v3.25.2
Shareholder Report
 6 Months Ended
Jun. 30, 2025
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, 2025
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, 2025 to June 30, 2025.
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 
$ 0 A
0.00%B
  
A Amount represents less than $.50
B Amount represents less than 0.005%
 
Expenses Paid, Amount $ 0
Expense Ratio, Percent 0.00%
Net Assets $ 3,119,027,408
Holdings Count | shares 1,907
Investment Company Portfolio Turnover 119.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of June 30, 2025)
 
KEY FACTS 
  
  
Fund Size
$3,119,027,408
  
Number of Holdings
1,907
  
Portfolio Turnover
119%
  
Holdings [Text Block]   U.S. Government and U.S. Government Agency Obligations 63.5 AAA 7.2 AA 0.4 A 9.1 BBB 14.4 BB 1.7 B 0.9 Not Rated 2.3 Short-Term Investments and Net Other Assets (Liabilities) 0.5 QUALITY DIVERSIFICATION (% of Fund's net assets)     U.S. Government and U.S. Government Agency Obligations - 63.5   AAA - 7.2                                  AA - 0.4                                   A - 9.1                                    BBB - 14.4                                 BB - 1.7                                   B - 0.9                                    Not Rated - 2.3                            Short-Term Investments and Net Other Assets (Liabilities) - 0.5   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 45.4 Corporate Bonds 24.1 U.S. Government Agency - Mortgage Securities 18.1 Asset-Backed Securities 8.0 CMOs and Other Mortgage Related Securities 3.8 Other Investments 0.1 Foreign Government and Government Agency Obligations 0.0 Short-Term Investments and Net Other Assets (Liabilities) 0.5 ASSET ALLOCATION (% of Fund's net assets)     U.S. Treasury Obligations - 45.4           Corporate Bonds - 24.1                     U.S. Government Agency - Mortgage Securities - 18.1   Asset-Backed Securities - 8.0              CMOs and Other Mortgage Related Securities - 3.8   Other Investments - 0.1                    Foreign Government and Government Agency Obligations - 0.0   Short-Term Investments and Net Other Assets (Liabilities) - 0.5   United States 90.4 Grand Cayman (UK Overseas Ter) 5.4 Mexico 0.8 United Kingdom 0.7 Ireland 0.5 Bailiwick Of Jersey 0.5 Netherlands 0.4 Switzerland 0.3 France 0.3 Others 0.7 GEOGRAPHIC DIVERSIFICATION (% of Fund's net assets)     United States - 90.4                       Grand Cayman (UK Overseas Ter) - 5.4       Mexico - 0.8                               United Kingdom - 0.7                       Ireland - 0.5                              Bailiwick Of Jersey - 0.5                  Netherlands - 0.4                          Switzerland - 0.3                          France - 0.3                               Others - 0.7                                  
Largest Holdings [Text Block] TOP HOLDINGS (% of Fund's net assets)     US Treasury Notes 33.8   US Treasury Bonds 11.6   Fannie Mae Mortgage pass-thru certificates 5.8   Ginnie Mae II Pool 4.8   Freddie Mac Gold Pool 4.2   Uniform Mortgage Backed Securities 2.2   Morgan Stanley 1.4   JPMorgan Chase & Co 1.4   Bank of America Corp 1.2   Wells Fargo & Co 0.9     67.3