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Shareholder Report
 6 Months Ended
Oct. 31, 2024
USD ($)
shares
Shareholder Report [Line Items]  
Document Type N-CSRS
Amendment Flag false
Registrant Name Fidelity Hereford Street Trust
Entity Central Index Key 0000917286
Entity Investment Company Type N-1A
Document Period End Date Oct. 31, 2024
Fidelity Government Money Market Fund - Capital Reserves Class  
Shareholder Report [Line Items]  
Fund Name Fidelity® Government Money Market Fund
Class Name Fidelity® Government Money Market Fund Capital Reserves Class
Trading Symbol FZAXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity® Government Money Market Fund for the period May 1, 2024 to October 31, 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 1-877-208-0098 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-208-0098
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
 
Capital Reserves Class 
$ 48 
0.95%
  
Expenses Paid, Amount $ 48
Expense Ratio, Percent 0.95%
Net Assets $ 365,130,039,759
Holdings Count | shares 542
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of October 31, 2024)
 
KEY FACTS 
  
  
Fund Size
$365,130,039,759
  
Number of Holdings
542
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 57.0   8-30 9.4   31-60 11.8   61-90 5.9   91-180 16.1   >180 3.2         Repurchase Agreements 40.4 U.S. Treasury Obligations 37.1 U.S. Government Agency - Debt 25.9 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (3.4)%
Fidelity Money Market Fund  
Shareholder Report [Line Items]  
Fund Name Fidelity® Money Market Fund
Class Name Fidelity® Money Market Fund
Trading Symbol SPRXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity® Money Market Fund for the period May 1, 2024 to October 31, 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 or by sending an e-mail to fidfunddocuments@fidelity.com.
Additional Information Phone Number 1-800-544-8544
Additional Information Email fidfunddocuments@fidelity.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
 
Fidelity® Money Market Fund 
$ 21 
0.42%
  
Expenses Paid, Amount $ 21
Expense Ratio, Percent 0.42%
Net Assets $ 118,680,964,291
Holdings Count | shares 284
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of October 31, 2024)
 
KEY FACTS 
  
  
Fund Size
$118,680,964,291
  
Number of Holdings
284
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 77.8   8-30 6.1   31-60 6.8   61-90 8.8   91-180 3.2   >180 0.2         Repurchase Agreements 53.9 Commercial Paper 17.0 U.S. Treasury Obligations 13.5 Certificates of Deposit 9.8 Non-Negotiable Time Deposit 8.7 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (2.9)%
Fidelity Flex Government Money Market Fund  
Shareholder Report [Line Items]  
Fund Name Fidelity Flex® Government Money Market Fund
Class Name Fidelity Flex® Government Money Market Fund
Trading Symbol FLGXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity Flex® Government Money Market Fund for the period May 1, 2024 to October 31, 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 1-800-544-3455 or by sending an e-mail to fidfunddocuments@fidelity.com.
Additional Information Phone Number 1-800-544-3455
Additional Information Email fidfunddocuments@fidelity.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
 
Fidelity Flex® Government Money Market 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 $ 144,051,394
Holdings Count | shares 133
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of October 31, 2024)
 
KEY FACTS 
  
  
Fund Size
$144,051,394
  
Number of Holdings
133
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 50.7   8-30 9.1   31-60 15.7   61-90 7.1   91-180 12.6   >180 2.1         U.S. Treasury Obligations 42.0 Repurchase Agreements 28.1 U.S. Government Agency - Debt 27.2 Net Other Assets (Liabilities) 2.7 ASSET ALLOCATION (% of Fund's net assets)
Fidelity Government Money Market Fund - Class S  
Shareholder Report [Line Items]  
Fund Name Fidelity® Government Money Market Fund
Class Name Fidelity® Government Money Market Fund Class S
Trading Symbol FZSXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity® Government Money Market Fund for the period May 1, 2024 to October 31, 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 or by sending an e-mail to fidfunddocuments@fidelity.com.
Additional Information Phone Number 1-800-544-8544
Additional Information Email fidfunddocuments@fidelity.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 S 
$ 21 
0.42%
  
Expenses Paid, Amount $ 21
Expense Ratio, Percent 0.42%
Net Assets $ 365,130,039,759
Holdings Count | shares 542
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of October 31, 2024)
 
KEY FACTS 
  
  
Fund Size
$365,130,039,759
  
Number of Holdings
542
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 57.0   8-30 9.4   31-60 11.8   61-90 5.9   91-180 16.1   >180 3.2         Repurchase Agreements 40.4 U.S. Treasury Obligations 37.1 U.S. Government Agency - Debt 25.9 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (3.4)%
Fidelity Government Money Market Fund - Class K6  
Shareholder Report [Line Items]  
Fund Name Fidelity® Government Money Market Fund
Class Name Fidelity® Government Money Market Fund Class K6
Trading Symbol FNBXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity® Government Money Market Fund for the period May 1, 2024 to October 31, 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 1-800-835-5092 or by sending an e-mail to fidfunddocuments@fidelity.com.
Additional Information Phone Number 1-800-835-5092
Additional Information Email fidfunddocuments@fidelity.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 K6 
$ 13 
0.25%
  
Expenses Paid, Amount $ 13
Expense Ratio, Percent 0.25%
Net Assets $ 365,130,039,759
Holdings Count | shares 542
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of October 31, 2024)
 
KEY FACTS 
  
  
Fund Size
$365,130,039,759
  
Number of Holdings
542
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 57.0   8-30 9.4   31-60 11.8   61-90 5.9   91-180 16.1   >180 3.2         Repurchase Agreements 40.4 U.S. Treasury Obligations 37.1 U.S. Government Agency - Debt 25.9 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (3.4)%
Fidelity Series Treasury Bill Index Fund  
Shareholder Report [Line Items]  
Fund Name Fidelity® Series Treasury Bill Index Fund
Class Name Fidelity® Series Treasury Bill Index Fund
Trading Symbol FHQFX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity® Series Treasury Bill Index Fund for the period May 1, 2024 to October 31, 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
 
Fidelity® Series Treasury Bill Index 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 $ 1,431,850,404
Holdings Count | shares 18
Investment Company Portfolio Turnover 0.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of October 31, 2024)
 
KEY FACTS 
  
  
Fund Size
$1,431,850,404
  
Number of Holdings
18
  
Portfolio Turnover
0%A
  
A Amount represents less than 1%
Holdings [Text Block]   COUPON DISTRIBUTION (% of Fund's net assets) Zero coupon bonds 99.9     U.S. Treasury Obligations 99.9 Short-Term Investments and Net Other Assets (Liabilities) 0.1 ASSET ALLOCATION (% of Fund's net assets)
Largest Holdings [Text Block] TOP HOLDINGS (% of Fund's net assets)     US Treasury Bill 99.9     99.9  
Fidelity Treasury Only Money Market Fund  
Shareholder Report [Line Items]  
Fund Name Fidelity® Treasury Only Money Market Fund
Class Name Fidelity® Treasury Only Money Market Fund
Trading Symbol FDLXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity® Treasury Only Money Market Fund for the period May 1, 2024 to October 31, 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 or by sending an e-mail to fidfunddocuments@fidelity.com.
Additional Information Phone Number 1-800-544-8544
Additional Information Email fidfunddocuments@fidelity.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
 
Fidelity® Treasury Only Money Market Fund 
$ 21 
0.42%
  
Expenses Paid, Amount $ 21
Expense Ratio, Percent 0.42%
Net Assets $ 6,684,674,417
Holdings Count | shares 47
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of October 31, 2024)
 
KEY FACTS 
  
  
Fund Size
$6,684,674,417
  
Number of Holdings
47
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 14.6   8-30 31.3   31-60 23.5   61-90 10.5   91-180 18.7   >180 1.0         U.S. Treasury Obligations 99.6 Net Other Assets (Liabilities) 0.4 ASSET ALLOCATION (% of Fund's net assets)
Fidelity Government Money Market Fund - Daily Money Class  
Shareholder Report [Line Items]  
Fund Name Fidelity® Government Money Market Fund
Class Name Fidelity® Government Money Market Fund Daily Money Class
Trading Symbol FZBXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity® Government Money Market Fund for the period May 1, 2024 to October 31, 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 1-877-208-0098 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-208-0098
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
 
Daily Money Class 
$ 36 
0.70%
  
Expenses Paid, Amount $ 36
Expense Ratio, Percent 0.70%
Net Assets $ 365,130,039,759
Holdings Count | shares 542
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of October 31, 2024)
 
KEY FACTS 
  
  
Fund Size
$365,130,039,759
  
Number of Holdings
542
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 57.0   8-30 9.4   31-60 11.8   61-90 5.9   91-180 16.1   >180 3.2         Repurchase Agreements 40.4 U.S. Treasury Obligations 37.1 U.S. Government Agency - Debt 25.9 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (3.4)%
Fidelity Government Money Market Fund - Class M  
Shareholder Report [Line Items]  
Fund Name Fidelity® Government Money Market Fund
Class Name Fidelity® Government Money Market Fund Advisor M Class
Trading Symbol FZGXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity® Government Money Market Fund for the period May 1, 2024 to October 31, 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 1-877-208-0098 or by sending an e-mail to funddocuments@fmr.com.
Additional Information Phone Number 1-877-208-0098
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
 
Advisor M Class 
$ 36 
0.70%
  
Expenses Paid, Amount $ 36
Expense Ratio, Percent 0.70%
Net Assets $ 365,130,039,759
Holdings Count | shares 542
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of October 31, 2024)
 
KEY FACTS 
  
  
Fund Size
$365,130,039,759
  
Number of Holdings
542
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 57.0   8-30 9.4   31-60 11.8   61-90 5.9   91-180 16.1   >180 3.2         Repurchase Agreements 40.4 U.S. Treasury Obligations 37.1 U.S. Government Agency - Debt 25.9 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (3.4)%
Fidelity Government Money Market Fund  
Shareholder Report [Line Items]  
Fund Name Fidelity® Government Money Market Fund
Class Name Fidelity® Government Money Market Fund
Trading Symbol SPAXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity® Government Money Market Fund for the period May 1, 2024 to October 31, 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 or by sending an e-mail to fidfunddocuments@fidelity.com.
Additional Information Phone Number 1-800-544-8544
Additional Information Email fidfunddocuments@fidelity.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
 
Fidelity® Government Money Market Fund 
$ 21 
0.42%
  
Expenses Paid, Amount $ 21
Expense Ratio, Percent 0.42%
Net Assets $ 365,130,039,759
Holdings Count | shares 542
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of October 31, 2024)
 
KEY FACTS 
  
  
Fund Size
$365,130,039,759
  
Number of Holdings
542
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 57.0   8-30 9.4   31-60 11.8   61-90 5.9   91-180 16.1   >180 3.2         Repurchase Agreements 40.4 U.S. Treasury Obligations 37.1 U.S. Government Agency - Debt 25.9 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (3.4)%
Fidelity Government Money Market Fund - Premium Class  
Shareholder Report [Line Items]  
Fund Name Fidelity® Government Money Market Fund
Class Name Fidelity® Government Money Market Fund Premium Class
Trading Symbol FZCXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity® Government Money Market Fund for the period May 1, 2024 to October 31, 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 or by sending an e-mail to fidfunddocuments@fidelity.com.
Additional Information Phone Number 1-800-544-8544
Additional Information Email fidfunddocuments@fidelity.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
 
Premium Class 
$ 16 
0.32%
  
Expenses Paid, Amount $ 16
Expense Ratio, Percent 0.32%
Net Assets $ 365,130,039,759
Holdings Count | shares 542
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of October 31, 2024)
 
KEY FACTS 
  
  
Fund Size
$365,130,039,759
  
Number of Holdings
542
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 57.0   8-30 9.4   31-60 11.8   61-90 5.9   91-180 16.1   >180 3.2         Repurchase Agreements 40.4 U.S. Treasury Obligations 37.1 U.S. Government Agency - Debt 25.9 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (3.4)%
Fidelity Money Market Fund - Premium Class  
Shareholder Report [Line Items]  
Fund Name Fidelity® Money Market Fund
Class Name Fidelity® Money Market Fund Premium Class
Trading Symbol FZDXX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity® Money Market Fund for the period May 1, 2024 to October 31, 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 or by sending an e-mail to fidfunddocuments@fidelity.com.
Additional Information Phone Number 1-800-544-8544
Additional Information Email fidfunddocuments@fidelity.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
 
Premium Class 
$ 15 
0.30%
  
Expenses Paid, Amount $ 15
Expense Ratio, Percent 0.30%
Net Assets $ 118,680,964,291
Holdings Count | shares 284
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of October 31, 2024)
 
KEY FACTS 
  
  
Fund Size
$118,680,964,291
  
Number of Holdings
284
  
Holdings [Text Block]   EFFECTIVE MATURITY DIVERSIFICATION (% of Fund's net assets)   Days   1-7 77.8   8-30 6.1   31-60 6.8   61-90 8.8   91-180 3.2   >180 0.2         Repurchase Agreements 53.9 Commercial Paper 17.0 U.S. Treasury Obligations 13.5 Certificates of Deposit 9.8 Non-Negotiable Time Deposit 8.7 ASSET ALLOCATION (% of Fund's net assets) Net Other Assets (Liabilities) - (2.9)%