

2012

rr_AnnualReturn2012

0.01%

2013

rr_AnnualReturn2013

0.01%

2014

rr_AnnualReturn2014

0.01%

2015

rr_AnnualReturn2015

0.01%

2016

rr_AnnualReturn2016

0.25%

2017

rr_AnnualReturn2017

0.80%

2018

rr_AnnualReturn2018

1.68%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.55%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.51%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2014

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

03.31 FIMM Funds Class 3 Combo PRO-06 | Prime Money Market Portfolio | Return Before Taxes | Prime Money Market Portfolio-Class III

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Prime Money Market Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Class III</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.68%

Past 5 years

rr_AverageAnnualReturnYear05

0.55%

Past 10 years

rr_AverageAnnualReturnYear10

0.31%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class III of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.43% (the Expense Cap). If at any time during the current fiscal year expenses for Class III of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds Class 3 Combo PRO-06 | Prime Reserves Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Prime Reserves Portfolio</b>/III</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Prime Reserves Portfolio}	03.31 FIMM Funds Class 3 Combo PRO-06 Prime Reserves Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Prime Reserves Portfolio}	03.31 FIMM Funds Class 3 Combo PRO-06 Prime Reserves Portfolio Prime Reserves Portfolio-Class III
Management fee	0.14%
Distribution and/or Service (12b-1) fees	0.25%
Other expenses	0.08%
Total annual operating expenses	0.47%
Fee waiver and/or expense reimbursement	0.04%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.43%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Prime Reserves Portfolio}	03.31 FIMM Funds Class 3 Combo PRO-06 Prime Reserves Portfolio Prime Reserves Portfolio-Class III USD ($)
1 year	$ 44
3 years	145
5 years	258
10 years	$ 586

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Although the fund is a money market fund, it will price and transact at a "floating" net asset value (NAV) that will fluctuate based on changes in the value of the fund's investments.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Floating NAV. The fund does not maintain a constant NAV per share of $1.00. The value of the fund's shares will be calculated to four decimal places and will vary as a result of changes in the value of the securities in which the fund invests.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.51%	December 31, 2018
Lowest Quarter Return	0.15%	March 31, 2017
Year-to-Date Return	0.56%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Prime Reserves Portfolio} - 03.31 FIMM Funds Class 3 Combo PRO-06 - Prime Reserves Portfolio - Prime Reserves Portfolio-Class III	Past 1 year	Since Inception	[1]	Inception Date
Before Taxes				Jun. 02, 2016
Return Before Taxes	1.71%	1.03%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class III of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.43% (the Expense Cap). If at any time during the current fiscal year expenses for Class III of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

[1] From June 2, 2016

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Class 3 Combo PRO-06 | Prime Reserves Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Prime Reserves Portfolio</b>/III</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Although the fund is a money market fund, it will price and transact at a "floating" net asset value (NAV) that will fluctuate based on changes in the value of the fund's investments.

Strategy Portfolio Concentration [Text]

rr_StrategyPortfolioConcentration

<font style="FONT-FAMILY: Times New Roman" size="3">Investing more than 25% of total assets in the financial services industries.</font>

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Floating NAV. The fund does not maintain a constant NAV per share of $1.00. The value of the fund's shares will be calculated to four decimal places and will vary as a result of changes in the value of the securities in which the fund invests.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Price Fluctuates [Text]

rr_RiskMoneyMarketFundPriceFluctuates

<font style="FONT-FAMILY: Times New Roman" size="3">Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them.</font>

Risk Money Market Fund May Impose Fees or Suspend Sales [Text]

rr_RiskMoneyMarketFundMayImposeFeesOrSuspendSales

<font style="FONT-FAMILY: Times New Roman" size="3">The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">institutional.fidelity.com</font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.51%	December 31, 2018
Lowest Quarter Return	0.15%	March 31, 2017
Year-to-Date Return	0.56%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Class 3 Combo PRO-06 | Prime Reserves Portfolio | Prime Reserves Portfolio-Class III

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.25%

Other expenses

rr_OtherExpensesOverAssets

0.08%

Total annual operating expenses

rr_ExpensesOverAssets

0.47%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.04%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.43%

1 year

rr_ExpenseExampleYear01

$ 44

3 years

rr_ExpenseExampleYear03

145

5 years

rr_ExpenseExampleYear05

258

10 years

rr_ExpenseExampleYear10

$ 586

2017

rr_AnnualReturn2017

0.82%

2018

rr_AnnualReturn2018

1.71%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.56%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.51%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2017

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

0.15%

Inception Date

rr_AverageAnnualReturnInceptionDate

Jun. 02, 2016

03.31 FIMM Funds Class 3 Combo PRO-06 | Prime Reserves Portfolio | Return Before Taxes | Prime Reserves Portfolio-Class III

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Prime Reserves Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Class III</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.71%

Since Inception

rr_AverageAnnualReturnSinceInception

1.03%

^[2]

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class III of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.43% (the Expense Cap). If at any time during the current fiscal year expenses for Class III of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

[2] From June 2, 2016

XML

03.31 FIMM Funds Class 3 Combo PRO-06 | Tax-Exempt Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Tax-Exempt Portfolio</b>/III</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of interest income exempt from federal income tax as is consistent with liquidity and stability of principal.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Tax-Exempt Portfolio}	03.31 FIMM Funds Class 3 Combo PRO-06 Tax-Exempt Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Tax-Exempt Portfolio}	03.31 FIMM Funds Class 3 Combo PRO-06 Tax-Exempt Portfolio Tax-Exempt Portfolio-Class III
Management fee	0.14%
Distribution and/or Service (12b-1) fees	0.25%
Other expenses	0.07%
Total annual operating expenses	0.46%
Fee waiver and/or expense reimbursement	0.03%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.43%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Tax-Exempt Portfolio}	03.31 FIMM Funds Class 3 Combo PRO-06 Tax-Exempt Portfolio Tax-Exempt Portfolio-Class III USD ($)
1 year	$ 44
3 years	144
5 years	254
10 years	$ 575

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Normally investing in municipal money market securities.

• Prior to June 30, 2016, normally not investing in any rated security that is rated in the second highest short-term rating category by Standard & Poor's (S&P) or Moody's Investors Service (Moody's) (beginning June 30, 2016, this investment strategy is no longer in effect).

• Normally investing at least 80% of assets in municipal securities whose interest is exempt from federal income tax.

• Normally not investing in municipal securities whose interest is subject to the federal alternative minimum tax.

• Potentially investing up to 20% of assets in securities subject to state and/or federal income tax.

• Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Municipal Market Volatility. The municipal market is volatile and can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities.

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Foreign Exposure. Entities providing credit support or a maturity-shortening structure that are located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.32%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2014
Year-to-Date Return	0.30%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Tax-Exempt Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds Class 3 Combo PRO-06 | Tax-Exempt Portfolio | Tax-Exempt Portfolio-Class III | Return Before Taxes	1.05%	0.36%	0.19%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class III of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.43% (the Expense Cap). If at any time during the current fiscal year expenses for Class III of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Class 3 Combo PRO-06 | Tax-Exempt Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Tax-Exempt Portfolio</b>/III</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of interest income exempt from federal income tax as is consistent with liquidity and stability of principal.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Normally investing in municipal money market securities.

• Prior to June 30, 2016, normally not investing in any rated security that is rated in the second highest short-term rating category by Standard & Poor's (S&P) or Moody's Investors Service (Moody's) (beginning June 30, 2016, this investment strategy is no longer in effect).

• Normally investing at least 80% of assets in municipal securities whose interest is exempt from federal income tax.

• Normally not investing in municipal securities whose interest is subject to the federal alternative minimum tax.

• Potentially investing up to 20% of assets in securities subject to state and/or federal income tax.

• Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Municipal Market Volatility. The municipal market is volatile and can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities.

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Foreign Exposure. Entities providing credit support or a maturity-shortening structure that are located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund May Impose Fees or Suspend Sales [Text]

rr_RiskMoneyMarketFundMayImposeFeesOrSuspendSales

<font style="FONT-FAMILY: Times New Roman" size="3">The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Money Market Fund May Not Preserve Dollar [Text]

rr_RiskMoneyMarketFundMayNotPreserveDollar

<font style="FONT-FAMILY: Times New Roman" size="3">Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">institutional.fidelity.com</font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.32%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2014
Year-to-Date Return	0.30%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Class 3 Combo PRO-06 | Tax-Exempt Portfolio | Tax-Exempt Portfolio-Class III

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.25%

Other expenses

rr_OtherExpensesOverAssets

0.07%

Total annual operating expenses

rr_ExpensesOverAssets

0.46%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.03%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.43%

1 year

rr_ExpenseExampleYear01

$ 44

3 years

rr_ExpenseExampleYear03

144

5 years

rr_ExpenseExampleYear05

254

10 years

rr_ExpenseExampleYear10

$ 575

2009

rr_AnnualReturn2009

0.07%

2010

rr_AnnualReturn2010

0.01%

2011

rr_AnnualReturn2011

0.01%

2012

rr_AnnualReturn2012

0.01%

2013

rr_AnnualReturn2013

0.02%

2014

rr_AnnualReturn2014

0.03%

2015

rr_AnnualReturn2015

0.08%

2016

rr_AnnualReturn2016

0.18%

2017

rr_AnnualReturn2017

0.46%

2018

rr_AnnualReturn2018

1.05%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.30%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.32%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2014

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

03.31 FIMM Funds Class 3 Combo PRO-06 | Tax-Exempt Portfolio | Return Before Taxes | Tax-Exempt Portfolio-Class III

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Tax-Exempt Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Class III</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.05%

Past 5 years

rr_AverageAnnualReturnYear05

0.36%

Past 10 years

rr_AverageAnnualReturnYear10

0.19%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class III of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.43% (the Expense Cap). If at any time during the current fiscal year expenses for Class III of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds Class 3 Combo PRO-06 | Treasury Only Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Treasury Only Portfolio</b>/III</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks as high a level of current income as is consistent with the security of principal and liquidity.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Treasury Only Portfolio}	03.31 FIMM Funds Class 3 Combo PRO-06 Treasury Only Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Treasury Only Portfolio}	03.31 FIMM Funds Class 3 Combo PRO-06 Treasury Only Portfolio Treasury Only Portfolio-Class III
Management fee	0.14%
Distribution and/or Service (12b-1) fees	0.25%
Other expenses	0.07%
Total annual operating expenses	0.46%
Fee waiver and/or expense reimbursement	0.03%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.43%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Treasury Only Portfolio}	03.31 FIMM Funds Class 3 Combo PRO-06 Treasury Only Portfolio Treasury Only Portfolio-Class III USD ($)
1 year	$ 44
3 years	144
5 years	254
10 years	$ 575

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Normally investing at least 99.5% of total assets in cash and U.S. Treasury securities.

• Normally investing in securities whose interest is exempt from state and local income taxes.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.46%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.50%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Treasury Only Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds Class 3 Combo PRO-06 | Treasury Only Portfolio | Treasury Only Portfolio-Class III | Return Before Taxes	1.46%	0.39%	0.20%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class III of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.43% (the Expense Cap). If at any time during the current fiscal year expenses for Class III of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Class 3 Combo PRO-06 | Treasury Only Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Treasury Only Portfolio</b>/III</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks as high a level of current income as is consistent with the security of principal and liquidity.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Normally investing at least 99.5% of total assets in cash and U.S. Treasury securities.

• Normally investing in securities whose interest is exempt from state and local income taxes.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities.

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Money Market Fund May Not Preserve Dollar [Text]

rr_RiskMoneyMarketFundMayNotPreserveDollar

<font style="FONT-FAMILY: Times New Roman" size="3">Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">institutional.fidelity.com</font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.46%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.50%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Class 3 Combo PRO-06 | Treasury Only Portfolio | Treasury Only Portfolio-Class III

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.25%

Other expenses

rr_OtherExpensesOverAssets

0.07%

Total annual operating expenses

rr_ExpensesOverAssets

0.46%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.03%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.43%

1 year

rr_ExpenseExampleYear01

$ 44

3 years

rr_ExpenseExampleYear03

144

5 years

rr_ExpenseExampleYear05

254

10 years

rr_ExpenseExampleYear10

$ 575

2009

rr_AnnualReturn2009

0.03%

2010

rr_AnnualReturn2010

0.01%

2011

rr_AnnualReturn2011

0.01%

2012

rr_AnnualReturn2012

0.01%

2013

rr_AnnualReturn2013

0.01%

2014

rr_AnnualReturn2014

0.01%

2015

rr_AnnualReturn2015

0.01%

2016

rr_AnnualReturn2016

0.01%

2017

rr_AnnualReturn2017

0.46%

2018

rr_AnnualReturn2018

1.46%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.50%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.46%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

03.31 FIMM Funds Class 3 Combo PRO-06 | Treasury Only Portfolio | Return Before Taxes | Treasury Only Portfolio-Class III

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Treasury Only Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Class III</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.46%

Past 5 years

rr_AverageAnnualReturnYear05

0.39%

Past 10 years

rr_AverageAnnualReturnYear10

0.20%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class III of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.43% (the Expense Cap). If at any time during the current fiscal year expenses for Class III of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds Class 3 Combo PRO-06 | Treasury Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Treasury Portfolio</b>/III</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Treasury Portfolio}	03.31 FIMM Funds Class 3 Combo PRO-06 Treasury Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Treasury Portfolio}	03.31 FIMM Funds Class 3 Combo PRO-06 Treasury Portfolio Treasury Portfolio-Class III
Management fee	0.14%
Distribution and/or Service (12b-1) fees	0.25%
Other expenses	0.07%
Total annual operating expenses	0.46%
Fee waiver and/or expense reimbursement	0.03%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.43%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Treasury Portfolio}	03.31 FIMM Funds Class 3 Combo PRO-06 Treasury Portfolio Treasury Portfolio-Class III USD ($)
1 year	$ 44
3 years	144
5 years	254
10 years	$ 575

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Normally investing at least 99.5% of total assets in cash, U.S. Treasury securities and/or repurchase agreements for those securities.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities and repurchase agreements for those securities.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.47%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.50%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Treasury Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds Class 3 Combo PRO-06 | Treasury Portfolio | Treasury Portfolio-Class III | Return Before Taxes	1.46%	0.40%	0.20%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class III of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.43% (the Expense Cap). If at any time during the current fiscal year expenses for Class III of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Class 3 Combo PRO-06 | Treasury Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Treasury Portfolio</b>/III</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Normally investing at least 99.5% of total assets in cash, U.S. Treasury securities and/or repurchase agreements for those securities.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities and repurchase agreements for those securities.

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Money Market Fund May Not Preserve Dollar [Text]

rr_RiskMoneyMarketFundMayNotPreserveDollar

<font style="FONT-FAMILY: Times New Roman" size="3">Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">institutional.fidelity.com</font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.47%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.50%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Class 3 Combo PRO-06 | Treasury Portfolio | Treasury Portfolio-Class III

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.25%

Other expenses

rr_OtherExpensesOverAssets

0.07%

Total annual operating expenses

rr_ExpensesOverAssets

0.46%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.03%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.43%

1 year

rr_ExpenseExampleYear01

$ 44

3 years

rr_ExpenseExampleYear03

144

5 years

rr_ExpenseExampleYear05

254

10 years

rr_ExpenseExampleYear10

$ 575

2009

rr_AnnualReturn2009

0.03%

2010

rr_AnnualReturn2010

0.01%

2011

rr_AnnualReturn2011

0.01%

2012

rr_AnnualReturn2012

0.01%

2013

rr_AnnualReturn2013

0.01%

2014

rr_AnnualReturn2014

0.01%

2015

rr_AnnualReturn2015

0.01%

2016

rr_AnnualReturn2016

0.01%

2017

rr_AnnualReturn2017

0.49%

2018

rr_AnnualReturn2018

1.46%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.50%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.47%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

03.31 FIMM Funds Class 3 Combo PRO-06 | Treasury Portfolio | Return Before Taxes | Treasury Portfolio-Class III

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Treasury Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Class III</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.46%

Past 5 years

rr_AverageAnnualReturnYear05

0.40%

Past 10 years

rr_AverageAnnualReturnYear10

0.20%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class III of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.43% (the Expense Cap). If at any time during the current fiscal year expenses for Class III of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds Class 2 Combo PRO-06 | Government Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Government Portfolio</b>/II</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Government Portfolio}	03.31 FIMM Funds Class 2 Combo PRO-06 Government Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Government Portfolio}	03.31 FIMM Funds Class 2 Combo PRO-06 Government Portfolio Government Portfolio-Class II
Management fee	0.14%
Distribution and/or Service (12b-1) fees	0.15%
Other expenses	0.07%
Total annual operating expenses	0.36%
Fee waiver and/or expense reimbursement	0.03%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.33%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Government Portfolio}	03.31 FIMM Funds Class 2 Combo PRO-06 Government Portfolio Government Portfolio-Class II USD ($)
1 year	$ 34
3 years	112
5 years	198
10 years	$ 452

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Normally investing at least 99.5% of total assets in cash, U.S. Government securities and/or repurchase agreements that are collateralized fully (i.e., collateralized by cash or government securities).

• Investing in U.S. Government securities issued by entities that are chartered or sponsored by Congress but whose securities are neither issued nor guaranteed by the U.S. Treasury.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Government securities and repurchase agreements for those securities.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.49%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.53%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Government Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds Class 2 Combo PRO-06 | Government Portfolio | Government Portfolio-Class II | Return Before Taxes	1.57%	0.46%	0.25%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class II of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.33% (the Expense Cap). If at any time during the current fiscal year expenses for Class II of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Class 2 Combo PRO-06 | Government Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Government Portfolio</b>/II</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Normally investing at least 99.5% of total assets in cash, U.S. Government securities and/or repurchase agreements that are collateralized fully (i.e., collateralized by cash or government securities).

• Investing in U.S. Government securities issued by entities that are chartered or sponsored by Congress but whose securities are neither issued nor guaranteed by the U.S. Treasury.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Government securities and repurchase agreements for those securities.

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Money Market Fund May Not Preserve Dollar [Text]

rr_RiskMoneyMarketFundMayNotPreserveDollar

<font style="FONT-FAMILY: Times New Roman" size="3">Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">institutional.fidelity.com </font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.49%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.53%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Class 2 Combo PRO-06 | Government Portfolio | Government Portfolio-Class II

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.15%

Other expenses

rr_OtherExpensesOverAssets

0.07%

Total annual operating expenses

rr_ExpensesOverAssets

0.36%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.03%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.33%

1 year

rr_ExpenseExampleYear01

$ 34

3 years

rr_ExpenseExampleYear03

112

5 years

rr_ExpenseExampleYear05

198

10 years

rr_ExpenseExampleYear10

$ 452

2009

rr_AnnualReturn2009

0.21%

2010

rr_AnnualReturn2010

0.01%

2011

rr_AnnualReturn2011

0.01%

2012

rr_AnnualReturn2012

0.01%

2013

rr_AnnualReturn2013

0.01%

2014

rr_AnnualReturn2014

0.01%

2015

rr_AnnualReturn2015

0.01%

2016

rr_AnnualReturn2016

0.10%

2017

rr_AnnualReturn2017

0.60%

2018

rr_AnnualReturn2018

1.57%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.53%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.49%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

03.31 FIMM Funds Class 2 Combo PRO-06 | Government Portfolio | Return Before Taxes | Government Portfolio-Class II

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Government Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Class II</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.57%

Past 5 years

rr_AverageAnnualReturnYear05

0.46%

Past 10 years

rr_AverageAnnualReturnYear10

0.25%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class II of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.33% (the Expense Cap). If at any time during the current fiscal year expenses for Class II of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds Class 2 Combo PRO-06 | Money Market Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Money Market Portfolio</b>/II</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Money Market Portfolio}	03.31 FIMM Funds Class 2 Combo PRO-06 Money Market Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Money Market Portfolio}	03.31 FIMM Funds Class 2 Combo PRO-06 Money Market Portfolio Money Market Portfolio-Class II
Management fee	0.14%
Distribution and/or Service (12b-1) fees	0.15%
Other expenses	0.07%
Total annual operating expenses	0.36%
Fee waiver and/or expense reimbursement	0.03%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.33%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Money Market Portfolio}	03.31 FIMM Funds Class 2 Combo PRO-06 Money Market Portfolio Money Market Portfolio-Class II USD ($)
1 year	$ 34
3 years	112
5 years	198
10 years	$ 452

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.54%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.58%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Money Market Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds Class 2 Combo PRO-06 | Money Market Portfolio | Money Market Portfolio-Class II | Return Before Taxes	1.85%	0.63%	0.39%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class II of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.33% (the Expense Cap). If at any time during the current fiscal year expenses for Class II of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Class 2 Combo PRO-06 | Money Market Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Money Market Portfolio</b>/II</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Strategy Portfolio Concentration [Text]

rr_StrategyPortfolioConcentration

<font style="FONT-FAMILY: Times New Roman" size="3">Investing more than 25% of total assets in the financial services industries.</font>

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund May Impose Fees or Suspend Sales [Text]

rr_RiskMoneyMarketFundMayImposeFeesOrSuspendSales

<font style="FONT-FAMILY: Times New Roman" size="3">The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Money Market Fund May Not Preserve Dollar [Text]

rr_RiskMoneyMarketFundMayNotPreserveDollar

<font style="FONT-FAMILY: Times New Roman" size="3">Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">institutional.fidelity.com </font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.54%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.58%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Class 2 Combo PRO-06 | Money Market Portfolio | Money Market Portfolio-Class II

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.15%

Other expenses

rr_OtherExpensesOverAssets

0.07%

Total annual operating expenses

rr_ExpensesOverAssets

0.36%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.03%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.33%

1 year

rr_ExpenseExampleYear01

$ 34

3 years

rr_ExpenseExampleYear03

112

5 years

rr_ExpenseExampleYear05

198

10 years

rr_ExpenseExampleYear10

$ 452

2009

rr_AnnualReturn2009

0.58%

2010

rr_AnnualReturn2010

0.09%

2011

rr_AnnualReturn2011

0.03%

2012

rr_AnnualReturn2012

0.04%

2013

rr_AnnualReturn2013

0.01%

2014

rr_AnnualReturn2014

0.01%

2015

rr_AnnualReturn2015

0.01%

2016

rr_AnnualReturn2016

0.33%

2017

rr_AnnualReturn2017

0.98%

2018

rr_AnnualReturn2018

1.85%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.58%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.54%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

03.31 FIMM Funds Class 2 Combo PRO-06 | Money Market Portfolio | Return Before Taxes | Money Market Portfolio-Class II

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Money Market Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Class II</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.85%

Past 5 years

rr_AverageAnnualReturnYear05

0.63%

Past 10 years

rr_AverageAnnualReturnYear10

0.39%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class II of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.33% (the Expense Cap). If at any time during the current fiscal year expenses for Class II of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds Class 2 Combo PRO-06 | Prime Money Market Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Prime Money Market Portfolio</b>/II</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Prime Money Market Portfolio}	03.31 FIMM Funds Class 2 Combo PRO-06 Prime Money Market Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Prime Money Market Portfolio}	03.31 FIMM Funds Class 2 Combo PRO-06 Prime Money Market Portfolio Prime Money Market Portfolio-Class II
Management fee	0.14%
Distribution and/or Service (12b-1) fees	0.15%
Other expenses	0.07%
Total annual operating expenses	0.36%
Fee waiver and/or expense reimbursement	0.03%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.33%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Prime Money Market Portfolio}	03.31 FIMM Funds Class 2 Combo PRO-06 Prime Money Market Portfolio Prime Money Market Portfolio-Class II USD ($)
1 year	$ 34
3 years	112
5 years	198
10 years	$ 452

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Although the fund is a money market fund, it will price and transact at a "floating" net asset value (NAV) that will fluctuate based on changes in the value of the fund's investments.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Floating NAV. The fund does not maintain a constant NAV per share of $1.00. The value of the fund's shares will be calculated to four decimal places and will vary as a result of changes in the value of the securities in which the fund invests.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.53%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2014
Year-to-Date Return	0.57%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Prime Money Market Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds Class 2 Combo PRO-06 | Prime Money Market Portfolio | Prime Money Market Portfolio-Class II | Return Before Taxes	1.79%	0.62%	0.36%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class II of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.33% (the Expense Cap). If at any time during the current fiscal year expenses for Class II of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Class 2 Combo PRO-06 | Prime Money Market Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Prime Money Market Portfolio</b>/II</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Although the fund is a money market fund, it will price and transact at a "floating" net asset value (NAV) that will fluctuate based on changes in the value of the fund's investments.

Strategy Portfolio Concentration [Text]

rr_StrategyPortfolioConcentration

<font style="FONT-FAMILY: Times New Roman" size="3">Investing more than 25% of total assets in the financial services industries.</font>

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Floating NAV. The fund does not maintain a constant NAV per share of $1.00. The value of the fund's shares will be calculated to four decimal places and will vary as a result of changes in the value of the securities in which the fund invests.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Price Fluctuates [Text]

rr_RiskMoneyMarketFundPriceFluctuates

<font style="FONT-FAMILY: Times New Roman" size="3">Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them.</font>

Risk Money Market Fund May Impose Fees or Suspend Sales [Text]

rr_RiskMoneyMarketFundMayImposeFeesOrSuspendSales

<font style="FONT-FAMILY: Times New Roman" size="3">The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">institutional.fidelity.com</font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.53%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2014
Year-to-Date Return	0.57%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Class 2 Combo PRO-06 | Prime Money Market Portfolio | Prime Money Market Portfolio-Class II

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.15%

Other expenses

rr_OtherExpensesOverAssets

0.07%

Total annual operating expenses

rr_ExpensesOverAssets

0.36%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.03%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.33%

1 year

rr_ExpenseExampleYear01

$ 34

3 years

rr_ExpenseExampleYear03

112

5 years

rr_ExpenseExampleYear05

198

10 years

rr_ExpenseExampleYear10

$ 452

2009

rr_AnnualReturn2009

0.43%

2010

rr_AnnualReturn2010

0.04%

2011

rr_AnnualReturn2011

0.01%

2012

rr_AnnualReturn2012

0.01%

2013

rr_AnnualReturn2013

0.01%

2014

rr_AnnualReturn2014

0.01%

2015

rr_AnnualReturn2015

0.02%

2016

rr_AnnualReturn2016

0.37%

2017

rr_AnnualReturn2017

0.91%

2018

rr_AnnualReturn2018

1.79%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.57%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.53%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2014

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

03.31 FIMM Funds Class 2 Combo PRO-06 | Prime Money Market Portfolio | Return Before Taxes | Prime Money Market Portfolio-Class II

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Prime Money Market Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Class II</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.79%

Past 5 years

rr_AverageAnnualReturnYear05

0.62%

Past 10 years

rr_AverageAnnualReturnYear10

0.36%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class II of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.33% (the Expense Cap). If at any time during the current fiscal year expenses for Class II of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds Class 2 Combo PRO-06 | Prime Reserves Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Prime Reserves Portfolio</b>/II</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Prime Reserves Portfolio}	03.31 FIMM Funds Class 2 Combo PRO-06 Prime Reserves Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Prime Reserves Portfolio}	03.31 FIMM Funds Class 2 Combo PRO-06 Prime Reserves Portfolio Prime Reserves Portfolio-Class II
Management fee	0.14%
Distribution and/or Service (12b-1) fees	0.15%
Other expenses	0.09%
Total annual operating expenses	0.38%
Fee waiver and/or expense reimbursement	0.05%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.33%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Prime Reserves Portfolio}	03.31 FIMM Funds Class 2 Combo PRO-06 Prime Reserves Portfolio Prime Reserves Portfolio-Class II USD ($)
1 year	$ 34
3 years	115
5 years	206
10 years	$ 474

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Although the fund is a money market fund, it will price and transact at a "floating" net asset value (NAV) that will fluctuate based on changes in the value of the fund's investments.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Floating NAV. The fund does not maintain a constant NAV per share of $1.00. The value of the fund's shares will be calculated to four decimal places and will vary as a result of changes in the value of the securities in which the fund invests.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.54%	December 31, 2018
Lowest Quarter Return	0.18%	March 31, 2017
Year-to-Date Return	0.58%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Prime Reserves Portfolio} - 03.31 FIMM Funds Class 2 Combo PRO-06 - Prime Reserves Portfolio - Prime Reserves Portfolio-Class II	Past 1 year	Since Inception	[1]	Inception Date
Before Taxes				Jun. 02, 2016
Return Before Taxes	1.81%	1.13%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class II of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.33% (the Expense Cap). If at any time during the current fiscal year expenses for Class II of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

[1] From June 2, 2016

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Class 2 Combo PRO-06 | Prime Reserves Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Prime Reserves Portfolio</b>/II</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Although the fund is a money market fund, it will price and transact at a "floating" net asset value (NAV) that will fluctuate based on changes in the value of the fund's investments.

Strategy Portfolio Concentration [Text]

rr_StrategyPortfolioConcentration

<font style="FONT-FAMILY: Times New Roman" size="3">Investing more than 25% of total assets in the financial services industries.</font>

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Floating NAV. The fund does not maintain a constant NAV per share of $1.00. The value of the fund's shares will be calculated to four decimal places and will vary as a result of changes in the value of the securities in which the fund invests.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Price Fluctuates [Text]

rr_RiskMoneyMarketFundPriceFluctuates

<font style="FONT-FAMILY: Times New Roman" size="3">Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them.</font>

Risk Money Market Fund May Impose Fees or Suspend Sales [Text]

rr_RiskMoneyMarketFundMayImposeFeesOrSuspendSales

<font style="FONT-FAMILY: Times New Roman" size="3">The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">institutional.fidelity.com</font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.54%	December 31, 2018
Lowest Quarter Return	0.18%	March 31, 2017
Year-to-Date Return	0.58%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Class 2 Combo PRO-06 | Prime Reserves Portfolio | Prime Reserves Portfolio-Class II

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.15%

Other expenses

rr_OtherExpensesOverAssets

0.09%

Total annual operating expenses

rr_ExpensesOverAssets

0.38%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.05%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.33%

1 year

rr_ExpenseExampleYear01

$ 34

3 years

rr_ExpenseExampleYear03

115

5 years

rr_ExpenseExampleYear05

206

10 years

rr_ExpenseExampleYear10

$ 474

2017

rr_AnnualReturn2017

0.92%

2018

rr_AnnualReturn2018

1.81%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.58%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.54%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2017

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

0.18%

Inception Date

rr_AverageAnnualReturnInceptionDate

Jun. 02, 2016

03.31 FIMM Funds Class 2 Combo PRO-06 | Prime Reserves Portfolio | Return Before Taxes | Prime Reserves Portfolio-Class II

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Prime Reserves Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Class II</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.81%

Since Inception

rr_AverageAnnualReturnSinceInception

1.13%

^[2]

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class II of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.33% (the Expense Cap). If at any time during the current fiscal year expenses for Class II of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

[2] From June 2, 2016

XML

03.31 FIMM Funds Class 2 Combo PRO-06 | Tax-Exempt Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Tax-Exempt Portfolio</b>/II</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of interest income exempt from federal income tax as is consistent with liquidity and stability of principal.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Tax-Exempt Portfolio}	03.31 FIMM Funds Class 2 Combo PRO-06 Tax-Exempt Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Tax-Exempt Portfolio}	03.31 FIMM Funds Class 2 Combo PRO-06 Tax-Exempt Portfolio Tax-Exempt Portfolio-Class II
Management fee	0.14%
Distribution and/or Service (12b-1) fees	0.15%
Other expenses	0.07%
Total annual operating expenses	0.36%
Fee waiver and/or expense reimbursement	0.03%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.33%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Tax-Exempt Portfolio}	03.31 FIMM Funds Class 2 Combo PRO-06 Tax-Exempt Portfolio Tax-Exempt Portfolio-Class II USD ($)
1 year	$ 34
3 years	112
5 years	198
10 years	$ 452

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Normally investing in municipal money market securities.

• Prior to June 30, 2016, normally not investing in any rated security that is rated in the second highest short-term rating category by Standard & Poor's (S&P) or Moody's Investors Service (Moody's) (beginning June 30, 2016, this investment strategy is no longer in effect).

• Normally investing at least 80% of assets in municipal securities whose interest is exempt from federal income tax.

• Normally not investing in municipal securities whose interest is subject to the federal alternative minimum tax.

• Potentially investing up to 20% of assets in securities subject to state and/or federal income tax.

• Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Municipal Market Volatility. The municipal market is volatile and can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities.

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Foreign Exposure. Entities providing credit support or a maturity-shortening structure that are located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.35%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2013
Year-to-Date Return	0.32%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Tax-Exempt Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds Class 2 Combo PRO-06 | Tax-Exempt Portfolio | Tax-Exempt Portfolio-Class II | Return Before Taxes	1.15%	0.41%	0.22%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class II of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.33% (the Expense Cap). If at any time during the current fiscal year expenses for Class II of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Class 2 Combo PRO-06 | Tax-Exempt Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Tax-Exempt Portfolio</b>/II</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of interest income exempt from federal income tax as is consistent with liquidity and stability of principal.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Normally investing in municipal money market securities.

• Prior to June 30, 2016, normally not investing in any rated security that is rated in the second highest short-term rating category by Standard & Poor's (S&P) or Moody's Investors Service (Moody's) (beginning June 30, 2016, this investment strategy is no longer in effect).

• Normally investing at least 80% of assets in municipal securities whose interest is exempt from federal income tax.

• Normally not investing in municipal securities whose interest is subject to the federal alternative minimum tax.

• Potentially investing up to 20% of assets in securities subject to state and/or federal income tax.

• Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Municipal Market Volatility. The municipal market is volatile and can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities.

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Foreign Exposure. Entities providing credit support or a maturity-shortening structure that are located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund May Impose Fees or Suspend Sales [Text]

rr_RiskMoneyMarketFundMayImposeFeesOrSuspendSales

<font style="FONT-FAMILY: Times New Roman" size="3">The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Money Market Fund May Not Preserve Dollar [Text]

rr_RiskMoneyMarketFundMayNotPreserveDollar

<font style="FONT-FAMILY: Times New Roman" size="3">Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">institutional.fidelity.com </font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.35%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2013
Year-to-Date Return	0.32%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Class 2 Combo PRO-06 | Tax-Exempt Portfolio | Tax-Exempt Portfolio-Class II

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.15%

Other expenses

rr_OtherExpensesOverAssets

0.07%

Total annual operating expenses

rr_ExpensesOverAssets

0.36%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.03%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.33%

1 year

rr_ExpenseExampleYear01

$ 34

3 years

rr_ExpenseExampleYear03

112

5 years

rr_ExpenseExampleYear05

198

10 years

rr_ExpenseExampleYear10

$ 452

2009

rr_AnnualReturn2009

0.12%

2010

rr_AnnualReturn2010

0.01%

2011

rr_AnnualReturn2011

0.01%

2012

rr_AnnualReturn2012

0.01%

2013

rr_AnnualReturn2013

0.02%

2014

rr_AnnualReturn2014

0.03%

2015

rr_AnnualReturn2015

0.08%

2016

rr_AnnualReturn2016

0.25%

2017

rr_AnnualReturn2017

0.56%

2018

rr_AnnualReturn2018

1.15%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.32%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.35%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2013

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

03.31 FIMM Funds Class 2 Combo PRO-06 | Tax-Exempt Portfolio | Return Before Taxes | Tax-Exempt Portfolio-Class II

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Tax-Exempt Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Class II</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.15%

Past 5 years

rr_AverageAnnualReturnYear05

0.41%

Past 10 years

rr_AverageAnnualReturnYear10

0.22%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class II of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.33% (the Expense Cap). If at any time during the current fiscal year expenses for Class II of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds Class 2 Combo PRO-06 | Treasury Only Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Treasury Only Portfolio</b>/II</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks as high a level of current income as is consistent with the security of principal and liquidity.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Treasury Only Portfolio}	03.31 FIMM Funds Class 2 Combo PRO-06 Treasury Only Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Treasury Only Portfolio}	03.31 FIMM Funds Class 2 Combo PRO-06 Treasury Only Portfolio Treasury Only Portfolio-Class II
Management fee	0.14%
Distribution and/or Service (12b-1) fees	0.15%
Other expenses	0.07%
Total annual operating expenses	0.36%
Fee waiver and/or expense reimbursement	0.03%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.33%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Treasury Only Portfolio}	03.31 FIMM Funds Class 2 Combo PRO-06 Treasury Only Portfolio Treasury Only Portfolio-Class II USD ($)
1 year	$ 34
3 years	112
5 years	198
10 years	$ 452

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Normally investing at least 99.5% of total assets in cash and U.S. Treasury securities.

• Normally investing in securities whose interest is exempt from state and local income taxes.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.49%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2014
Year-to-Date Return	0.52%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Treasury Only Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds Class 2 Combo PRO-06 | Treasury Only Portfolio | Treasury Only Portfolio-Class II | Return Before Taxes	1.56%	0.43%	0.23%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class II of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.33% (the Expense Cap). If at any time during the current fiscal year expenses for Class II of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Class 2 Combo PRO-06 | Treasury Only Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Treasury Only Portfolio</b>/II</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks as high a level of current income as is consistent with the security of principal and liquidity.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Normally investing at least 99.5% of total assets in cash and U.S. Treasury securities.

• Normally investing in securities whose interest is exempt from state and local income taxes.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities.

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Money Market Fund May Not Preserve Dollar [Text]

rr_RiskMoneyMarketFundMayNotPreserveDollar

<font style="FONT-FAMILY: Times New Roman" size="3">Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">institutional.fidelity.com</font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.49%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2014
Year-to-Date Return	0.52%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Class 2 Combo PRO-06 | Treasury Only Portfolio | Treasury Only Portfolio-Class II

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.15%

Other expenses

rr_OtherExpensesOverAssets

0.07%

Total annual operating expenses

rr_ExpensesOverAssets

0.36%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.03%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.33%

1 year

rr_ExpenseExampleYear01

$ 34

3 years

rr_ExpenseExampleYear03

112

5 years

rr_ExpenseExampleYear05

198

10 years

rr_ExpenseExampleYear10

$ 452

2009

rr_AnnualReturn2009

0.06%

2010

rr_AnnualReturn2010

0.01%

2011

rr_AnnualReturn2011

0.01%

2012

rr_AnnualReturn2012

0.01%

2013

rr_AnnualReturn2013

0.01%

2014

rr_AnnualReturn2014

0.01%

2015

rr_AnnualReturn2015

0.01%

2016

rr_AnnualReturn2016

0.03%

2017

rr_AnnualReturn2017

0.56%

2018

rr_AnnualReturn2018

1.56%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.52%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.49%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2014

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

03.31 FIMM Funds Class 2 Combo PRO-06 | Treasury Only Portfolio | Return Before Taxes | Treasury Only Portfolio-Class II

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Treasury Only Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Class II</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.56%

Past 5 years

rr_AverageAnnualReturnYear05

0.43%

Past 10 years

rr_AverageAnnualReturnYear10

0.23%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class II of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.33% (the Expense Cap). If at any time during the current fiscal year expenses for Class II of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds Class 2 Combo PRO-06 | Treasury Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Treasury Portfolio</b>/II</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Treasury Portfolio}	03.31 FIMM Funds Class 2 Combo PRO-06 Treasury Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Treasury Portfolio}	03.31 FIMM Funds Class 2 Combo PRO-06 Treasury Portfolio Treasury Portfolio-Class II
Management fee	0.14%
Distribution and/or Service (12b-1) fees	0.15%
Other expenses	0.07%
Total annual operating expenses	0.36%
Fee waiver and/or expense reimbursement	0.03%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.33%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Treasury Portfolio}	03.31 FIMM Funds Class 2 Combo PRO-06 Treasury Portfolio Treasury Portfolio-Class II USD ($)
1 year	$ 34
3 years	112
5 years	198
10 years	$ 452

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Normally investing at least 99.5% of total assets in cash, U.S. Treasury securities and/or repurchase agreements for those securities.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities and repurchase agreements for those securities.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.49%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.52%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Treasury Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds Class 2 Combo PRO-06 | Treasury Portfolio | Treasury Portfolio-Class II | Return Before Taxes	1.56%	0.44%	0.23%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class II of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.33% (the Expense Cap). If at any time during the current fiscal year expenses for Class II of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Class 2 Combo PRO-06 | Treasury Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Treasury Portfolio</b>/II</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Normally investing at least 99.5% of total assets in cash, U.S. Treasury securities and/or repurchase agreements for those securities.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities and repurchase agreements for those securities.

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Money Market Fund May Not Preserve Dollar [Text]

rr_RiskMoneyMarketFundMayNotPreserveDollar

<font style="FONT-FAMILY: Times New Roman" size="3">Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">institutional.fidelity.com</font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.49%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.52%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Class 2 Combo PRO-06 | Treasury Portfolio | Treasury Portfolio-Class II

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.15%

Other expenses

rr_OtherExpensesOverAssets

0.07%

Total annual operating expenses

rr_ExpensesOverAssets

0.36%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.03%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.33%

1 year

rr_ExpenseExampleYear01

$ 34

3 years

rr_ExpenseExampleYear03

112

5 years

rr_ExpenseExampleYear05

198

10 years

rr_ExpenseExampleYear10

$ 452

2009

rr_AnnualReturn2009

0.08%

2010

rr_AnnualReturn2010

0.01%

2011

rr_AnnualReturn2011

0.01%

2012

rr_AnnualReturn2012

0.01%

2013

rr_AnnualReturn2013

0.01%

2014

rr_AnnualReturn2014

0.01%

2015

rr_AnnualReturn2015

0.01%

2016

rr_AnnualReturn2016

0.06%

2017

rr_AnnualReturn2017

0.59%

2018

rr_AnnualReturn2018

1.56%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.52%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.49%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

03.31 FIMM Funds Class 2 Combo PRO-06 | Treasury Portfolio | Return Before Taxes | Treasury Portfolio-Class II

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Treasury Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Class II</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.56%

Past 5 years

rr_AverageAnnualReturnYear05

0.44%

Past 10 years

rr_AverageAnnualReturnYear10

0.23%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class II of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.33% (the Expense Cap). If at any time during the current fiscal year expenses for Class II of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds Class 1 Combo PRO-06 | Government Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Government Portfolio</b>/I</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Government Portfolio}	03.31 FIMM Funds Class 1 Combo PRO-06 Government Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Government Portfolio}	03.31 FIMM Funds Class 1 Combo PRO-06 Government Portfolio Government Portfolio-Class I
Management fee	0.14%
Distribution and/or Service (12b-1) fees	none
Other expenses	0.07%
Total annual operating expenses	0.21%
Fee waiver and/or expense reimbursement	0.03%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.18%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Government Portfolio}	03.31 FIMM Funds Class 1 Combo PRO-06 Government Portfolio Government Portfolio-Class I USD ($)
1 year	$ 18
3 years	64
5 years	114
10 years	$ 264

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Normally investing at least 99.5% of total assets in cash, U.S. Government securities and/or repurchase agreements that are collateralized fully (i.e., collateralized by cash or government securities).

• Investing in U.S. Government securities issued by entities that are chartered or sponsored by Congress but whose securities are neither issued nor guaranteed by the U.S. Treasury.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Government securities and repurchase agreements for those securities.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.53%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.56%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Government Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds Class 1 Combo PRO-06 | Government Portfolio | Government Portfolio-Class I | Return Before Taxes	1.72%	0.55%	0.32%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class I of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.18% (the Expense Cap). If at any time during the current fiscal year expenses for Class I of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Class 1 Combo PRO-06 | Government Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Government Portfolio</b>/I</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Normally investing at least 99.5% of total assets in cash, U.S. Government securities and/or repurchase agreements that are collateralized fully (i.e., collateralized by cash or government securities).

• Investing in U.S. Government securities issued by entities that are chartered or sponsored by Congress but whose securities are neither issued nor guaranteed by the U.S. Treasury.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Government securities and repurchase agreements for those securities.

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Money Market Fund May Not Preserve Dollar [Text]

rr_RiskMoneyMarketFundMayNotPreserveDollar

<font style="FONT-FAMILY: Times New Roman" size="3">Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">www.fidelity.com or institutional.fidelity.com </font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.53%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.56%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Class 1 Combo PRO-06 | Government Portfolio | Government Portfolio-Class I

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.07%

Total annual operating expenses

rr_ExpensesOverAssets

0.21%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.03%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.18%

1 year

rr_ExpenseExampleYear01

$ 18

3 years

rr_ExpenseExampleYear03

64

5 years

rr_ExpenseExampleYear05

114

10 years

rr_ExpenseExampleYear10

$ 264

2009

rr_AnnualReturn2009

0.34%

2010

rr_AnnualReturn2010

0.06%

2011

rr_AnnualReturn2011

0.01%

2012

rr_AnnualReturn2012

0.01%

2013

rr_AnnualReturn2013

0.01%

2014

rr_AnnualReturn2014

0.01%

2015

rr_AnnualReturn2015

0.01%

2016

rr_AnnualReturn2016

0.25%

2017

rr_AnnualReturn2017

0.75%

2018

rr_AnnualReturn2018

1.72%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.56%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.53%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

03.31 FIMM Funds Class 1 Combo PRO-06 | Government Portfolio | Return Before Taxes | Government Portfolio-Class I

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Government Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Class I</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.72%

Past 5 years

rr_AverageAnnualReturnYear05

0.55%

Past 10 years

rr_AverageAnnualReturnYear10

0.32%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class I of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.18% (the Expense Cap). If at any time during the current fiscal year expenses for Class I of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds Class 1 Combo PRO-06 | Money Market Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Money Market Portfolio</b>/I</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Money Market Portfolio}	03.31 FIMM Funds Class 1 Combo PRO-06 Money Market Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Money Market Portfolio}	03.31 FIMM Funds Class 1 Combo PRO-06 Money Market Portfolio Money Market Portfolio-Class I
Management fee	0.14%
Distribution and/or Service (12b-1) fees	none
Other expenses	0.07%
Total annual operating expenses	0.21%
Fee waiver and/or expense reimbursement	0.03%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.18%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Money Market Portfolio}	03.31 FIMM Funds Class 1 Combo PRO-06 Money Market Portfolio Money Market Portfolio-Class I USD ($)
1 year	$ 18
3 years	64
5 years	114
10 years	$ 264

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.58%	December 31, 2018
Lowest Quarter Return	0.01%	March 31, 2014
Year-to-Date Return	0.62%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Money Market Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds Class 1 Combo PRO-06 | Money Market Portfolio | Money Market Portfolio-Class I | Return Before Taxes	2.00%	0.75%	0.52%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class I of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.18% (the Expense Cap). If at any time during the current fiscal year expenses for Class I of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Class 1 Combo PRO-06 | Money Market Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Money Market Portfolio</b>/I</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Strategy Portfolio Concentration [Text]

rr_StrategyPortfolioConcentration

<font style="FONT-FAMILY: Times New Roman" size="3">Investing more than 25% of total assets in the financial services industries.</font>

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund May Impose Fees or Suspend Sales [Text]

rr_RiskMoneyMarketFundMayImposeFeesOrSuspendSales

<font style="FONT-FAMILY: Times New Roman" size="3">The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Money Market Fund May Not Preserve Dollar [Text]

rr_RiskMoneyMarketFundMayNotPreserveDollar

<font style="FONT-FAMILY: Times New Roman" size="3">Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">www.fidelity.com or institutional.fidelity.com </font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.58%	December 31, 2018
Lowest Quarter Return	0.01%	March 31, 2014
Year-to-Date Return	0.62%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Class 1 Combo PRO-06 | Money Market Portfolio | Money Market Portfolio-Class I

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.07%

Total annual operating expenses

rr_ExpensesOverAssets

0.21%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.03%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.18%

1 year

rr_ExpenseExampleYear01

$ 18

3 years

rr_ExpenseExampleYear03

64

5 years

rr_ExpenseExampleYear05

114

10 years

rr_ExpenseExampleYear10

$ 264

2009

rr_AnnualReturn2009

0.73%

2010

rr_AnnualReturn2010

0.24%

2011

rr_AnnualReturn2011

0.16%

2012

rr_AnnualReturn2012

0.18%

2013

rr_AnnualReturn2013

0.08%

2014

rr_AnnualReturn2014

0.05%

2015

rr_AnnualReturn2015

0.11%

2016

rr_AnnualReturn2016

0.48%

2017

rr_AnnualReturn2017

1.13%

2018

rr_AnnualReturn2018

2.00%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.62%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.58%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2014

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

0.01%

03.31 FIMM Funds Class 1 Combo PRO-06 | Money Market Portfolio | Return Before Taxes | Money Market Portfolio-Class I

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Money Market Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Class I</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

2.00%

Past 5 years

rr_AverageAnnualReturnYear05

0.75%

Past 10 years

rr_AverageAnnualReturnYear10

0.52%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class I of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.18% (the Expense Cap). If at any time during the current fiscal year expenses for Class I of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds Class 1 Combo PRO-06 | Prime Money Market Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Prime Money Market Portfolio</b>/I</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Prime Money Market Portfolio}	03.31 FIMM Funds Class 1 Combo PRO-06 Prime Money Market Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Prime Money Market Portfolio}	03.31 FIMM Funds Class 1 Combo PRO-06 Prime Money Market Portfolio Prime Money Market Portfolio-Class I
Management fee	0.14%
Distribution and/or Service (12b-1) fees	none
Other expenses	0.07%
Total annual operating expenses	0.21%
Fee waiver and/or expense reimbursement	0.03%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.18%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Prime Money Market Portfolio}	03.31 FIMM Funds Class 1 Combo PRO-06 Prime Money Market Portfolio Prime Money Market Portfolio-Class I USD ($)
1 year	$ 18
3 years	64
5 years	114
10 years	$ 264

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Although the fund is a money market fund, it will price and transact at a "floating" net asset value (NAV) that will fluctuate based on changes in the value of the fund's investments.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Floating NAV. The fund does not maintain a constant NAV per share of $1.00. The value of the fund's shares will be calculated to four decimal places and will vary as a result of changes in the value of the securities in which the fund invests.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.57%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2014
Year-to-Date Return	0.61%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Prime Money Market Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds Class 1 Combo PRO-06 | Prime Money Market Portfolio | Prime Money Market Portfolio-Class I | Return Before Taxes	1.94%	0.72%	0.46%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class I of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.18% (the Expense Cap). If at any time during the current fiscal year expenses for Class I of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Class 1 Combo PRO-06 | Prime Money Market Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Prime Money Market Portfolio</b>/I</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Although the fund is a money market fund, it will price and transact at a "floating" net asset value (NAV) that will fluctuate based on changes in the value of the fund's investments.

Strategy Portfolio Concentration [Text]

rr_StrategyPortfolioConcentration

<font style="FONT-FAMILY: Times New Roman" size="3">Investing more than 25% of total assets in the financial services industries.</font>

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Floating NAV. The fund does not maintain a constant NAV per share of $1.00. The value of the fund's shares will be calculated to four decimal places and will vary as a result of changes in the value of the securities in which the fund invests.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Price Fluctuates [Text]

rr_RiskMoneyMarketFundPriceFluctuates

<font style="FONT-FAMILY: Times New Roman" size="3">Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them.</font>

Risk Money Market Fund May Impose Fees or Suspend Sales [Text]

rr_RiskMoneyMarketFundMayImposeFeesOrSuspendSales

<font style="FONT-FAMILY: Times New Roman" size="3">The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">www.fidelity.com or institutional.fidelity.com </font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.57%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2014
Year-to-Date Return	0.61%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Class 1 Combo PRO-06 | Prime Money Market Portfolio | Prime Money Market Portfolio-Class I

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.07%

Total annual operating expenses

rr_ExpensesOverAssets

0.21%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.03%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.18%

1 year

rr_ExpenseExampleYear01

$ 18

3 years

rr_ExpenseExampleYear03

64

5 years

rr_ExpenseExampleYear05

114

10 years

rr_ExpenseExampleYear10

$ 264

2009

rr_AnnualReturn2009

0.58%

2010

rr_AnnualReturn2010

0.19%

2011

rr_AnnualReturn2011

0.11%

2012

rr_AnnualReturn2012

0.11%

2013

rr_AnnualReturn2013

0.02%

2014

rr_AnnualReturn2014

0.01%

2015

rr_AnnualReturn2015

0.08%

2016

rr_AnnualReturn2016

0.52%

2017

rr_AnnualReturn2017

1.06%

2018

rr_AnnualReturn2018

1.94%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.61%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.57%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2014

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

03.31 FIMM Funds Class 1 Combo PRO-06 | Prime Money Market Portfolio | Return Before Taxes | Prime Money Market Portfolio-Class I

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Prime Money Market Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Class I</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.94%

Past 5 years

rr_AverageAnnualReturnYear05

0.72%

Past 10 years

rr_AverageAnnualReturnYear10

0.46%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class I of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.18% (the Expense Cap). If at any time during the current fiscal year expenses for Class I of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds Class 1 Combo PRO-06 | Prime Reserves Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Prime Reserves Portfolio</b>/I</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Prime Reserves Portfolio}	03.31 FIMM Funds Class 1 Combo PRO-06 Prime Reserves Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Prime Reserves Portfolio}	03.31 FIMM Funds Class 1 Combo PRO-06 Prime Reserves Portfolio Prime Reserves Portfolio-Class I
Management fee	0.14%
Distribution and/or Service (12b-1) fees	none
Other expenses	0.08%
Total annual operating expenses	0.22%
Fee waiver and/or expense reimbursement	0.04%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.18%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Prime Reserves Portfolio}	03.31 FIMM Funds Class 1 Combo PRO-06 Prime Reserves Portfolio Prime Reserves Portfolio-Class I USD ($)
1 year	$ 18
3 years	65
5 years	118
10 years	$ 275

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Although the fund is a money market fund, it will price and transact at a "floating" net asset value (NAV) that will fluctuate based on changes in the value of the fund's investments.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Floating NAV. The fund does not maintain a constant NAV per share of $1.00. The value of the fund's shares will be calculated to four decimal places and will vary as a result of changes in the value of the securities in which the fund invests.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.57%	December 31, 2018
Lowest Quarter Return	0.22%	March 31, 2017
Year-to-Date Return	0.62%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Prime Reserves Portfolio} - 03.31 FIMM Funds Class 1 Combo PRO-06 - Prime Reserves Portfolio - Prime Reserves Portfolio-Class I	Past 1 year	Since Inception	[1]	Inception Date
Before Taxes				Jun. 02, 2016
Return Before Taxes	1.95%	1.28%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class I of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.18% (the Expense Cap). If at any time during the current fiscal year expenses for Class I of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

[1] From June 2, 2016

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Class 1 Combo PRO-06 | Prime Reserves Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Prime Reserves Portfolio</b>/I</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Although the fund is a money market fund, it will price and transact at a "floating" net asset value (NAV) that will fluctuate based on changes in the value of the fund's investments.

Strategy Portfolio Concentration [Text]

rr_StrategyPortfolioConcentration

<font style="FONT-FAMILY: Times New Roman" size="3">Investing more than 25% of total assets in the financial services industries.</font>

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Floating NAV. The fund does not maintain a constant NAV per share of $1.00. The value of the fund's shares will be calculated to four decimal places and will vary as a result of changes in the value of the securities in which the fund invests.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Price Fluctuates [Text]

rr_RiskMoneyMarketFundPriceFluctuates

<font style="FONT-FAMILY: Times New Roman" size="3">Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them.</font>

Risk Money Market Fund May Impose Fees or Suspend Sales [Text]

rr_RiskMoneyMarketFundMayImposeFeesOrSuspendSales

<font style="FONT-FAMILY: Times New Roman" size="3">The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">www.fidelity.com or institutional.fidelity.com</font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.57%	December 31, 2018
Lowest Quarter Return	0.22%	March 31, 2017
Year-to-Date Return	0.62%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Class 1 Combo PRO-06 | Prime Reserves Portfolio | Prime Reserves Portfolio-Class I

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.08%

Total annual operating expenses

rr_ExpensesOverAssets

0.22%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.04%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.18%

1 year

rr_ExpenseExampleYear01

$ 18

3 years

rr_ExpenseExampleYear03

65

5 years

rr_ExpenseExampleYear05

118

10 years

rr_ExpenseExampleYear10

$ 275

2017

rr_AnnualReturn2017

1.08%

2018

rr_AnnualReturn2018

1.95%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.62%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.57%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2017

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

0.22%

Inception Date

rr_AverageAnnualReturnInceptionDate

Jun. 02, 2016

03.31 FIMM Funds Class 1 Combo PRO-06 | Prime Reserves Portfolio | Return Before Taxes | Prime Reserves Portfolio-Class I

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Prime Reserves Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Class I</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.95%

Since Inception

rr_AverageAnnualReturnSinceInception

1.28%

^[2]

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class I of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.18% (the Expense Cap). If at any time during the current fiscal year expenses for Class I of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

[2] From June 2, 2016

XML

03.31 FIMM Funds Class 1 Combo PRO-06 | Tax-Exempt Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Tax-Exempt Portfolio</b>/I</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of interest income exempt from federal income tax as is consistent with liquidity and stability of principal.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Tax-Exempt Portfolio}	03.31 FIMM Funds Class 1 Combo PRO-06 Tax-Exempt Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Tax-Exempt Portfolio}	03.31 FIMM Funds Class 1 Combo PRO-06 Tax-Exempt Portfolio Tax-Exempt Portfolio-Class I
Management fee	0.14%
Distribution and/or Service (12b-1) fees	none
Other expenses	0.08%
Total annual operating expenses	0.22%
Fee waiver and/or expense reimbursement	0.04%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.18%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Tax-Exempt Portfolio}	03.31 FIMM Funds Class 1 Combo PRO-06 Tax-Exempt Portfolio Tax-Exempt Portfolio-Class I USD ($)
1 year	$ 18
3 years	65
5 years	118
10 years	$ 275

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Normally investing in municipal money market securities.

• Prior to June 30, 2016, normally not investing in any rated security that is rated in the second highest short-term rating category by Standard & Poor's (S&P) or Moody's Investors Service (Moody's) (beginning June 30, 2016, this investment strategy is no longer in effect).

• Normally investing at least 80% of assets in municipal securities whose interest is exempt from federal income tax.

• Normally not investing in municipal securities whose interest is subject to the federal alternative minimum tax.

• Potentially investing up to 20% of assets in securities subject to state and/or federal income tax.

• Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Municipal Market Volatility. The municipal market is volatile and can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities.

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Foreign Exposure. Entities providing credit support or a maturity-shortening structure that are located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.38%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2014
Year-to-Date Return	0.36%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Tax-Exempt Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds Class 1 Combo PRO-06 | Tax-Exempt Portfolio | Tax-Exempt Portfolio-Class I | Return Before Taxes	1.30%	0.50%	0.29%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class I of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.18% (the Expense Cap). If at any time during the current fiscal year expenses for Class I of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Class 1 Combo PRO-06 | Tax-Exempt Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Tax-Exempt Portfolio</b>/I</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of interest income exempt from federal income tax as is consistent with liquidity and stability of principal.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Normally investing in municipal money market securities.

• Prior to June 30, 2016, normally not investing in any rated security that is rated in the second highest short-term rating category by Standard & Poor's (S&P) or Moody's Investors Service (Moody's) (beginning June 30, 2016, this investment strategy is no longer in effect).

• Normally investing at least 80% of assets in municipal securities whose interest is exempt from federal income tax.

• Normally not investing in municipal securities whose interest is subject to the federal alternative minimum tax.

• Potentially investing up to 20% of assets in securities subject to state and/or federal income tax.

• Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Municipal Market Volatility. The municipal market is volatile and can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities.

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Foreign Exposure. Entities providing credit support or a maturity-shortening structure that are located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund May Impose Fees or Suspend Sales [Text]

rr_RiskMoneyMarketFundMayImposeFeesOrSuspendSales

<font style="FONT-FAMILY: Times New Roman" size="3">The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Money Market Fund May Not Preserve Dollar [Text]

rr_RiskMoneyMarketFundMayNotPreserveDollar

<font style="FONT-FAMILY: Times New Roman" size="3">Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">www.fidelity.com or institutional.fidelity.com </font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.38%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2014
Year-to-Date Return	0.36%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Class 1 Combo PRO-06 | Tax-Exempt Portfolio | Tax-Exempt Portfolio-Class I

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.08%

Total annual operating expenses

rr_ExpensesOverAssets

0.22%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.04%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.18%

1 year

rr_ExpenseExampleYear01

$ 18

3 years

rr_ExpenseExampleYear03

65

5 years

rr_ExpenseExampleYear05

118

10 years

rr_ExpenseExampleYear10

$ 275

2009

rr_AnnualReturn2009

0.25%

2010

rr_AnnualReturn2010

0.08%

2011

rr_AnnualReturn2011

0.03%

2012

rr_AnnualReturn2012

0.01%

2013

rr_AnnualReturn2013

0.02%

2014

rr_AnnualReturn2014

0.03%

2015

rr_AnnualReturn2015

0.08%

2016

rr_AnnualReturn2016

0.36%

2017

rr_AnnualReturn2017

0.71%

2018

rr_AnnualReturn2018

1.30%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.36%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.38%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2014

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

03.31 FIMM Funds Class 1 Combo PRO-06 | Tax-Exempt Portfolio | Return Before Taxes | Tax-Exempt Portfolio-Class I

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Tax-Exempt Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Class I</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.30%

Past 5 years

rr_AverageAnnualReturnYear05

0.50%

Past 10 years

rr_AverageAnnualReturnYear10

0.29%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class I of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.18% (the Expense Cap). If at any time during the current fiscal year expenses for Class I of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds Class 1 Combo PRO-06 | Treasury Only Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Treasury Only Portfolio</b>/I</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks as high a level of current income as is consistent with the security of principal and liquidity.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Treasury Only Portfolio}	03.31 FIMM Funds Class 1 Combo PRO-06 Treasury Only Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Treasury Only Portfolio}	03.31 FIMM Funds Class 1 Combo PRO-06 Treasury Only Portfolio Treasury Only Portfolio-Class I
Management fee	0.14%
Distribution and/or Service (12b-1) fees	none
Other expenses	0.07%
Total annual operating expenses	0.21%
Fee waiver and/or expense reimbursement	0.03%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.18%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Treasury Only Portfolio}	03.31 FIMM Funds Class 1 Combo PRO-06 Treasury Only Portfolio Treasury Only Portfolio-Class I USD ($)
1 year	$ 18
3 years	64
5 years	114
10 years	$ 264

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Normally investing at least 99.5% of total assets in cash and U.S. Treasury securities.

• Normally investing in securities whose interest is exempt from state and local income taxes.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.53%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.56%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Treasury Only Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds Class 1 Combo PRO-06 | Treasury Only Portfolio | Treasury Only Portfolio-Class I | Return Before Taxes	1.71%	0.52%	0.28%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class I of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.18% (the Expense Cap). If at any time during the current fiscal year expenses for Class I of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Class 1 Combo PRO-06 | Treasury Only Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Treasury Only Portfolio</b>/I</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks as high a level of current income as is consistent with the security of principal and liquidity.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Normally investing at least 99.5% of total assets in cash and U.S. Treasury securities.

• Normally investing in securities whose interest is exempt from state and local income taxes.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities.

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Money Market Fund May Not Preserve Dollar [Text]

rr_RiskMoneyMarketFundMayNotPreserveDollar

<font style="FONT-FAMILY: Times New Roman" size="3">Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">www.fidelity.com or institutional.fidelity.com </font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.53%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.56%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Class 1 Combo PRO-06 | Treasury Only Portfolio | Treasury Only Portfolio-Class I

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.07%

Total annual operating expenses

rr_ExpensesOverAssets

0.21%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.03%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.18%

1 year

rr_ExpenseExampleYear01

$ 18

3 years

rr_ExpenseExampleYear03

64

5 years

rr_ExpenseExampleYear05

114

10 years

rr_ExpenseExampleYear10

$ 264

2009

rr_AnnualReturn2009

0.16%

2010

rr_AnnualReturn2010

0.01%

2011

rr_AnnualReturn2011

0.01%

2012

rr_AnnualReturn2012

0.01%

2013

rr_AnnualReturn2013

0.01%

2014

rr_AnnualReturn2014

0.01%

2015

rr_AnnualReturn2015

0.01%

2016

rr_AnnualReturn2016

0.17%

2017

rr_AnnualReturn2017

0.71%

2018

rr_AnnualReturn2018

1.71%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.56%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.53%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

03.31 FIMM Funds Class 1 Combo PRO-06 | Treasury Only Portfolio | Return Before Taxes | Treasury Only Portfolio-Class I

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Treasury Only Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Class I</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.71%

Past 5 years

rr_AverageAnnualReturnYear05

0.52%

Past 10 years

rr_AverageAnnualReturnYear10

0.28%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class I of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.18% (the Expense Cap). If at any time during the current fiscal year expenses for Class I of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds Class 1 Combo PRO-06 | Treasury Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Treasury Portfolio</b>/I</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Treasury Portfolio}	03.31 FIMM Funds Class 1 Combo PRO-06 Treasury Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Treasury Portfolio}	03.31 FIMM Funds Class 1 Combo PRO-06 Treasury Portfolio Treasury Portfolio-Class I
Management fee	0.14%
Distribution and/or Service (12b-1) fees	none
Other expenses	0.07%
Total annual operating expenses	0.21%
Fee waiver and/or expense reimbursement	0.03%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.18%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Treasury Portfolio}	03.31 FIMM Funds Class 1 Combo PRO-06 Treasury Portfolio Treasury Portfolio-Class I USD ($)
1 year	$ 18
3 years	64
5 years	114
10 years	$ 264

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Normally investing at least 99.5% of total assets in cash, U.S. Treasury securities and/or repurchase agreements for those securities.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities and repurchase agreements for those securities.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.53%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.56%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Treasury Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds Class 1 Combo PRO-06 | Treasury Portfolio | Treasury Portfolio-Class I | Return Before Taxes	1.72%	0.53%	0.29%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class I of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.18% (the Expense Cap). If at any time during the current fiscal year expenses for Class I of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Class 1 Combo PRO-06 | Treasury Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Treasury Portfolio</b>/I</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Normally investing at least 99.5% of total assets in cash, U.S. Treasury securities and/or repurchase agreements for those securities.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities and repurchase agreements for those securities.

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Money Market Fund May Not Preserve Dollar [Text]

rr_RiskMoneyMarketFundMayNotPreserveDollar

<font style="FONT-FAMILY: Times New Roman" size="3">Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit www.fidelity.com or institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">www.fidelity.com or institutional.fidelity.com</font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.53%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.56%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Class 1 Combo PRO-06 | Treasury Portfolio | Treasury Portfolio-Class I

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.07%

Total annual operating expenses

rr_ExpensesOverAssets

0.21%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.03%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.18%

1 year

rr_ExpenseExampleYear01

$ 18

3 years

rr_ExpenseExampleYear03

64

5 years

rr_ExpenseExampleYear05

114

10 years

rr_ExpenseExampleYear10

$ 264

2009

rr_AnnualReturn2009

0.18%

2010

rr_AnnualReturn2010

0.03%

2011

rr_AnnualReturn2011

0.01%

2012

rr_AnnualReturn2012

0.01%

2013

rr_AnnualReturn2013

0.01%

2014

rr_AnnualReturn2014

0.01%

2015

rr_AnnualReturn2015

0.01%

2016

rr_AnnualReturn2016

0.20%

2017

rr_AnnualReturn2017

0.74%

2018

rr_AnnualReturn2018

1.72%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.56%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.53%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

03.31 FIMM Funds Class 1 Combo PRO-06 | Treasury Portfolio | Return Before Taxes | Treasury Portfolio-Class I

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Treasury Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Class I</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.72%

Past 5 years

rr_AverageAnnualReturnYear05

0.53%

Past 10 years

rr_AverageAnnualReturnYear10

0.29%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class I of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.18% (the Expense Cap). If at any time during the current fiscal year expenses for Class I of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds Class 4 Combo PRO-06 | Prime Money Market Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Prime Money Market Portfolio</b>/IV</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Prime Money Market Portfolio}	03.31 FIMM Funds Class 4 Combo PRO-06 Prime Money Market Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Prime Money Market Portfolio}	03.31 FIMM Funds Class 4 Combo PRO-06 Prime Money Market Portfolio Prime Money Market Portfolio-Class IV
Management fee	0.14%
Distribution and/or Service (12b-1) fees	0.50%
Other expenses	0.07%
Total annual operating expenses	0.71%
Fee waiver and/or expense reimbursement	0.03%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.68%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Prime Money Market Portfolio}	03.31 FIMM Funds Class 4 Combo PRO-06 Prime Money Market Portfolio Prime Money Market Portfolio-Class IV USD ($)
1 year	$ 69
3 years	223
5 years	391
10 years	$ 879

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Although the fund is a money market fund, it will price and transact at a "floating" net asset value (NAV) that will fluctuate based on changes in the value of the fund's investments.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Floating NAV. The fund does not maintain a constant NAV per share of $1.00. The value of the fund's shares will be calculated to four decimal places and will vary as a result of changes in the value of the securities in which the fund invests.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.43%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.49%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Prime Money Market Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds Class 4 Combo PRO-06 | Prime Money Market Portfolio | Prime Money Market Portfolio-Class IV | Return Before Taxes	1.42%	0.42%	0.24%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class IV of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.68% (the Expense Cap). If at any time during the current fiscal year expenses for Class IV of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Class 4 Combo PRO-06 | Prime Money Market Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Prime Money Market Portfolio</b>/IV</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Although the fund is a money market fund, it will price and transact at a "floating" net asset value (NAV) that will fluctuate based on changes in the value of the fund's investments.

Strategy Portfolio Concentration [Text]

rr_StrategyPortfolioConcentration

<font style="FONT-FAMILY: Times New Roman" size="3">Investing more than 25% of total assets in the financial services industries.</font>

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Floating NAV. The fund does not maintain a constant NAV per share of $1.00. The value of the fund's shares will be calculated to four decimal places and will vary as a result of changes in the value of the securities in which the fund invests.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Price Fluctuates [Text]

rr_RiskMoneyMarketFundPriceFluctuates

<font style="FONT-FAMILY: Times New Roman" size="3">Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them.</font>

Risk Money Market Fund May Impose Fees or Suspend Sales [Text]

rr_RiskMoneyMarketFundMayImposeFeesOrSuspendSales

<font style="FONT-FAMILY: Times New Roman" size="3">The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">institutional.fidelity.com</font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.43%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.49%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Class 4 Combo PRO-06 | Prime Money Market Portfolio | Prime Money Market Portfolio-Class IV

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.50%

Other expenses

rr_OtherExpensesOverAssets

0.07%

Total annual operating expenses

rr_ExpensesOverAssets

0.71%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.03%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.68%

1 year

rr_ExpenseExampleYear01

$ 69

3 years

rr_ExpenseExampleYear03

223

5 years

rr_ExpenseExampleYear05

391

10 years

rr_ExpenseExampleYear10

$ 879

2009

rr_AnnualReturn2009

0.20%

2010

rr_AnnualReturn2010

0.01%

2011

rr_AnnualReturn2011

0.01%

2012

rr_AnnualReturn2012

0.01%

2013

rr_AnnualReturn2013

0.01%

2014

rr_AnnualReturn2014

0.01%

2015

rr_AnnualReturn2015

0.01%

2016

rr_AnnualReturn2016

0.12%

2017

rr_AnnualReturn2017

0.57%

2018

rr_AnnualReturn2018

1.42%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.49%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.43%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

03.31 FIMM Funds Class 4 Combo PRO-06 | Prime Money Market Portfolio | Return Before Taxes | Prime Money Market Portfolio-Class IV

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Prime Money Market Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Class IV</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.42%

Past 5 years

rr_AverageAnnualReturnYear05

0.42%

Past 10 years

rr_AverageAnnualReturnYear10

0.24%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class IV of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.68% (the Expense Cap). If at any time during the current fiscal year expenses for Class IV of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds Class 4 Combo PRO-06 | Treasury Only Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Treasury Only Portfolio</b>/IV</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks as high a level of current income as is consistent with the security of principal and liquidity.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Treasury Only Portfolio}	03.31 FIMM Funds Class 4 Combo PRO-06 Treasury Only Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Treasury Only Portfolio}	03.31 FIMM Funds Class 4 Combo PRO-06 Treasury Only Portfolio Treasury Only Portfolio-Class IV
Management fee	0.14%
Distribution and/or Service (12b-1) fees	0.50%
Other expenses	0.07%
Total annual operating expenses	0.71%
Fee waiver and/or expense reimbursement	0.03%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.68%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Treasury Only Portfolio}	03.31 FIMM Funds Class 4 Combo PRO-06 Treasury Only Portfolio Treasury Only Portfolio-Class IV USD ($)
1 year	$ 69
3 years	223
5 years	391
10 years	$ 879

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Normally investing at least 99.5% of total assets in cash and U.S. Treasury securities.

• Normally investing in securities whose interest is exempt from state and local income taxes.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.40%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2017
Year-to-Date Return	0.43%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Treasury Only Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds Class 4 Combo PRO-06 | Treasury Only Portfolio | Treasury Only Portfolio-Class IV | Return Before Taxes	1.20%	0.29%	0.15%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class IV of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.68% (the Expense Cap). If at any time during the current fiscal year expenses for Class IV of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Class 4 Combo PRO-06 | Treasury Only Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Treasury Only Portfolio</b>/IV</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks as high a level of current income as is consistent with the security of principal and liquidity.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Normally investing at least 99.5% of total assets in cash and U.S. Treasury securities.

• Normally investing in securities whose interest is exempt from state and local income taxes.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities.

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Money Market Fund May Not Preserve Dollar [Text]

rr_RiskMoneyMarketFundMayNotPreserveDollar

<font style="FONT-FAMILY: Times New Roman" size="3">Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">institutional.fidelity.com</font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.40%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2017
Year-to-Date Return	0.43%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Class 4 Combo PRO-06 | Treasury Only Portfolio | Treasury Only Portfolio-Class IV

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.50%

Other expenses

rr_OtherExpensesOverAssets

0.07%

Total annual operating expenses

rr_ExpensesOverAssets

0.71%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.03%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.68%

1 year

rr_ExpenseExampleYear01

$ 69

3 years

rr_ExpenseExampleYear03

223

5 years

rr_ExpenseExampleYear05

391

10 years

rr_ExpenseExampleYear10

$ 879

2009

rr_AnnualReturn2009

0.01%

2010

rr_AnnualReturn2010

0.01%

2011

rr_AnnualReturn2011

0.01%

2012

rr_AnnualReturn2012

0.01%

2013

rr_AnnualReturn2013

0.01%

2014

rr_AnnualReturn2014

0.01%

2015

rr_AnnualReturn2015

0.01%

2016

rr_AnnualReturn2016

0.01%

2017

rr_AnnualReturn2017

0.24%

2018

rr_AnnualReturn2018

1.20%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.43%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.40%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2017

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

03.31 FIMM Funds Class 4 Combo PRO-06 | Treasury Only Portfolio | Return Before Taxes | Treasury Only Portfolio-Class IV

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Treasury Only Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Class IV</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.20%

Past 5 years

rr_AverageAnnualReturnYear05

0.29%

Past 10 years

rr_AverageAnnualReturnYear10

0.15%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class IV of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.68% (the Expense Cap). If at any time during the current fiscal year expenses for Class IV of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds Class 4 Combo PRO-06 | Treasury Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Treasury Portfolio</b>/IV</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Treasury Portfolio}	03.31 FIMM Funds Class 4 Combo PRO-06 Treasury Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Treasury Portfolio}	03.31 FIMM Funds Class 4 Combo PRO-06 Treasury Portfolio Treasury Portfolio-Class IV
Management fee	0.14%
Distribution and/or Service (12b-1) fees	0.50%
Other expenses	0.07%
Total annual operating expenses	0.71%
Fee waiver and/or expense reimbursement	0.03%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.68%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Treasury Portfolio}	03.31 FIMM Funds Class 4 Combo PRO-06 Treasury Portfolio Treasury Portfolio-Class IV USD ($)
1 year	$ 69
3 years	223
5 years	391
10 years	$ 879

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Normally investing at least 99.5% of total assets in cash, U.S. Treasury securities and/or repurchase agreements for those securities.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities and repurchase agreements for those securities.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.40%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.44%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Treasury Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds Class 4 Combo PRO-06 | Treasury Portfolio | Treasury Portfolio-Class IV | Return Before Taxes	1.21%	0.30%	0.15%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class IV of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.68% (the Expense Cap). If at any time during the current fiscal year expenses for Class IV of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Class 4 Combo PRO-06 | Treasury Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Treasury Portfolio</b>/IV</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Normally investing at least 99.5% of total assets in cash, U.S. Treasury securities and/or repurchase agreements for those securities.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities and repurchase agreements for those securities.

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Money Market Fund May Not Preserve Dollar [Text]

rr_RiskMoneyMarketFundMayNotPreserveDollar

<font style="FONT-FAMILY: Times New Roman" size="3">Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">institutional.fidelity.com</font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.40%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.44%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Class 4 Combo PRO-06 | Treasury Portfolio | Treasury Portfolio-Class IV

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.50%

Other expenses

rr_OtherExpensesOverAssets

0.07%

Total annual operating expenses

rr_ExpensesOverAssets

0.71%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.03%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.68%

1 year

rr_ExpenseExampleYear01

$ 69

3 years

rr_ExpenseExampleYear03

223

5 years

rr_ExpenseExampleYear05

391

10 years

rr_ExpenseExampleYear10

$ 879

2009

rr_AnnualReturn2009

0.01%

2010

rr_AnnualReturn2010

0.01%

2011

rr_AnnualReturn2011

0.01%

2012

rr_AnnualReturn2012

0.01%

2013

rr_AnnualReturn2013

0.01%

2014

rr_AnnualReturn2014

0.01%

2015

rr_AnnualReturn2015

0.01%

2016

rr_AnnualReturn2016

0.01%

2017

rr_AnnualReturn2017

0.26%

2018

rr_AnnualReturn2018

1.21%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.44%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.40%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

03.31 FIMM Funds Class 4 Combo PRO-06 | Treasury Portfolio | Return Before Taxes | Treasury Portfolio-Class IV

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Treasury Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Class IV</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.21%

Past 5 years

rr_AverageAnnualReturnYear05

0.30%

Past 10 years

rr_AverageAnnualReturnYear10

0.15%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Class IV of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.68% (the Expense Cap). If at any time during the current fiscal year expenses for Class IV of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds Select Combo PRO-06 | Government Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Government Portfolio</b>/Select</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Government Portfolio}	03.31 FIMM Funds Select Combo PRO-06 Government Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Government Portfolio}	03.31 FIMM Funds Select Combo PRO-06 Government Portfolio Government Portfolio-Select FIMM
Management fee	0.14%
Distribution and/or Service (12b-1) fees	0.05%
Other expenses	0.06%
Total annual operating expenses	0.25%
Fee waiver and/or expense reimbursement	0.02%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.23%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Government Portfolio}	03.31 FIMM Funds Select Combo PRO-06 Government Portfolio Government Portfolio-Select FIMM USD ($)
1 year	$ 24
3 years	78
5 years	138
10 years	$ 315

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Normally investing at least 99.5% of total assets in cash, U.S. Government securities and/or repurchase agreements that are collateralized fully (i.e., collateralized by cash or government securities).

• Investing in U.S. Government securities issued by entities that are chartered or sponsored by Congress but whose securities are neither issued nor guaranteed by the U.S. Treasury.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Government securities and repurchase agreements for those securities.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.52%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.55%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Government Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds Select Combo PRO-06 | Government Portfolio | Government Portfolio-Select FIMM | Return Before Taxes	1.67%	0.52%	0.29%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Select Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.23% (the Expense Cap). If at any time during the current fiscal year expenses for Select Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Select Combo PRO-06 | Government Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Government Portfolio</b>/Select</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Normally investing at least 99.5% of total assets in cash, U.S. Government securities and/or repurchase agreements that are collateralized fully (i.e., collateralized by cash or government securities).

• Investing in U.S. Government securities issued by entities that are chartered or sponsored by Congress but whose securities are neither issued nor guaranteed by the U.S. Treasury.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Government securities and repurchase agreements for those securities.

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Money Market Fund May Not Preserve Dollar [Text]

rr_RiskMoneyMarketFundMayNotPreserveDollar

<font style="FONT-FAMILY: Times New Roman" size="3">Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">institutional.fidelity.com </font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.52%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.55%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Select Combo PRO-06 | Government Portfolio | Government Portfolio-Select FIMM

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.05%

Other expenses

rr_OtherExpensesOverAssets

0.06%

Total annual operating expenses

rr_ExpensesOverAssets

0.25%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.02%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.23%

1 year

rr_ExpenseExampleYear01

$ 24

3 years

rr_ExpenseExampleYear03

78

5 years

rr_ExpenseExampleYear05

138

10 years

rr_ExpenseExampleYear10

$ 315

2009

rr_AnnualReturn2009

0.29%

2010

rr_AnnualReturn2010

0.02%

2011

rr_AnnualReturn2011

0.01%

2012

rr_AnnualReturn2012

0.01%

2013

rr_AnnualReturn2013

0.01%

2014

rr_AnnualReturn2014

0.01%

2015

rr_AnnualReturn2015

0.01%

2016

rr_AnnualReturn2016

0.20%

2017

rr_AnnualReturn2017

0.70%

2018

rr_AnnualReturn2018

1.67%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.55%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.52%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

03.31 FIMM Funds Select Combo PRO-06 | Government Portfolio | Return Before Taxes | Government Portfolio-Select FIMM

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Government Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Select Class</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.67%

Past 5 years

rr_AverageAnnualReturnYear05

0.52%

Past 10 years

rr_AverageAnnualReturnYear10

0.29%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Select Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.23% (the Expense Cap). If at any time during the current fiscal year expenses for Select Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds Select Combo PRO-06 | Money Market Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Money Market Portfolio</b>/Select</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Money Market Portfolio}	03.31 FIMM Funds Select Combo PRO-06 Money Market Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Money Market Portfolio}	03.31 FIMM Funds Select Combo PRO-06 Money Market Portfolio Money Market Portfolio-Select FIMM
Management fee	0.14%
Distribution and/or Service (12b-1) fees	0.05%
Other expenses	0.07%
Total annual operating expenses	0.26%
Fee waiver and/or expense reimbursement	0.03%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.23%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Money Market Portfolio}	03.31 FIMM Funds Select Combo PRO-06 Money Market Portfolio Money Market Portfolio-Select FIMM USD ($)
1 year	$ 24
3 years	80
5 years	142
10 years	$ 327

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.57%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2014
Year-to-Date Return	0.60%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Money Market Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds Select Combo PRO-06 | Money Market Portfolio | Money Market Portfolio-Select FIMM | Return Before Taxes	1.95%	0.70%	0.47%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Select Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.23% (the Expense Cap). If at any time during the current fiscal year expenses for Select Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Select Combo PRO-06 | Money Market Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Money Market Portfolio</b>/Select</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Strategy Portfolio Concentration [Text]

rr_StrategyPortfolioConcentration

<font style="FONT-FAMILY: Times New Roman" size="3">Investing more than 25% of total assets in the financial services industries.</font>

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund May Impose Fees or Suspend Sales [Text]

rr_RiskMoneyMarketFundMayImposeFeesOrSuspendSales

<font style="FONT-FAMILY: Times New Roman" size="3">The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Money Market Fund May Not Preserve Dollar [Text]

rr_RiskMoneyMarketFundMayNotPreserveDollar

<font style="FONT-FAMILY: Times New Roman" size="3">Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">institutional.fidelity.com </font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.57%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2014
Year-to-Date Return	0.60%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Select Combo PRO-06 | Money Market Portfolio | Money Market Portfolio-Select FIMM

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.05%

Other expenses

rr_OtherExpensesOverAssets

0.07%

Total annual operating expenses

rr_ExpensesOverAssets

0.26%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.03%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.23%

1 year

rr_ExpenseExampleYear01

$ 24

3 years

rr_ExpenseExampleYear03

80

5 years

rr_ExpenseExampleYear05

142

10 years

rr_ExpenseExampleYear10

$ 327

2009

rr_AnnualReturn2009

0.68%

2010

rr_AnnualReturn2010

0.19%

2011

rr_AnnualReturn2011

0.11%

2012

rr_AnnualReturn2012

0.13%

2013

rr_AnnualReturn2013

0.03%

2014

rr_AnnualReturn2014

0.01%

2015

rr_AnnualReturn2015

0.06%

2016

rr_AnnualReturn2016

0.43%

2017

rr_AnnualReturn2017

1.08%

2018

rr_AnnualReturn2018

1.95%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.60%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.57%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2014

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

03.31 FIMM Funds Select Combo PRO-06 | Money Market Portfolio | Return Before Taxes | Money Market Portfolio-Select FIMM

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Money Market Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Select Class</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.95%

Past 5 years

rr_AverageAnnualReturnYear05

0.70%

Past 10 years

rr_AverageAnnualReturnYear10

0.47%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Select Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.23% (the Expense Cap). If at any time during the current fiscal year expenses for Select Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds Select Combo PRO-06 | Prime Money Market Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Prime Money Market Portfolio</b>/Select</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Prime Money Market Portfolio}	03.31 FIMM Funds Select Combo PRO-06 Prime Money Market Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Prime Money Market Portfolio}	03.31 FIMM Funds Select Combo PRO-06 Prime Money Market Portfolio Prime Money Market Portfolio-Select FIMM
Management fee	0.14%
Distribution and/or Service (12b-1) fees	0.05%
Other expenses	0.07%
Total annual operating expenses	0.26%
Fee waiver and/or expense reimbursement	0.03%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.23%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Prime Money Market Portfolio}	03.31 FIMM Funds Select Combo PRO-06 Prime Money Market Portfolio Prime Money Market Portfolio-Select FIMM USD ($)
1 year	$ 24
3 years	80
5 years	142
10 years	$ 327

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Although the fund is a money market fund, it will price and transact at a "floating" net asset value (NAV) that will fluctuate based on changes in the value of the fund's investments.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Floating NAV. The fund does not maintain a constant NAV per share of $1.00. The value of the fund's shares will be calculated to four decimal places and will vary as a result of changes in the value of the securities in which the fund invests.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.55%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.60%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Prime Money Market Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds Select Combo PRO-06 | Prime Money Market Portfolio | Prime Money Market Portfolio-Select FIMM | Return Before Taxes	1.88%	0.68%	0.42%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Select Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.23% (the Expense Cap). If at any time during the current fiscal year expenses for Select Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Select Combo PRO-06 | Prime Money Market Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Prime Money Market Portfolio</b>/Select</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Although the fund is a money market fund, it will price and transact at a "floating" net asset value (NAV) that will fluctuate based on changes in the value of the fund's investments.

Strategy Portfolio Concentration [Text]

rr_StrategyPortfolioConcentration

<font style="FONT-FAMILY: Times New Roman" size="3">Investing more than 25% of total assets in the financial services industries.</font>

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Floating NAV. The fund does not maintain a constant NAV per share of $1.00. The value of the fund's shares will be calculated to four decimal places and will vary as a result of changes in the value of the securities in which the fund invests.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Price Fluctuates [Text]

rr_RiskMoneyMarketFundPriceFluctuates

<font style="FONT-FAMILY: Times New Roman" size="3">Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them.</font>

Risk Money Market Fund May Impose Fees or Suspend Sales [Text]

rr_RiskMoneyMarketFundMayImposeFeesOrSuspendSales

<font style="FONT-FAMILY: Times New Roman" size="3">The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3"> institutional.fidelity.com </font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.55%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.60%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Select Combo PRO-06 | Prime Money Market Portfolio | Prime Money Market Portfolio-Select FIMM

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.05%

Other expenses

rr_OtherExpensesOverAssets

0.07%

Total annual operating expenses

rr_ExpensesOverAssets

0.26%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.03%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.23%

1 year

rr_ExpenseExampleYear01

$ 24

3 years

rr_ExpenseExampleYear03

80

5 years

rr_ExpenseExampleYear05

142

10 years

rr_ExpenseExampleYear10

$ 327

2009

rr_AnnualReturn2009

0.53%

2010

rr_AnnualReturn2010

0.14%

2011

rr_AnnualReturn2011

0.06%

2012

rr_AnnualReturn2012

0.06%

2013

rr_AnnualReturn2013

0.01%

2014

rr_AnnualReturn2014

0.01%

2015

rr_AnnualReturn2015

0.04%

2016

rr_AnnualReturn2016

0.45%

2017

rr_AnnualReturn2017

1.02%

2018

rr_AnnualReturn2018

1.88%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.60%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.55%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

03.31 FIMM Funds Select Combo PRO-06 | Prime Money Market Portfolio | Return Before Taxes | Prime Money Market Portfolio-Select FIMM

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Prime Money Market Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Select Class</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.88%

Past 5 years

rr_AverageAnnualReturnYear05

0.68%

Past 10 years

rr_AverageAnnualReturnYear10

0.42%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Select Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.23% (the Expense Cap). If at any time during the current fiscal year expenses for Select Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds Select Combo PRO-06 | Prime Reserves Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Prime Reserves Portfolio</b>/Select</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Prime Reserves Portfolio}	03.31 FIMM Funds Select Combo PRO-06 Prime Reserves Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Prime Reserves Portfolio}	03.31 FIMM Funds Select Combo PRO-06 Prime Reserves Portfolio Prime Reserves Portfolio-Select FIMM
Management fee	0.14%
Distribution and/or Service (12b-1) fees	0.05%
Other expenses	0.08%
Total annual operating expenses	0.27%
Fee waiver and/or expense reimbursement	0.04%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.23%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Prime Reserves Portfolio}	03.31 FIMM Funds Select Combo PRO-06 Prime Reserves Portfolio Prime Reserves Portfolio-Select FIMM USD ($)
1 year	$ 24
3 years	81
5 years	146
10 years	$ 338

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Although the fund is a money market fund, it will price and transact at a "floating" net asset value (NAV) that will fluctuate based on changes in the value of the fund's investments.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Floating NAV. The fund does not maintain a constant NAV per share of $1.00. The value of the fund's shares will be calculated to four decimal places and will vary as a result of changes in the value of the securities in which the fund invests.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.56%	December 31, 2018
Lowest Quarter Return	0.20%	March 31, 2017
Year-to-Date Return	0.61%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Prime Reserves Portfolio} - 03.31 FIMM Funds Select Combo PRO-06 - Prime Reserves Portfolio - Prime Reserves Portfolio-Select FIMM	Past 1 year	Since Inception	[1]	Inception Date
Before Taxes				Jun. 02, 2016
Return Before Taxes	1.91%	1.23%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Select Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.23% (the Expense Cap). If at any time during the current fiscal year expenses for Select Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

[1] From June 2, 2016

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Select Combo PRO-06 | Prime Reserves Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Prime Reserves Portfolio</b>/Select</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Investing in U.S. dollar-denominated money market securities of domestic and foreign issuers, U.S. Government securities, and repurchase agreements.

• Potentially entering into reverse repurchase agreements.

• Investing more than 25% of total assets in the financial services industries.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Although the fund is a money market fund, it will price and transact at a "floating" net asset value (NAV) that will fluctuate based on changes in the value of the fund's investments.

Strategy Portfolio Concentration [Text]

rr_StrategyPortfolioConcentration

<font style="FONT-FAMILY: Times New Roman" size="3">Investing more than 25% of total assets in the financial services industries.</font>

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Floating NAV. The fund does not maintain a constant NAV per share of $1.00. The value of the fund's shares will be calculated to four decimal places and will vary as a result of changes in the value of the securities in which the fund invests.

• Foreign Exposure. Entities located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Financial Services Concentration. Changes in government regulation and interest rates and economic downturns can have a significant negative effect on issuers in the financial services sector, including the price of their securities or their ability to meet their payment obligations.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Price Fluctuates [Text]

rr_RiskMoneyMarketFundPriceFluctuates

<font style="FONT-FAMILY: Times New Roman" size="3">Because the share price of the fund will fluctuate, when you sell your shares they may be worth more or less than what you originally paid for them.</font>

Risk Money Market Fund May Impose Fees or Suspend Sales [Text]

rr_RiskMoneyMarketFundMayImposeFeesOrSuspendSales

<font style="FONT-FAMILY: Times New Roman" size="3">The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3"> institutional.fidelity.com</font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.56%	December 31, 2018
Lowest Quarter Return	0.20%	March 31, 2017
Year-to-Date Return	0.61%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Select Combo PRO-06 | Prime Reserves Portfolio | Prime Reserves Portfolio-Select FIMM

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.05%

Other expenses

rr_OtherExpensesOverAssets

0.08%

Total annual operating expenses

rr_ExpensesOverAssets

0.27%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.04%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.23%

1 year

rr_ExpenseExampleYear01

$ 24

3 years

rr_ExpenseExampleYear03

81

5 years

rr_ExpenseExampleYear05

146

10 years

rr_ExpenseExampleYear10

$ 338

2017

rr_AnnualReturn2017

1.02%

2018

rr_AnnualReturn2018

1.91%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.61%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.56%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2017

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

0.20%

Inception Date

rr_AverageAnnualReturnInceptionDate

Jun. 02, 2016

03.31 FIMM Funds Select Combo PRO-06 | Prime Reserves Portfolio | Return Before Taxes | Prime Reserves Portfolio-Select FIMM

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Prime Reserves Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Select Class</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.91%

Since Inception

rr_AverageAnnualReturnSinceInception

1.23%

^[2]

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Select Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.23% (the Expense Cap). If at any time during the current fiscal year expenses for Select Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

[2] From June 2, 2016

XML

03.31 FIMM Funds Select Combo PRO-06 | Tax-Exempt Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Tax-Exempt Portfolio</b>/Select</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of interest income exempt from federal income tax as is consistent with liquidity and stability of principal.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Tax-Exempt Portfolio}	03.31 FIMM Funds Select Combo PRO-06 Tax-Exempt Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Tax-Exempt Portfolio}	03.31 FIMM Funds Select Combo PRO-06 Tax-Exempt Portfolio Tax-Exempt Portfolio-Select FIMM
Management fee	0.14%
Distribution and/or Service (12b-1) fees	0.05%
Other expenses	0.08%
Total annual operating expenses	0.27%
Fee waiver and/or expense reimbursement	0.04%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.23%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Tax-Exempt Portfolio}	03.31 FIMM Funds Select Combo PRO-06 Tax-Exempt Portfolio Tax-Exempt Portfolio-Select FIMM USD ($)
1 year	$ 24
3 years	81
5 years	146
10 years	$ 338

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Normally investing in municipal money market securities.

• Prior to June 30, 2016, normally not investing in any rated security that is rated in the second highest short-term rating category by Standard & Poor's (S&P) or Moody's Investors Service (Moody's) (beginning June 30, 2016, this investment strategy is no longer in effect).

• Normally investing at least 80% of assets in municipal securities whose interest is exempt from federal income tax.

• Normally not investing in municipal securities whose interest is subject to the federal alternative minimum tax.

• Potentially investing up to 20% of assets in securities subject to state and/or federal income tax.

• Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Municipal Market Volatility. The municipal market is volatile and can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities.

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Foreign Exposure. Entities providing credit support or a maturity-shortening structure that are located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.37%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.35%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Tax-Exempt Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds Select Combo PRO-06 | Tax-Exempt Portfolio | Tax-Exempt Portfolio-Select FIMM | Return Before Taxes	1.25%	0.47%	0.26%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Select Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.23% (the Expense Cap). If at any time during the current fiscal year expenses for Select Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Select Combo PRO-06 | Tax-Exempt Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Tax-Exempt Portfolio</b>/Select</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of interest income exempt from federal income tax as is consistent with liquidity and stability of principal.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Normally investing in municipal money market securities.

• Prior to June 30, 2016, normally not investing in any rated security that is rated in the second highest short-term rating category by Standard & Poor's (S&P) or Moody's Investors Service (Moody's) (beginning June 30, 2016, this investment strategy is no longer in effect).

• Normally investing at least 80% of assets in municipal securities whose interest is exempt from federal income tax.

• Normally not investing in municipal securities whose interest is subject to the federal alternative minimum tax.

• Potentially investing up to 20% of assets in securities subject to state and/or federal income tax.

• Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Municipal Market Volatility. The municipal market is volatile and can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities.

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Foreign Exposure. Entities providing credit support or a maturity-shortening structure that are located in foreign countries can be affected by adverse political, regulatory, market, or economic developments in those countries.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund May Impose Fees or Suspend Sales [Text]

rr_RiskMoneyMarketFundMayImposeFeesOrSuspendSales

<font style="FONT-FAMILY: Times New Roman" size="3">The fund may impose a fee upon the sale of your shares or may temporarily suspend your ability to sell shares if the fund's liquidity falls below required minimums because of market conditions or other factors.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Money Market Fund May Not Preserve Dollar [Text]

rr_RiskMoneyMarketFundMayNotPreserveDollar

<font style="FONT-FAMILY: Times New Roman" size="3">Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">institutional.fidelity.com </font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.37%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.35%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Select Combo PRO-06 | Tax-Exempt Portfolio | Tax-Exempt Portfolio-Select FIMM

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.05%

Other expenses

rr_OtherExpensesOverAssets

0.08%

Total annual operating expenses

rr_ExpensesOverAssets

0.27%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.04%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.23%

1 year

rr_ExpenseExampleYear01

$ 24

3 years

rr_ExpenseExampleYear03

81

5 years

rr_ExpenseExampleYear05

146

10 years

rr_ExpenseExampleYear10

$ 338

2009

rr_AnnualReturn2009

0.20%

2010

rr_AnnualReturn2010

0.05%

2011

rr_AnnualReturn2011

0.02%

2012

rr_AnnualReturn2012

0.01%

2013

rr_AnnualReturn2013

0.02%

2014

rr_AnnualReturn2014

0.03%

2015

rr_AnnualReturn2015

0.08%

2016

rr_AnnualReturn2016

0.32%

2017

rr_AnnualReturn2017

0.66%

2018

rr_AnnualReturn2018

1.25%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.35%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.37%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

03.31 FIMM Funds Select Combo PRO-06 | Tax-Exempt Portfolio | Return Before Taxes | Tax-Exempt Portfolio-Select FIMM

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Tax-Exempt Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Select Class</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.25%

Past 5 years

rr_AverageAnnualReturnYear05

0.47%

Past 10 years

rr_AverageAnnualReturnYear10

0.26%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Select Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.23% (the Expense Cap). If at any time during the current fiscal year expenses for Select Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds Select Combo PRO-06 | Treasury Only Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Treasury Only Portfolio</b>/Select</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks as high a level of current income as is consistent with the security of principal and liquidity.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Treasury Only Portfolio}	03.31 FIMM Funds Select Combo PRO-06 Treasury Only Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Treasury Only Portfolio}	03.31 FIMM Funds Select Combo PRO-06 Treasury Only Portfolio Treasury Only Portfolio-Select FIMM
Management fee	0.14%
Distribution and/or Service (12b-1) fees	0.05%
Other expenses	0.07%
Total annual operating expenses	0.26%
Fee waiver and/or expense reimbursement	0.03%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.23%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Treasury Only Portfolio}	03.31 FIMM Funds Select Combo PRO-06 Treasury Only Portfolio Treasury Only Portfolio-Select FIMM USD ($)
1 year	$ 24
3 years	80
5 years	142
10 years	$ 327

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Normally investing at least 99.5% of total assets in cash and U.S. Treasury securities.

• Normally investing in securities whose interest is exempt from state and local income taxes.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.51%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2014
Year-to-Date Return	0.55%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Treasury Only Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds Select Combo PRO-06 | Treasury Only Portfolio | Treasury Only Portfolio-Select FIMM | Return Before Taxes	1.66%	0.49%	0.26%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Select Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.23% (the Expense Cap). If at any time during the current fiscal year expenses for Select Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Select Combo PRO-06 | Treasury Only Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Treasury Only Portfolio</b>/Select</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks as high a level of current income as is consistent with the security of principal and liquidity.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Normally investing at least 99.5% of total assets in cash and U.S. Treasury securities.

• Normally investing in securities whose interest is exempt from state and local income taxes.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities.

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Money Market Fund May Not Preserve Dollar [Text]

rr_RiskMoneyMarketFundMayNotPreserveDollar

<font style="FONT-FAMILY: Times New Roman" size="3">Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3"> institutional.fidelity.com</font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.51%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2014
Year-to-Date Return	0.55%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Select Combo PRO-06 | Treasury Only Portfolio | Treasury Only Portfolio-Select FIMM

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.05%

Other expenses

rr_OtherExpensesOverAssets

0.07%

Total annual operating expenses

rr_ExpensesOverAssets

0.26%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.03%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.23%

1 year

rr_ExpenseExampleYear01

$ 24

3 years

rr_ExpenseExampleYear03

80

5 years

rr_ExpenseExampleYear05

142

10 years

rr_ExpenseExampleYear10

$ 327

2009

rr_AnnualReturn2009

0.12%

2010

rr_AnnualReturn2010

0.01%

2011

rr_AnnualReturn2011

0.01%

2012

rr_AnnualReturn2012

0.01%

2013

rr_AnnualReturn2013

0.01%

2014

rr_AnnualReturn2014

0.01%

2015

rr_AnnualReturn2015

0.01%

2016

rr_AnnualReturn2016

0.12%

2017

rr_AnnualReturn2017

0.66%

2018

rr_AnnualReturn2018

1.66%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.55%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.51%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2014

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

03.31 FIMM Funds Select Combo PRO-06 | Treasury Only Portfolio | Return Before Taxes | Treasury Only Portfolio-Select FIMM

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Treasury Only Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Select Class</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.66%

Past 5 years

rr_AverageAnnualReturnYear05

0.49%

Past 10 years

rr_AverageAnnualReturnYear10

0.26%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Select Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.23% (the Expense Cap). If at any time during the current fiscal year expenses for Select Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

03.31 FIMM Funds Select Combo PRO-06 | Treasury Portfolio

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Treasury Portfolio</b>/Select</p>

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

Shareholder Fees {- Treasury Portfolio}	03.31 FIMM Funds Select Combo PRO-06 Treasury Portfolio USD ($)
(fees paid directly from your investment)	none

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Annual Operating Expenses {- Treasury Portfolio}	03.31 FIMM Funds Select Combo PRO-06 Treasury Portfolio Treasury Portfolio-Select FIMM
Management fee	0.14%
Distribution and/or Service (12b-1) fees	0.05%
Other expenses	0.07%
Total annual operating expenses	0.26%
Fee waiver and/or expense reimbursement	0.03%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.23%

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Expense Example {- Treasury Portfolio}	03.31 FIMM Funds Select Combo PRO-06 Treasury Portfolio Treasury Portfolio-Select FIMM USD ($)
1 year	$ 24
3 years	80
5 years	142
10 years	$ 327

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

• Normally investing at least 99.5% of total assets in cash, U.S. Treasury securities and/or repurchase agreements for those securities.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities and repurchase agreements for those securities.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.52%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.55%	March 31, 2019

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

For the periods ended December 31, 2018

Average Annual Total Returns{- Treasury Portfolio}	Past 1 year	Past 5 years	Past 10 years
03.31 FIMM Funds Select Combo PRO-06 | Treasury Portfolio | Treasury Portfolio-Select FIMM | Return Before Taxes	1.66%	0.50%	0.27%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Select Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.23% (the Expense Cap). If at any time during the current fiscal year expenses for Select Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2019

03.31 FIMM Funds Select Combo PRO-06 | Treasury Portfolio

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

<p style="font: bold 20pt Arial, Helvetica, sans-serif;">Fund Summary</p><p style="font: normal 12pt Arial, Helvetica, sans-serif;"><b>Fund/Class</b>:<br/><b>Treasury Portfolio</b>/Select</p>

Objective [Heading]

rr_ObjectiveHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;">Investment Objective</font>

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to obtain as high a level of current income as is consistent with the preservation of principal and liquidity within the limitations prescribed for the fund.

Expense [Heading]

rr_ExpenseHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Fee Table</b></font>

Expense Narrative [Text Block]

rr_ExpenseNarrativeTextBlock

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Shareholder fees</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px"></p>

(fees paid directly from your investment)

rr_ShareholderFeeOther

none

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

<p style="font: bold 11pt Arial, sans-serif; margin-bottom: 0px;">Annual Operating Expenses</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">(expenses that you pay each year as a % of the value of your investment)</p>

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

<font style="FONT-FAMILY: Times New Roman" size="3">July 31, 2020</font>

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

<p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Sell All Shares</p>

Strategy [Heading]

rr_StrategyHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Strategies</b></font>

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Normally investing at least 99.5% of total assets in cash, U.S. Treasury securities and/or repurchase agreements for those securities.

• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.

In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities and repurchase agreements for those securities.

Risk [Heading]

rr_RiskHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Principal Investment Risks</b></font>

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Interest Rate Changes. Interest rate increases can cause the price of a money market security to decrease.

• Issuer-Specific Changes. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a money market security to decrease.

You could lose money by investing in the fund. Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so. An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.

The fund will not impose a fee upon the sale of your shares, nor temporarily suspend your ability to sell shares if the fund's weekly liquid assets fall below 30% of its total assets because of market conditions or other factors.

Risk Lose Money [Text]

rr_RiskLoseMoney

<font style="FONT-FAMILY: Times New Roman" size="3">You could lose money by investing in the fund.</font>

Risk Money Market Fund Sponsor May Not Provide Support [Text]

rr_RiskMoneyMarketFundSponsorMayNotProvideSupport

<font style="FONT-FAMILY: Times New Roman" size="3">Fidelity Investments and its affiliates, the fund’s sponsor, have no legal obligation to provide financial support to the fund, and you should not expect that the sponsor will provide financial support to the fund at any time.</font>

Risk Money Market Fund May Not Preserve Dollar [Text]

rr_RiskMoneyMarketFundMayNotPreserveDollar

<font style="FONT-FAMILY: Times New Roman" size="3">Although the fund seeks to preserve the value of your investment at $1.00 per share, it cannot guarantee it will do so.</font>

Risk Not Insured Depository Institution [Text]

rr_RiskNotInsuredDepositoryInstitution

<font style="FONT-FAMILY: Times New Roman" size="3">An investment in the fund is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.</font>

Bar Chart and Performance Table [Heading]

rr_BarChartAndPerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Performance</b></font>

Performance Narrative [Text Block]

rr_PerformanceNarrativeTextBlock

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year. Past performance is not an indication of future performance.

Visit institutional.fidelity.com for more recent performance information.

Performance Information Illustrates Variability of Returns [Text]

rr_PerformanceInformationIllustratesVariabilityOfReturns

<font style="FONT-FAMILY: Times New Roman" size="3">The information illustrates the changes in the performance of the fund's shares from year to year.</font>

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

<font style="FONT-FAMILY: Times New Roman" size="3">institutional.fidelity.com </font>

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

<font style="FONT-FAMILY: Times New Roman" size="3">Past performance is not an indication of future performance.</font>

Bar Chart [Heading]

rr_BarChartHeading

<p style="font:bold 11pt Arial, sans-serif; margin-bottom: 0px;">Year-by-Year Returns</p><p style="font: bold 9pt Arial, sans-serif; margin-top: 0px">Calendar Years</p>

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	0.52%	December 31, 2018
Lowest Quarter Return	0.00%	March 31, 2015
Year-to-Date Return	0.55%	March 31, 2019

Performance Table Heading

rr_PerformanceTableHeading

<font style="font: bold 16pt Arial, Helvetica, sans-serif;"><b>Average Annual Returns</b></font>

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2018

03.31 FIMM Funds Select Combo PRO-06 | Treasury Portfolio | Treasury Portfolio-Select FIMM

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.14%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

0.05%

Other expenses

rr_OtherExpensesOverAssets

0.07%

Total annual operating expenses

rr_ExpensesOverAssets

0.26%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.03%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.23%

1 year

rr_ExpenseExampleYear01

$ 24

3 years

rr_ExpenseExampleYear03

80

5 years

rr_ExpenseExampleYear05

142

10 years

rr_ExpenseExampleYear10

$ 327

2009

rr_AnnualReturn2009

0.14%

2010

rr_AnnualReturn2010

0.01%

2011

rr_AnnualReturn2011

0.01%

2012

rr_AnnualReturn2012

0.01%

2013

rr_AnnualReturn2013

0.01%

2014

rr_AnnualReturn2014

0.01%

2015

rr_AnnualReturn2015

0.01%

2016

rr_AnnualReturn2016

0.15%

2017

rr_AnnualReturn2017

0.69%

2018

rr_AnnualReturn2018

1.66%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2019

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.55%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Dec. 31, 2018

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

0.52%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Mar. 31, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

none

03.31 FIMM Funds Select Combo PRO-06 | Treasury Portfolio | Return Before Taxes | Treasury Portfolio-Select FIMM

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Treasury Portfolio

Label

rr_AverageAnnualReturnLabel

<font style="FONT-FAMILY: Times New Roman" ><b>Select Class</b></font>

Past 1 year

rr_AverageAnnualReturnYear01

1.66%

Past 5 years

rr_AverageAnnualReturnYear05

0.50%

Past 10 years

rr_AverageAnnualReturnYear10

0.27%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Select Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.23% (the Expense Cap). If at any time during the current fiscal year expenses for Select Class of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through July 31, 2020. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	Fidelity Colchester Street Trust
Prospectus Date	rr_ProspectusDate	May 30, 2019
Document Creation Date	dei_DocumentCreationDate	May 28, 2019