XML 46 R2.htm IDEA: XBRL DOCUMENT v3.24.3
Shareholder Report
 6 Months Ended
Sep. 30, 2024
USD ($)
shares
Shareholder Report [Line Items]  
Document Type N-CSRS
Amendment Flag false
Registrant Name Fidelity Colchester Street Trust
Entity Central Index Key 0000356173
Entity Investment Company Type N-1A
Document Period End Date Sep. 30, 2024
Fidelity Investments Money Market - Money Market Portfolio - Select Class  
Shareholder Report [Line Items]  
Fund Name Money Market Portfolio
Class Name Money Market Portfolio Select Class
Trading Symbol FMYXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Money Market Portfolio for the period April 1, 2024 to September 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 1-877-297-2952 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-297-2952
Additional Information Email funddocuments@fmr.com
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
 
Select Class 
$ 12 
0.23%
  
Expenses Paid, Amount $ 12
Expense Ratio, Percent 0.23%
Net Assets $ 135,270,705,349
Holdings Count | shares 247
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of September 30, 2024)
 
KEY FACTS 
  
  
Fund Size
$135,270,705,349
 
  
Number of Holdings
247
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 90.8   8-30 4.3   31-60 2.4   61-90 0.9   91-180 2.9         Repurchase Agreements 65.1 Commercial Paper 15.2 Non-Negotiable Time Deposit 11.5 Certificates of Deposit 7.8 U.S. Treasury Obligations 1.7 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (1.3)%
Fidelity Investments Money Market Treasury Only Portfolio - Institutional Class  
Shareholder Report [Line Items]  
Fund Name Treasury Only Portfolio
Class Name Treasury Only Portfolio Institutional Class
Trading Symbol FRSXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Treasury Only Portfolio for the period April 1, 2024 to September 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 1-877-297-2952 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-297-2952
Additional Information Email funddocuments@fmr.com
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
 
Institutional Class 
$ 7 
0.14%
  
Expenses Paid, Amount $ 7
Expense Ratio, Percent 0.14%
Net Assets $ 87,593,462,799
Holdings Count | shares 43
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of September 30, 2024)
 
KEY FACTS 
  
  
Fund Size
$87,593,462,799
 
  
Number of Holdings
43
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 20.2   8-30 41.5   31-60 20.7   61-90 9.8   91-180 13.5   >180 0.8         U.S. Treasury Obligations 106.5 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (6.5)%
Fidelity Investments Money Market Treasury Portfolio - Class III  
Shareholder Report [Line Items]  
Fund Name Treasury Portfolio
Class Name Treasury Portfolio Class III
Trading Symbol FCSXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Treasury Portfolio for the period April 1, 2024 to September 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 1-877-297-2952 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-297-2952
Additional Information Email funddocuments@fmr.com
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
 
Class III 
$ 22 
0.43%
  
Expenses Paid, Amount $ 22
Expense Ratio, Percent 0.43%
Net Assets $ 69,262,336,441
Holdings Count | shares 137
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of September 30, 2024)
 
KEY FACTS 
  
  
Fund Size
$69,262,336,441
 
  
Number of Holdings
137
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 61.8   8-30 16.0   31-60 9.6   61-90 4.8   91-180 8.0   >180 1.4         Repurchase Agreements 51.5 U.S. Treasury Obligations 50.1 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (1.6)%
Fidelity Investments Money Market Tax Exempt Portfolio - Class III  
Shareholder Report [Line Items]  
Fund Name Tax-Exempt Portfolio
Class Name Tax-Exempt Portfolio Class III
Trading Symbol FETXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Tax-Exempt Portfolio for the period April 1, 2024 to September 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 1-877-297-2952 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-297-2952
Additional Information Email funddocuments@fmr.com
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
 
Class III 
$ 22 
0.43%
  
Expenses Paid, Amount $ 22
Expense Ratio, Percent 0.43%
Net Assets $ 9,532,805,489
Holdings Count | shares 781
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of September 30, 2024)
 
KEY FACTS 
  
  
Fund Size
$9,532,805,489
 
  
Number of Holdings
781
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 81.2   8-30 2.6   31-60 3.7   61-90 1.9   91-180 4.6   >180 7.0         Variable Rate Demand Notes (VRDNs) 42.8 Tender Option Bond 22.9 Municipal Securities 11.1 Commercial Paper 8.5 Net Other Assets (Liabilities) 14.7 ASSET ALLOCATION (% of Fund's net assets)
Fidelity Investments Money Market Treasury Portfolio - Class IV  
Shareholder Report [Line Items]  
Fund Name Treasury Portfolio
Class Name Treasury Portfolio Class IV
Trading Symbol FTVXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Treasury Portfolio for the period April 1, 2024 to September 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 1-877-297-2952 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-297-2952
Additional Information Email funddocuments@fmr.com
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
 
Class IV 
$ 35 
0.68%
  
Expenses Paid, Amount $ 35
Expense Ratio, Percent 0.68%
Net Assets $ 69,262,336,441
Holdings Count | shares 137
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of September 30, 2024)
 
KEY FACTS 
  
  
Fund Size
$69,262,336,441
 
  
Number of Holdings
137
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 61.8   8-30 16.0   31-60 9.6   61-90 4.8   91-180 8.0   >180 1.4         Repurchase Agreements 51.5 U.S. Treasury Obligations 50.1 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (1.6)%
Fidelity Investments Money Market Tax Exempt Portfolio - Select Class  
Shareholder Report [Line Items]  
Fund Name Tax-Exempt Portfolio
Class Name Tax-Exempt Portfolio Select Class
Trading Symbol FSXXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Tax-Exempt Portfolio for the period April 1, 2024 to September 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 1-877-297-2952 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-297-2952
Additional Information Email funddocuments@fmr.com
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
 
Select Class 
$ 12 
0.23%
  
Expenses Paid, Amount $ 12
Expense Ratio, Percent 0.23%
Net Assets $ 9,532,805,489
Holdings Count | shares 781
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of September 30, 2024)
 
KEY FACTS 
  
  
Fund Size
$9,532,805,489
 
  
Number of Holdings
781
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 81.2   8-30 2.6   31-60 3.7   61-90 1.9   91-180 4.6   >180 7.0         Variable Rate Demand Notes (VRDNs) 42.8 Tender Option Bond 22.9 Municipal Securities 11.1 Commercial Paper 8.5 Net Other Assets (Liabilities) 14.7 ASSET ALLOCATION (% of Fund's net assets)
Fidelity Investments Money Market Treasury Portfolio - Class II  
Shareholder Report [Line Items]  
Fund Name Treasury Portfolio
Class Name Treasury Portfolio Class II
Trading Symbol FCEXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Treasury Portfolio for the period April 1, 2024 to September 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 1-877-297-2952 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-297-2952
Additional Information Email funddocuments@fmr.com
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
 
Class II 
$ 17 
0.33%
  
Expenses Paid, Amount $ 17
Expense Ratio, Percent 0.33%
Net Assets $ 69,262,336,441
Holdings Count | shares 137
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of September 30, 2024)
 
KEY FACTS 
  
  
Fund Size
$69,262,336,441
 
  
Number of Holdings
137
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 61.8   8-30 16.0   31-60 9.6   61-90 4.8   91-180 8.0   >180 1.4         Repurchase Agreements 51.5 U.S. Treasury Obligations 50.1 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (1.6)%
Fidelity Investments Money Market Government Portfolio - Class II  
Shareholder Report [Line Items]  
Fund Name Government Portfolio
Class Name Government Portfolio Class II
Trading Symbol FCVXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Government Portfolio for the period April 1, 2024 to September 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 1-877-297-2952 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-297-2952
Additional Information Email funddocuments@fmr.com
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
 
Class II 
$ 17 
0.33%
  
Expenses Paid, Amount $ 17
Expense Ratio, Percent 0.33%
Net Assets $ 225,755,117,004
Holdings Count | shares 518
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of September 30, 2024)
 
KEY FACTS 
  
  
Fund Size
$225,755,117,004
 
  
Number of Holdings
518
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 70.0   8-30 13.5   31-60 6.3   61-90 3.5   91-180 7.1   >180 1.1         Repurchase Agreements 50.6 U.S. Treasury Obligations 27.4 U.S. Government Agency - Debt 23.5 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (1.5)%
Fidelity Investments Money Market Treasury Only Portfolio - Select Class  
Shareholder Report [Line Items]  
Fund Name Treasury Only Portfolio
Class Name Treasury Only Portfolio Select Class
Trading Symbol FTYXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Treasury Only Portfolio for the period April 1, 2024 to September 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 1-877-297-2952 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-297-2952
Additional Information Email funddocuments@fmr.com
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
 
Select Class 
$ 12 
0.23%
  
Expenses Paid, Amount $ 12
Expense Ratio, Percent 0.23%
Net Assets $ 87,593,462,799
Holdings Count | shares 43
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of September 30, 2024)
 
KEY FACTS 
  
  
Fund Size
$87,593,462,799
 
  
Number of Holdings
43
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 20.2   8-30 41.5   31-60 20.7   61-90 9.8   91-180 13.5   >180 0.8         U.S. Treasury Obligations 106.5 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (6.5)%
Fidelity Investments Money Market Treasury Portfolio - Institutional Class  
Shareholder Report [Line Items]  
Fund Name Treasury Portfolio
Class Name Treasury Portfolio Institutional Class
Trading Symbol FRBXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Treasury Portfolio for the period April 1, 2024 to September 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 1-877-297-2952 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-297-2952
Additional Information Email funddocuments@fmr.com
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
 
Institutional Class 
$ 7 
0.14%
  
Expenses Paid, Amount $ 7
Expense Ratio, Percent 0.14%
Net Assets $ 69,262,336,441
Holdings Count | shares 137
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of September 30, 2024)
 
KEY FACTS 
  
  
Fund Size
$69,262,336,441
 
  
Number of Holdings
137
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 61.8   8-30 16.0   31-60 9.6   61-90 4.8   91-180 8.0   >180 1.4         Repurchase Agreements 51.5 U.S. Treasury Obligations 50.1 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (1.6)%
Fidelity Investments Money Market Treasury Portfolio - Select Class  
Shareholder Report [Line Items]  
Fund Name Treasury Portfolio
Class Name Treasury Portfolio Select Class
Trading Symbol FTUXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Treasury Portfolio for the period April 1, 2024 to September 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 1-877-297-2952 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-297-2952
Additional Information Email funddocuments@fmr.com
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
 
Select Class 
$ 12 
0.23%
  
Expenses Paid, Amount $ 12
Expense Ratio, Percent 0.23%
Net Assets $ 69,262,336,441
Holdings Count | shares 137
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of September 30, 2024)
 
KEY FACTS 
  
  
Fund Size
$69,262,336,441
 
  
Number of Holdings
137
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 61.8   8-30 16.0   31-60 9.6   61-90 4.8   91-180 8.0   >180 1.4         Repurchase Agreements 51.5 U.S. Treasury Obligations 50.1 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (1.6)%
Fidelity Investments Money Market Tax Exempt Portfolio - Class II  
Shareholder Report [Line Items]  
Fund Name Tax-Exempt Portfolio
Class Name Tax-Exempt Portfolio Class II
Trading Symbol FEXXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Tax-Exempt Portfolio for the period April 1, 2024 to September 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 1-877-297-2952 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-297-2952
Additional Information Email funddocuments@fmr.com
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
 
Class II 
$ 17 
0.33%
  
Expenses Paid, Amount $ 17
Expense Ratio, Percent 0.33%
Net Assets $ 9,532,805,489
Holdings Count | shares 781
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of September 30, 2024)
 
KEY FACTS 
  
  
Fund Size
$9,532,805,489
 
  
Number of Holdings
781
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 81.2   8-30 2.6   31-60 3.7   61-90 1.9   91-180 4.6   >180 7.0         Variable Rate Demand Notes (VRDNs) 42.8 Tender Option Bond 22.9 Municipal Securities 11.1 Commercial Paper 8.5 Net Other Assets (Liabilities) 14.7 ASSET ALLOCATION (% of Fund's net assets)
Fidelity Investments Money Market Government Portfolio - Select Class  
Shareholder Report [Line Items]  
Fund Name Government Portfolio
Class Name Government Portfolio Select Class
Trading Symbol FGEXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Government Portfolio for the period April 1, 2024 to September 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 1-877-297-2952 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-297-2952
Additional Information Email funddocuments@fmr.com
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
 
Select Class 
$ 12 
0.23%
  
Expenses Paid, Amount $ 12
Expense Ratio, Percent 0.23%
Net Assets $ 225,755,117,004
Holdings Count | shares 518
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of September 30, 2024)
 
KEY FACTS 
  
  
Fund Size
$225,755,117,004
 
  
Number of Holdings
518
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 70.0   8-30 13.5   31-60 6.3   61-90 3.5   91-180 7.1   >180 1.1         Repurchase Agreements 50.6 U.S. Treasury Obligations 27.4 U.S. Government Agency - Debt 23.5 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (1.5)%
Fidelity Investments Money Market Government Portfolio - Institutional Class  
Shareholder Report [Line Items]  
Fund Name Government Portfolio
Class Name Government Portfolio Institutional Class
Trading Symbol FRGXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Government Portfolio for the period April 1, 2024 to September 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 1-877-297-2952 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-297-2952
Additional Information Email funddocuments@fmr.com
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
 
Institutional Class 
$ 7 
0.14%
  
Expenses Paid, Amount $ 7
Expense Ratio, Percent 0.14%
Net Assets $ 225,755,117,004
Holdings Count | shares 518
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of September 30, 2024)
 
KEY FACTS 
  
  
Fund Size
$225,755,117,004
 
  
Number of Holdings
518
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 70.0   8-30 13.5   31-60 6.3   61-90 3.5   91-180 7.1   >180 1.1         Repurchase Agreements 50.6 U.S. Treasury Obligations 27.4 U.S. Government Agency - Debt 23.5 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (1.5)%
Fidelity Investments Money Market Treasury Only Portfolio - Class IV  
Shareholder Report [Line Items]  
Fund Name Treasury Only Portfolio
Class Name Treasury Only Portfolio Class IV
Trading Symbol FOPXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Treasury Only Portfolio for the period April 1, 2024 to September 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 1-877-297-2952 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-297-2952
Additional Information Email funddocuments@fmr.com
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
 
Class IV 
$ 35 
0.68%
  
Expenses Paid, Amount $ 35
Expense Ratio, Percent 0.68%
Net Assets $ 87,593,462,799
Holdings Count | shares 43
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of September 30, 2024)
 
KEY FACTS 
  
  
Fund Size
$87,593,462,799
 
  
Number of Holdings
43
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 20.2   8-30 41.5   31-60 20.7   61-90 9.8   91-180 13.5   >180 0.8         U.S. Treasury Obligations 106.5 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (6.5)%
Fidelity Investments Money Market Tax Exempt Portfolio - Class I  
Shareholder Report [Line Items]  
Fund Name Tax-Exempt Portfolio
Class Name Tax-Exempt Portfolio Class I
Trading Symbol FTCXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Tax-Exempt Portfolio for the period April 1, 2024 to September 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 1-877-297-2952 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-297-2952
Additional Information Email funddocuments@fmr.com
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
 
Class I 
$ 9 
0.18%
  
Expenses Paid, Amount $ 9
Expense Ratio, Percent 0.18%
Net Assets $ 9,532,805,489
Holdings Count | shares 781
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of September 30, 2024)
 
KEY FACTS 
  
  
Fund Size
$9,532,805,489
 
  
Number of Holdings
781
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 81.2   8-30 2.6   31-60 3.7   61-90 1.9   91-180 4.6   >180 7.0         Variable Rate Demand Notes (VRDNs) 42.8 Tender Option Bond 22.9 Municipal Securities 11.1 Commercial Paper 8.5 Net Other Assets (Liabilities) 14.7 ASSET ALLOCATION (% of Fund's net assets)
Fidelity Investments Money Market - Money Market Portfolio - Class I  
Shareholder Report [Line Items]  
Fund Name Money Market Portfolio
Class Name Money Market Portfolio Class I
Trading Symbol FMPXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Money Market Portfolio for the period April 1, 2024 to September 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 1-877-297-2952 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-297-2952
Additional Information Email funddocuments@fmr.com
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
 
Class I 
$ 9 
0.18%
  
Expenses Paid, Amount $ 9
Expense Ratio, Percent 0.18%
Net Assets $ 135,270,705,349
Holdings Count | shares 247
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of September 30, 2024)
 
KEY FACTS 
  
  
Fund Size
$135,270,705,349
 
  
Number of Holdings
247
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 90.8   8-30 4.3   31-60 2.4   61-90 0.9   91-180 2.9         Repurchase Agreements 65.1 Commercial Paper 15.2 Non-Negotiable Time Deposit 11.5 Certificates of Deposit 7.8 U.S. Treasury Obligations 1.7 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (1.3)%
Fidelity Investments Money Market - Money Market Portfolio - Class II  
Shareholder Report [Line Items]  
Fund Name Money Market Portfolio
Class Name Money Market Portfolio Class II
Trading Symbol FCIXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Money Market Portfolio for the period April 1, 2024 to September 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 1-877-297-2952 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-297-2952
Additional Information Email funddocuments@fmr.com
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
 
Class II 
$ 17 
0.33%
  
Expenses Paid, Amount $ 17
Expense Ratio, Percent 0.33%
Net Assets $ 135,270,705,349
Holdings Count | shares 247
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of September 30, 2024)
 
KEY FACTS 
  
  
Fund Size
$135,270,705,349
 
  
Number of Holdings
247
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 90.8   8-30 4.3   31-60 2.4   61-90 0.9   91-180 2.9         Repurchase Agreements 65.1 Commercial Paper 15.2 Non-Negotiable Time Deposit 11.5 Certificates of Deposit 7.8 U.S. Treasury Obligations 1.7 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (1.3)%
Fidelity Investments Money Market Treasury Only Portfolio - Class II  
Shareholder Report [Line Items]  
Fund Name Treasury Only Portfolio
Class Name Treasury Only Portfolio Class II
Trading Symbol FOXXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Treasury Only Portfolio for the period April 1, 2024 to September 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 1-877-297-2952 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-297-2952
Additional Information Email funddocuments@fmr.com
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
 
Class II 
$ 17 
0.33%
  
Expenses Paid, Amount $ 17
Expense Ratio, Percent 0.33%
Net Assets $ 87,593,462,799
Holdings Count | shares 43
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of September 30, 2024)
 
KEY FACTS 
  
  
Fund Size
$87,593,462,799
 
  
Number of Holdings
43
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 20.2   8-30 41.5   31-60 20.7   61-90 9.8   91-180 13.5   >180 0.8         U.S. Treasury Obligations 106.5 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (6.5)%
Fidelity Investments Money Market Treasury Only Portfolio - Class III  
Shareholder Report [Line Items]  
Fund Name Treasury Only Portfolio
Class Name Treasury Only Portfolio Class III
Trading Symbol FOIXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Treasury Only Portfolio for the period April 1, 2024 to September 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 1-877-297-2952 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-297-2952
Additional Information Email funddocuments@fmr.com
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
 
Class III 
$ 22 
0.43%
  
Expenses Paid, Amount $ 22
Expense Ratio, Percent 0.43%
Net Assets $ 87,593,462,799
Holdings Count | shares 43
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of September 30, 2024)
 
KEY FACTS 
  
  
Fund Size
$87,593,462,799
 
  
Number of Holdings
43
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 20.2   8-30 41.5   31-60 20.7   61-90 9.8   91-180 13.5   >180 0.8         U.S. Treasury Obligations 106.5 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (6.5)%
Fidelity Investments Money Market Government Portfolio - Class I  
Shareholder Report [Line Items]  
Fund Name Government Portfolio
Class Name Government Portfolio Class I
Trading Symbol FIGXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Government Portfolio for the period April 1, 2024 to September 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 1-877-297-2952 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-297-2952
Additional Information Email funddocuments@fmr.com
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
 
Class I 
$ 9 
0.18%
  
Expenses Paid, Amount $ 9
Expense Ratio, Percent 0.18%
Net Assets $ 225,755,117,004
Holdings Count | shares 518
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of September 30, 2024)
 
KEY FACTS 
  
  
Fund Size
$225,755,117,004
 
  
Number of Holdings
518
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 70.0   8-30 13.5   31-60 6.3   61-90 3.5   91-180 7.1   >180 1.1         Repurchase Agreements 50.6 U.S. Treasury Obligations 27.4 U.S. Government Agency - Debt 23.5 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (1.5)%
Fidelity Investments Money Market Treasury Portfolio - Class I  
Shareholder Report [Line Items]  
Fund Name Treasury Portfolio
Class Name Treasury Portfolio Class I
Trading Symbol FISXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Treasury Portfolio for the period April 1, 2024 to September 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 1-877-297-2952 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-297-2952
Additional Information Email funddocuments@fmr.com
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
 
Class I 
$ 9 
0.18%
  
Expenses Paid, Amount $ 9
Expense Ratio, Percent 0.18%
Net Assets $ 69,262,336,441
Holdings Count | shares 137
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of September 30, 2024)
 
KEY FACTS 
  
  
Fund Size
$69,262,336,441
 
  
Number of Holdings
137
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 61.8   8-30 16.0   31-60 9.6   61-90 4.8   91-180 8.0   >180 1.4         Repurchase Agreements 51.5 U.S. Treasury Obligations 50.1 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (1.6)%
Fidelity Investments Money Market - Money Market Portfolio - Institutional Class  
Shareholder Report [Line Items]  
Fund Name Money Market Portfolio
Class Name Money Market Portfolio Institutional Class
Trading Symbol FNSXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Money Market Portfolio for the period April 1, 2024 to September 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 1-877-297-2952 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-297-2952
Additional Information Email funddocuments@fmr.com
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
 
Institutional Class 
$ 7 
0.14%
  
Expenses Paid, Amount $ 7
Expense Ratio, Percent 0.14%
Net Assets $ 135,270,705,349
Holdings Count | shares 247
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of September 30, 2024)
 
KEY FACTS 
  
  
Fund Size
$135,270,705,349
 
  
Number of Holdings
247
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 90.8   8-30 4.3   31-60 2.4   61-90 0.9   91-180 2.9         Repurchase Agreements 65.1 Commercial Paper 15.2 Non-Negotiable Time Deposit 11.5 Certificates of Deposit 7.8 U.S. Treasury Obligations 1.7 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (1.3)%
Fidelity Investments Money Market - Money Market Portfolio - Class III  
Shareholder Report [Line Items]  
Fund Name Money Market Portfolio
Class Name Money Market Portfolio Class III
Trading Symbol FCOXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Money Market Portfolio for the period April 1, 2024 to September 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 1-877-297-2952 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-297-2952
Additional Information Email funddocuments@fmr.com
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
 
Class III 
$ 22 
0.43%
  
Expenses Paid, Amount $ 22
Expense Ratio, Percent 0.43%
Net Assets $ 135,270,705,349
Holdings Count | shares 247
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of September 30, 2024)
 
KEY FACTS 
  
  
Fund Size
$135,270,705,349
 
  
Number of Holdings
247
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 90.8   8-30 4.3   31-60 2.4   61-90 0.9   91-180 2.9         Repurchase Agreements 65.1 Commercial Paper 15.2 Non-Negotiable Time Deposit 11.5 Certificates of Deposit 7.8 U.S. Treasury Obligations 1.7 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (1.3)%
Fidelity Investments Money Market Government Portfolio - Class III  
Shareholder Report [Line Items]  
Fund Name Government Portfolio
Class Name Government Portfolio Class III
Trading Symbol FCGXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Government Portfolio for the period April 1, 2024 to September 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 1-877-297-2952 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-297-2952
Additional Information Email funddocuments@fmr.com
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
 
Class III 
$ 22 
0.43%
  
Expenses Paid, Amount $ 22
Expense Ratio, Percent 0.43%
Net Assets $ 225,755,117,004
Holdings Count | shares 518
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of September 30, 2024)
 
KEY FACTS 
  
  
Fund Size
$225,755,117,004
 
  
Number of Holdings
518
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 70.0   8-30 13.5   31-60 6.3   61-90 3.5   91-180 7.1   >180 1.1         Repurchase Agreements 50.6 U.S. Treasury Obligations 27.4 U.S. Government Agency - Debt 23.5 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (1.5)%
Fidelity Investments Money Market Treasury Only Portfolio - Class I  
Shareholder Report [Line Items]  
Fund Name Treasury Only Portfolio
Class Name Treasury Only Portfolio Class I
Trading Symbol FSIXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Treasury Only Portfolio for the period April 1, 2024 to September 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 1-877-297-2952 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-297-2952
Additional Information Email funddocuments@fmr.com
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
 
Class I 
$ 9 
0.18%
  
Expenses Paid, Amount $ 9
Expense Ratio, Percent 0.18%
Net Assets $ 87,593,462,799
Holdings Count | shares 43
Additional Fund Statistics [Text Block] Key Fund Statistics  
(as of September 30, 2024)
 
KEY FACTS 
  
  
Fund Size
$87,593,462,799
 
  
Number of Holdings
43
 
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 20.2   8-30 41.5   31-60 20.7   61-90 9.8   91-180 13.5   >180 0.8         U.S. Treasury Obligations 106.5 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (6.5)%