Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	Fidelity Colchester Street Trust
Prospectus Date	rr_ProspectusDate	May 30, 2015
Inst \| Government Portfolio
Risk/Return:	rr_RiskReturnAbstract
Supplement [Text Block]	fcst_SupplementTextBlock	Supplement to the Fidelity^® Institutional Money Market Funds Institutional Class May 30, 2015 Prospectus The following information replaces the similar information for Government Portfolio found in the "Fund Summary" section under the heading "Principal Investment Strategies" on page 4. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments. The following information supplements information for Government Portfolio found in the "Fund Summary" section under the heading "Principal Investment Strategies" on page 4. In addition, the Adviser normally invests at least 80% of the fund's assets in U.S. Government securities and repurchase agreements for those securities. The following information supplements information for Government Portfolio, Treasury Only Portfolio, and Treasury Portfolio found in the "Fund Summary" section under the heading "Principal Investment Risks" on pages 4, 16, and 20, respectively. 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. The following information replaces the similar information for Money Market Portfolio found in the "Fund Summary" section under the heading "Principal Investment Strategies" on page 8. 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. The following information replaces the similar information for Money Market Portfolio and Prime Money Market Portfolio found in the "Fund Summary" section under the heading "Principal Investment Risks" on pages 9 and 12, respectively. 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 following information replaces the similar information for Prime Money Market Portfolio found in the "Fund Summary" section under the heading "Principal Investment Strategies" on page 12. 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. The following information replaces the similar information for Treasury Only Portfolio found in the "Fund Summary" section under the heading "Principal Investment Strategies" on page 16. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments. The following information supplements information for Treasury Only Portfolio found in the "Fund Summary" section under the heading "Principal Investment Strategies" on page 16. In addition, the Adviser normally invests at least 80% of the fund's assets in U.S. Treasury securities. The following information replaces the similar information for Treasury Portfolio found in the "Fund Summary" section under the heading "Principal Investment Strategies" on page 20. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments. The following information supplements information for Treasury Portfolio found in the "Fund Summary" section under the heading "Principal Investment Strategies" on page 20. In addition, the Adviser normally invests at least 80% of the fund's assets in U.S. Treasury securities and repurchase agreements for those securities.

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

Fidelity Colchester Street Trust

Prospectus Date

rr_ProspectusDate

May 30, 2015

Inst | Government Portfolio

Risk/Return:

rr_RiskReturnAbstract

Supplement [Text Block]

fcst_SupplementTextBlock

Supplement to the
Fidelity^® Institutional Money Market Funds
Institutional Class
May 30, 2015
Prospectus

The following information replaces the similar information for Government Portfolio found in the "Fund Summary" section under the heading "Principal Investment Strategies" on page 4.

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

The following information supplements information for Government Portfolio found in the "Fund Summary" section under the heading "Principal Investment Strategies" on page 4.

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

The following information supplements information for Government Portfolio, Treasury Only Portfolio, and Treasury Portfolio found in the "Fund Summary" section under the heading "Principal Investment Risks" on pages 4, 16, and 20, respectively.

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.

The following information replaces the similar information for Money Market Portfolio found in the "Fund Summary" section under the heading "Principal Investment Strategies" on page 8.

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.

The following information replaces the similar information for Money Market Portfolio and Prime Money Market Portfolio found in the "Fund Summary" section under the heading "Principal Investment Risks" on pages 9 and 12, respectively.

The following information replaces the similar information for Prime Money Market Portfolio found in the "Fund Summary" section under the heading "Principal Investment Strategies" on page 12.

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.

The following information replaces the similar information for Treasury Only Portfolio found in the "Fund Summary" section under the heading "Principal Investment Strategies" on page 16.

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

The following information supplements information for Treasury Only Portfolio found in the "Fund Summary" section under the heading "Principal Investment Strategies" on page 16.

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

The following information replaces the similar information for Treasury Portfolio found in the "Fund Summary" section under the heading "Principal Investment Strategies" on page 20.

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

The following information supplements information for Treasury Portfolio found in the "Fund Summary" section under the heading "Principal Investment Strategies" on page 20.

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