| Form N-CSR Cover | 12 Months Ended |
| Aug. 31, 2024 | |
| Shareholder Report [Line Items] | |
| Document Type | N-CSR |
| Amendment Flag | false |
| Registrant Name | Fidelity Garrison Street Trust |
| Entity Central Index Key | 0000803013 |
| Entity Investment Company Type | N-1A |
| Document Period End Date | Aug. 31, 2024 |
| Shareholder Report | 12 Months Ended |
| Aug. 31, 2024 USD ($) shares | |
| Shareholder Report [Line Items] | |
| Document Type | N-CSR |
| Amendment Flag | false |
| Registrant Name | Fidelity Garrison Street Trust |
| Entity Central Index Key | 0000803013 |
| Entity Investment Company Type | N-1A |
| Document Period End Date | Aug. 31, 2024 |
| Fidelity Education Income Fund | |
| Shareholder Report [Line Items] | |
| Fund Name | Fidelity® Education Income Fund |
| Class Name | Fidelity® Education Income Fund |
| No Trading Symbol Flag | true |
| Annual or Semi-Annual Statement [Text Block] | This annual shareholder report contains information about Fidelity® Education Income Fund for the period September 1, 2023 to August 31, 2024. |
| Shareholder Report Annual or Semi-Annual | 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 year? (based on hypothetical $10,000 investment) FUND COST (PREVIOUS YEAR) Costs of a $10,000 investment Costs paid as a percentage of a $10,000 investment Fidelity® Education Income 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% |
| Factors Affecting Performance [Text Block] | What affected the Fund's performance this period? • • • • • |
| Performance Past Does Not Indicate Future [Text] | The Fund's past performance is not a good predictor of the Fund's future performance.</span>" id="sjs-B24"><span style="font-family:'Fidelity Sans';font-size:11px;color:#000000;font-style:normal;font-weight:normal;">The Fund's past performance is not a good predictor of the Fund's future performance.</span> |
| Line Graph [Table Text Block] | How did the Fund perform over the life of Fund? CUMULATIVE PERFORMANCE March 16, 2021 through August 31, 2024. Initial investment of $10,000. Fidelity® Education Income Fund $10,000 $10,089 $9,448 $9,575 Fidelity Education Income Composite Index℠ $10,000 $10,059 $9,437 $9,515 Bloomberg U.S. 1-5 Year Government/Credit Bond Index $10,000 $10,055 $9,467 $9,580 Bloomberg U.S. Aggregate Bond Index $10,000 $10,268 $9,085 $8,977 2021 2022 2023 2024 |
| Average Annual Return [Table Text Block] | AVERAGE ANNUAL TOTAL RETURNS: 1 Year Life of Fund A Fidelity® Education Income Fund 6.83% 0.66% Fidelity Education Income Composite Index℠ 6.48% 0.38% Bloomberg U.S. 1-5 Year Government/Credit Bond Index 6.67% 0.63% Bloomberg U.S. Aggregate Bond Index 7.30% -1.08% A From March 16, 2021 Visit www.fidelity.com for more recent performance information. The Fund's past performance is not a good predictor of the Fund's future performance. The graph and table do not reflect the deduction of taxes that a shareholder would pay on fund distributions or redemption of fund shares. |
| Performance Inception Date | Mar. 16, 2021 |
| No Deduction of Taxes [Text Block] | The graph and table do not reflect the deduction of taxes that a shareholder would pay on fund distributions or redemption of fund shares. |
| Net Assets | $ 691,570,222 |
| Holdings Count | shares | 327 |
| Advisory Fees Paid, Amount | $ 0 |
| Investment Company Portfolio Turnover | 77% |
| Additional Fund Statistics [Text Block] | Key Fund Statistics (as of August 31, 2024) KEY FACTS Fund Size $ Number of Holdings 327 Total Advisory Fee $ Portfolio Turnover 77 % |
| Holdings [Text Block] | U.S. Government and U.S. Government Agency Obligations 50.2 AAA 15.3 AA 1.8 A 18.2 BBB 12.3 Not Rated 0.7 Short-Term Investments and Net Other Assets (Liabilities) 1.5 QUALITY DIVERSIFICATION (% of Fund's net assets) 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 48.4 Corporate Bonds 32.3 Asset-Backed Securities 13.3 CMOs and Other Mortgage Related Securities 2.7 U.S. Government Agency - Mortgage Securities 1.8 Short-Term Investments and Net Other Assets (Liabilities) 1.5 ASSET ALLOCATION (% of Fund's net assets) United States 90.1 United Kingdom 2.9 Germany 1.9 Japan 1.0 Switzerland 1.0 Canada 0.8 Norway 0.7 France 0.7 Netherlands 0.5 Others 0.4 GEOGRAPHIC DIVERSIFICATION (% of Fund's net assets) |
| Largest Holdings [Text Block] | TOP HOLDINGS (% of Fund's net assets) US Treasury Notes 48.4 JPMorgan Chase & Co 2.1 Bank of America Corp 1.6 Fannie Mae Mortgage pass-thru certificates 1.3 American Express Cr Acc Mst Tr 1.3 Morgan Stanley 1.2 Wells Fargo & Co 1.1 Chase Issuance Trust 1.0 Citigroup Inc 1.0 UBS Group AG 1.0 60.0 |
| Shareholder Report, Line Graph (Details) - USD ($) | 12 Months Ended | 48 Months Ended | |||
| Aug. 31, 2024 | Aug. 31, 2024 | Aug. 31, 2023 | Aug. 31, 2022 | Aug. 31, 2021 | |
| C000227568_0_LG_BBIndexMember_4YRMember | |||||
| Account Value [Line Items] | |||||
| Line Graph and Table Measure Name | Bloomberg U.S. Aggregate Bond Index</span>" id="sjs-C5"><span style="font-family:'Fidelity Sans';font-size:9px;color:#000000;font-style:normal;font-weight:normal;">Bloomberg U.S. Aggregate Bond Index</span> | ||||
| C000227568_0_BBIndexMember_AsOf_2021Member | |||||
| Account Value [Line Items] | |||||
| Account Value | $ 10,000 | ||||
| C000227568_0_BBIndexMember_AsOf_2022Member | |||||
| Account Value [Line Items] | |||||
| Account Value | $ 10,268 | ||||
| C000227568_0_BBIndexMember_AsOf_2023Member | |||||
| Account Value [Line Items] | |||||
| Account Value | $ 9,085 | ||||
| C000227568_0_BBIndexMember_AsOf_2024Member | |||||
| Account Value [Line Items] | |||||
| Account Value | $ 8,977 | $ 8,977 | |||
| C000227568_2_BBIndexMember_1YRMember | |||||
| Account Value [Line Items] | |||||
| Average Annual Return, Percent | 7.30% | ||||
| C000227568_2_BBIndexMember_LOCMember | |||||
| Account Value [Line Items] | |||||
| Average Annual Return, Percent | (1.08%) | ||||
| C000227568_LG_ClassMember_4YRMember | |||||
| Account Value [Line Items] | |||||
| Line Graph and Table Measure Name | Fidelity® Education Income Fund</span>" id="sjs-C26"><span style="font-family:'Fidelity Sans';font-size:9px;color:#000000;font-style:normal;font-weight:normal;">Fidelity® Education Income Fund</span> | ||||
| C000227568_ClassMember_AsOf_2021Member | |||||
| Account Value [Line Items] | |||||
| Account Value | 10,000 | ||||
| C000227568_ClassMember_AsOf_2022Member | |||||
| Account Value [Line Items] | |||||
| Account Value | 10,089 | ||||
| C000227568_ClassMember_AsOf_2023Member | |||||
| Account Value [Line Items] | |||||
| Account Value | 9,448 | ||||
| C000227568_ClassMember_AsOf_2024Member | |||||
| Account Value [Line Items] | |||||
| Account Value | $ 9,575 | $ 9,575 | |||
| C000227568_LG_1_AdditionalIndexMember_4YRMember | |||||
| Account Value [Line Items] | |||||
| Line Graph and Table Measure Name | Fidelity Education Income Composite Index℠</span>" id="sjs-C41"><span style="font-family:'Fidelity Sans';font-size:9px;color:#000000;font-style:normal;font-weight:normal;">Fidelity Education Income Composite Index℠</span> | ||||
| C000227568_1_AdditionalIndexMember_AsOf_2021Member | |||||
| Account Value [Line Items] | |||||
| Account Value | 10,000 | ||||
| C000227568_1_AdditionalIndexMember_AsOf_2022Member | |||||
| Account Value [Line Items] | |||||
| Account Value | 10,059 | ||||
| C000227568_1_AdditionalIndexMember_AsOf_2023Member | |||||
| Account Value [Line Items] | |||||
| Account Value | 9,437 | ||||
| C000227568_1_AdditionalIndexMember_AsOf_2024Member | |||||
| Account Value [Line Items] | |||||
| Account Value | 9,515 | $ 9,515 | |||
| C000227568_LG_2_AdditionalIndexMember_4YRMember | |||||
| Account Value [Line Items] | |||||
| Line Graph and Table Measure Name | Bloomberg U.S. 1-5 Year Government/Credit Bond Index</span>" id="sjs-C56"><span style="font-family:'Fidelity Sans';font-size:9px;color:#000000;font-style:normal;font-weight:normal;">Bloomberg U.S. 1-5 Year Government/Credit Bond Index</span> | ||||
| C000227568_2_AdditionalIndexMember_AsOf_2021Member | |||||
| Account Value [Line Items] | |||||
| Account Value | $ 10,000 | ||||
| C000227568_2_AdditionalIndexMember_AsOf_2022Member | |||||
| Account Value [Line Items] | |||||
| Account Value | $ 10,055 | ||||
| C000227568_2_AdditionalIndexMember_AsOf_2023Member | |||||
| Account Value [Line Items] | |||||
| Account Value | $ 9,467 | ||||
| C000227568_2_AdditionalIndexMember_AsOf_2024Member | |||||
| Account Value [Line Items] | |||||
| Account Value | $ 9,580 | $ 9,580 | |||
| C000227568_NoLoadClassMember_1YRMember | |||||
| Account Value [Line Items] | |||||
| Average Annual Return, Percent | 6.83% | ||||
| C000227568_NoLoadClassMember_2023-09-01_2024-08-31Member | |||||
| Account Value [Line Items] | |||||
| Average Annual Return, Percent | 0.66% | ||||
| C000227568_0_AdditionalIndexMember_1YRMember | |||||
| Account Value [Line Items] | |||||
| Average Annual Return, Percent | 6.48% | ||||
| C000227568_0_AdditionalIndexMember_LOCMember | |||||
| Account Value [Line Items] | |||||
| Average Annual Return, Percent | 0.38% | ||||
| C000227568_1_AdditionalIndexMember_1YRMember | |||||
| Account Value [Line Items] | |||||
| Average Annual Return, Percent | 6.67% | ||||
| C000227568_1_AdditionalIndexMember_LOCMember | |||||
| Account Value [Line Items] | |||||
| Average Annual Return, Percent | 0.63% |
| Shareholder Report, Holdings (Details) - Fidelity Education Income Fund | Aug. 31, 2024 |
| TH0Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 48.40% |
| TH1Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.10% |
| TH2Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.60% |
| TH3Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| TH4Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.30% |
| TH5Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.20% |
| TH6Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.10% |
| TH7Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1% |
| TH8Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1% |
| TH9Member | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1% |
| QDUSGovernmentandUSGovernmentAgencyObligationsMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 50.20% |
| QDAAAMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 15.30% |
| QDAAMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| QDAMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 18.20% |
| QDBBBMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 12.30% |
| QDNotRatedMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.70% |
| QDShortTermInvestmentsandNetOtherAssetsLiabilitiesMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.50% |
| United States | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 90.10% |
| United Kingdom | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.90% |
| Germany | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.90% |
| Japan | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1% |
| Switzerland | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1% |
| Canada | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.80% |
| Norway | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.70% |
| France | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.70% |
| Netherlands | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.50% |
| OtherMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 0.40% |
| AAUSTreasuryObligationsMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 48.40% |
| AACorporateBondsMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 32.30% |
| AAAssetBackedSecuritiesMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 13.30% |
| AACMOsandOtherMortgageRelatedSecuritiesMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 2.70% |
| AAUSGovernmentAgencyMortgageSecuritiesMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.80% |
| AAShortTermInvestmentsandNetOtherAssetsLiabilitiesMember | |
| Holdings [Line Items] | |
| Percent of Net Asset Value | 1.50% |
