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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, 2017
03.31 FIMM Funds Class 3 Combo PRO-03 | Prime Money Market Portfolio  
Risk/Return: rr_RiskReturnAbstract  
Risk/Return [Heading] rr_RiskReturnHeading

Fund Summary

Fund/Class:
Prime Money Market Portfolio/III
Objective [Heading] rr_ObjectiveHeading Investment Objective
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 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) rr_ShareholderFeeOther none
Operating Expenses Caption [Text] rr_OperatingExpensesCaption

Annual Operating Expenses

(expenses that you pay each year as a % of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination May 31, 2018
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

Sell All Shares
Strategy [Heading] rr_StrategyHeading Principal Investment Strategies
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 Investing more than 25% of total assets in the financial services industries.
Risk [Heading] rr_RiskHeading Principal Investment Risks
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 You could lose money by investing in the fund.
Risk Not Insured Depository Institution [Text] rr_RiskNotInsuredDepositoryInstitution 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.
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 institutional.fidelity.com for more recent performance 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 institutional.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:ReturnsQuarter ended
Highest Quarter Return1.27%September 30, 2007
Lowest Quarter Return0.00%March 31, 2014
Year-to-Date Return0.14%March 31, 2017
Performance Table Heading rr_PerformanceTableHeading Average Annual Returns
Performance Table Narrative rr_PerformanceTableNarrativeTextBlock

For the periods ended December 31, 2016
03.31 FIMM Funds Class 3 Combo PRO-03 | 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 0.06%
Total annual operating expenses rr_ExpensesOverAssets 0.45%
Fee waiver and/or expense reimbursement rr_FeeWaiverOrReimbursementOverAssets 0.02% [1]
Total annual operating expenses after fee waiver and/or expense reimbursement rr_NetExpensesOverAssets 0.43%
1 year rr_ExpenseExampleYear01 $ 44
3 years rr_ExpenseExampleYear03 142
5 years rr_ExpenseExampleYear05 250
10 years rr_ExpenseExampleYear10 $ 565
2007 rr_AnnualReturn2007 5.02%
2008 rr_AnnualReturn2008 2.69%
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%
Year to Date Return, Label rr_YearToDateReturnLabel Year-to-Date Return
Bar Chart, Year to Date Return, Date rr_BarChartYearToDateReturnDate Mar. 31, 2017
Bar Chart, Year to Date Return rr_BarChartYearToDateReturn 0.14%
Highest Quarterly Return, Label rr_HighestQuarterlyReturnLabel Highest Quarter Return
Highest Quarterly Return, Date rr_BarChartHighestQuarterlyReturnDate Sep. 30, 2007
Highest Quarterly Return rr_BarChartHighestQuarterlyReturn 1.27%
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-03 | 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 Class III
Past 1 year rr_AverageAnnualReturnYear01 0.25%
Past 5 years rr_AverageAnnualReturnYear05 0.06%
Past 10 years rr_AverageAnnualReturnYear10 0.82%
[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, certain securities lending costs, brokerage commissions, extraordinary expenses, and acquired fund fees and expenses, if any), as a percentage of its average net assets, exceed 0.43%. This arrangement will remain in effect through May 31, 2018. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees.