| Form N-CSR Cover | 6 Months Ended |
| Jun. 30, 2024 | |
| Shareholder Report [Line Items] | |
| Document Type | N-CSRS |
| Amendment Flag | false |
| Registrant Name | Fidelity Garrison Street Trust |
| Entity Central Index Key | 0000803013 |
| Entity Investment Company Type | N-1A |
| Document Period End Date | Jun. 30, 2024 |
| Shareholder Report | 6 Months Ended |
| Jun. 30, 2024 USD ($) shares | |
| Shareholder Report [Line Items] | |
| Document Type | N-CSRS |
| Amendment Flag | false |
| Registrant Name | Fidelity Garrison Street Trust |
| Entity Central Index Key | 0000803013 |
| Entity Investment Company Type | N-1A |
| Document Period End Date | Jun. 30, 2024 |
| VIP Investment Grade Central Fund | |
| Shareholder Report [Line Items] | |
| Fund Name | Fidelity® VIP Investment Grade Central Fund |
| Class Name | VIP Investment Grade Central Fund |
| No Trading Symbol Flag | true |
| Annual or Semi-Annual Statement [Text Block] | This semi-annual shareholder report contains information about Fidelity® VIP Investment Grade Central Fund for the period January 1, 2024 to June 30, 2024. |
| Shareholder Report Annual or Semi-Annual | semi-annual shareholder report |
| Additional Information [Text Block] | You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-800-544-8544. |
| Additional Information Phone Number | 1-800-544-8544 |
| Additional Information Website | fundresearch.fidelity.com/prospectus/sec |
| Expenses [Text Block] | What were your Fund costs for the last six months? (based on hypothetical $10,000 investment) Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment VIP Investment Grade Central Fund $ A 0.00 % B A Amount represents less than $.50 B Amount represents less than 0.005% |
| Expenses Paid, Amount | $ 0 |
| Expense Ratio, Percent | 0% |
| Net Assets | $ 2,975,910,228 |
| Holdings Count | shares | 1,927 |
| Investment Company Portfolio Turnover | 201% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics (as of June 30, 2024) KEY FACTS Fund Size $ Number of Holdings 1,927 Portfolio Turnover 201 % |
| Holdings [Text Block] | U.S. Government and U.S. Government Agency Obligations 66.3 AAA 7.6 AA 0.3 A 8.5 BBB 15.0 BB 2.2 B 0.8 Not Rated 1.2 QUALITY DIVERSIFICATION (% of Fund's net assets) Short-Term Investments and Net Other Assets (Liabilities) - (1.9)%* We have used ratings from Moody's Investors Service, Inc. Where Moody's® ratings are not available, we have used S&P® ratings. All ratings are as of the date indicated and do not reflect subsequent changes. U.S. Treasury Obligations 44.2 Corporate Bonds 25.1 U.S. Government Agency - Mortgage Securities 22.1 Asset-Backed Securities 6.5 CMOs and Other Mortgage Related Securities 3.9 Other Investments 0.1 Foreign Government and Government Agency Obligations 0.0 ASSET ALLOCATION (% of Fund's net assets) Short-Term Investments and Net Other Assets (Liabilities) - (1.9)% United States 91.1 Grand Cayman (Uk Overseas Ter) 4.8 United Kingdom 0.9 Mexico 0.8 Ireland 0.6 Switzerland 0.4 Germany 0.2 Belgium 0.2 Bailiwick Of Jersey 0.2 Others 0.8 GEOGRAPHIC DIVERSIFICATION (% of Fund's net assets) |
| Largest Holdings [Text Block] | TOP HOLDINGS (% of Fund's net assets) US Treasury Notes 33.2 US Treasury Bonds 11.0 Uniform Mortgage Backed Securities 7.9 Ginnie Mae II Pool 5.4 Fannie Mae Mortgage pass-thru certificates 4.4 Freddie Mac Gold Pool 4.0 JPMorgan Chase & Co 1.3 Bank of America Corp 1.3 Morgan Stanley 1.2 Citigroup Inc 0.9 70.6 |
| Shareholder Report, Holdings (Details) - VIP Investment Grade Central Fund | Jun. 30, 2024 |
| TH0Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 33.20% |
| TH1Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 11% |
| TH2Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 7.90% |
| TH3Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 5.40% |
| TH4Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.40% |
| TH5Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4% |
| TH6Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| TH7Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| TH8Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| TH9Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.90% |
| QDUSGovernmentandUSGovernmentAgencyObligationsMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 66.30% |
| QDAMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 8.50% |
| QDBBBMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 15% |
| QDBBMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.20% |
| QDBMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.80% |
| QDNotRatedMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| United States | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 91.10% |
| GrandCaymanUkOverseasTerMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 4.80% |
| United Kingdom | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.90% |
| Mexico | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.80% |
| Ireland | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.60% |
| Switzerland | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.40% |
| Germany | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.20% |
| Belgium | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.20% |
| BailiwickOfJerseyMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.20% |
| OtherMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.80% |
| AAUSTreasuryObligationsMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 44.20% |
| AACorporateBondsMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 25.10% |
| AAUSGovernmentAgencyMortgageSecuritiesMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 22.10% |
| AAAssetBackedSecuritiesMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 6.50% |
| AACMOsandOtherMortgageRelatedSecuritiesMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 3.90% |
| AAOtherInvestmentsMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.10% |
| AAForeignGovernmentandGovernmentAgencyObligationsMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0% |
| Label | Element | Value |
| C000035576 [Member] | QDAAMember [Member] | ||
| Percent of Net Asset Value | oef_PctOfNav | 0.30% |
| C000035576 [Member] | QDAAAMember [Member] | ||
| Percent of Net Asset Value | oef_PctOfNav | 7.60% |
