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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:ReturnsQuarter ended
Highest Quarter Return0.56%December 31, 2018
Lowest Quarter Return0.20%March 31, 2017
Year-to-Date Return0.61%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