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Label Element Value
Risk/Return: rr_RiskReturnAbstract  
Registrant Name dei_EntityRegistrantName Fidelity Colchester Street Trust
Prospectus Date rr_ProspectusDate May 30, 2014
II | Tax-Exempt Portfolio
  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading Fund Summary

Fund/Class:
Tax-Exempt Portfolio /II
Objective [Heading] rr_ObjectiveHeading Investment Objective
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 Fee Table
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 Shareholder fees
(fees paid directly from your investment)
Operating Expenses Caption [Text] rr_OperatingExpensesCaption Annual operating expenses
(expenses that you pay each year as a % of the value of your investment)
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:
Strategy [Heading] rr_StrategyHeading Principal Investment Strategies
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock
  • Normally investing in municipal money market securities rated in the highest category by at least one nationally recognized rating service or in one of the two highest categories by another rating service if rated by more than one, or, if unrated, determined to be of equivalent quality to the highest category by Fidelity Management & Research Company (FMR).
  • 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).
  • 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 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, and diversification of investments.
Risk [Heading] rr_RiskHeading Principal Investment Risks
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.
An investment in the fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Money Market Fund [Text] rr_RiskMoneyMarketFund Although the fund seeks to preserve the value of your investment at $1.00 per share, it is possible to lose money by investing in the fund.
Risk Not Insured Depository Institution [Text] rr_RiskNotInsuredDepositoryInstitution An investment in the fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading Performance
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.advisor.fidelity.com for updated return information.
Performance Information Illustrates Variability of Returns [Text] rr_PerformanceInformationIllustratesVariabilityOfReturns The information illustrates the changes in the performance of the fund's shares from year to year.
Performance Availability Website Address [Text] rr_PerformanceAvailabilityWebSiteAddress www.advisor.fidelity.com
Performance Past Does Not Indicate Future [Text] rr_PerformancePastDoesNotIndicateFuture Past performance is not an indication of future performance.
Bar Chart [Heading] rr_BarChartHeading Year-by-Year Returns
Calendar Years
Bar Chart Closing [Text Block] rr_BarChartClosingTextBlock
During the periods shown in the chart:
Returns
Quarter ended
Highest Quarter Return
0.86%
June 30, 2007
Lowest Quarter Return
0.00%
March 31, 2013
Year-to-Date Return
0.00%
March 31, 2014
Performance Table Heading rr_PerformanceTableHeading Average Annual Returns
For the periods ended
December 31, 2013
II | Tax-Exempt Portfolio | Class II
  
Risk/Return: rr_RiskReturnAbstract  
Shareholder fees (fees paid directly from your investment) rr_ShareholderFeeOther none
Management fee rr_ManagementFeesOverAssets 0.14%
Distribution and/or Service (12b-1) fees rr_DistributionAndService12b1FeesOverAssets 0.15%
Other expenses rr_OtherExpensesOverAssets 0.08%
Total annual operating expenses rr_ExpensesOverAssets 0.37%
1 year rr_ExpenseExampleYear01 38
3 years rr_ExpenseExampleYear03 119
5 years rr_ExpenseExampleYear05 208
10 years rr_ExpenseExampleYear10 $ 468
2004 rr_AnnualReturn2004 0.94%
2005 rr_AnnualReturn2005 2.14%
2006 rr_AnnualReturn2006 3.16%
2007 rr_AnnualReturn2007 3.38%
2008 rr_AnnualReturn2008 1.90%
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%
Year to Date Return, Label rr_YearToDateReturnLabel Year-to-Date Return
Bar Chart, Year to Date Return, Date rr_BarChartYearToDateReturnDate Mar. 31, 2014
Bar Chart, Year to Date Return rr_BarChartYearToDateReturn 0.00%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarter Return
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Jun. 30, 2007
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 0.86%
Lowest Quarterly Return, Label rr_LowestQuarterlyReturnLabel Lowest Quarter Return
Lowest Quarterly Return, Date rr_BarChartLowestQuarterlyReturnDate Mar. 31, 2013
Lowest Quarterly Return rr_BarChartLowestQuarterlyReturn 0.00%
Label rr_AverageAnnualReturnLabel Class II
Past 1 year rr_AverageAnnualReturnYear01 0.02%
Past 5 years rr_AverageAnnualReturnYear05 0.03%
Past 10 years rr_AverageAnnualReturnYear10 1.16%