v3.24.4
Label Element Value
Prospectus [Line Items] rr_ProspectusLineItems  
Document Type dei_DocumentType 485BPOS
Document Period End Date dei_DocumentPeriodEndDate Apr. 30, 2024
Entity Registrant Name dei_EntityRegistrantName Fidelity Hereford Street Trust
Entity Central Index Key dei_EntityCentralIndexKey 0000917286
Entity Inv Company Type dei_EntityInvCompanyType N-1A
Amendment Flag dei_AmendmentFlag false
Document Creation Date dei_DocumentCreationDate Dec. 20, 2024
Document Effective Date dei_DocumentEffectiveDate Dec. 20, 2024
Prospectus Date rr_ProspectusDate Dec. 20, 2024
(Fidelity Treasury Digital Fund)  
Prospectus [Line Items] rr_ProspectusLineItems  
Objective [Heading] rr_ObjectiveHeading Investment Objective
Objective, Primary [Text Block] rr_ObjectivePrimaryTextBlock
Fidelity® Treasury Digital Fund seeks to obtain as high a level of current income as is consistent with the preservation of capital and liquidity.
Expense [Heading] rr_ExpenseHeading Fee Table
Expense Narrative [Text Block] rr_ExpenseNarrativeTextBlock The following table describes the fees
and expenses that may be incurred when you buy, hold, and sell shares of the fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below.
Shareholder Fees Caption [Text] rr_ShareholderFeesCaption Shareholder fees(fees paid directly from your investment)
Operating Expenses Caption [Text] rr_OperatingExpensesCaption Annual Operating Expenses(expenses that you pay each year as a % of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination August 31, 2026
Other Expenses, New Fund, Based on Estimates [Text] rr_OtherExpensesNewFundBasedOnEstimates Based on estimated amounts for the current fiscal year.
Expense Example Narrative [Text Block] rr_ExpenseExampleNarrativeTextBlock This example helps compare the cost of investing in the fund with the cost of investing in other funds.
Let’s say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the
effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here’s how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:
Strategy [Heading] rr_StrategyHeading Principal Investment Strategies
Strategy Narrative [Text Block] rr_StrategyNarrativeTextBlock
• Normally investing at least 99.5% of total assets in cash and U.S. Treasury securities.
• Normally investing in securities whose interest is exempt from state and local income taxes.
• Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments.
In addition, the fund normally invests at least 80% of its assets in U.S. Treasury securities.
Treasury securities are high-quality securities issued or guaranteed by the U.S. Treasury. Treasury securities are backed by the full faith and credit of the U.S. Treasury. Treasury securities usually pay a fixed, variable, or floating rate of interest, and must repay the amount borrowed, usually at the maturity of the security.
Risk [Heading] rr_RiskHeading Principal Investment Risks
Bar Chart and Performance Table [Heading] rr_BarChartAndPerformanceTableHeading Performance
Performance Narrative [Text Block] rr_PerformanceNarrativeTextBlock
Performance history will be available for the fund after the fund has been in operation for one calendar year.
(Fidelity Treasury Digital Fund) | Fidelity Treasury Digital Fund  
Prospectus [Line Items] rr_ProspectusLineItems  
Shareholder Fee, Other rr_ShareholderFeeOther none
Management Fees (as a percentage of Assets) rr_ManagementFeesOverAssets 0.25%
Distribution and Service (12b-1) Fees rr_DistributionAndService12b1FeesOverAssets none
Other Expenses (as a percentage of Assets): rr_OtherExpensesOverAssets none [1]
Expenses (as a percentage of Assets) rr_ExpensesOverAssets 0.25%
Fee Waiver or Reimbursement rr_FeeWaiverOrReimbursementOverAssets 0.07% [2]
Net Expenses (as a percentage of Assets) rr_NetExpensesOverAssets 0.18%
Expense Example, with Redemption, 1 Year rr_ExpenseExampleYear01 $ 18
Expense Example, with Redemption, 3 Years rr_ExpenseExampleYear03 $ 68
(Fidelity Treasury Digital Fund) | Interest Rate Changes [Member]  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock
• Interest Rate Changes.
Interest rate increases can cause the price of a money market security to decrease.
(Fidelity Treasury Digital Fund) | Income Risk [Member]  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock
• Income Risk.
A low or negative interest rate environment can adversely affect the fund’s yield.
(Fidelity Treasury Digital Fund) | Issuer-Specific Changes [Member]  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock
• Issuer-Specific Changes.
A decline in the credit quality of an issuer can cause the price of a money market security to decrease.
(Fidelity Treasury Digital Fund) | U.S. Treasury Obligations [Member]  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock
• U.S. Treasury Obligations.
U.S. Treasury obligations are high-quality securities issued or guaranteed by the U.S. Treasury providing minimal risk of loss of principal if held to maturity. Fluctuations in interest rates may cause the market value of such securities to vary.
(Fidelity Treasury Digital Fund) | Risk Lose Money [Member]  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock You could lose money by investing in the fund.
(Fidelity Treasury Digital Fund) | Risk Not Insured Depository Institution [Member]  
Prospectus [Line Items] rr_ProspectusLineItems  
Risk [Text Block] rr_RiskTextBlock An investment in the fund is not a bank account and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
[1] Based on estimated amounts for the current fiscal year.
[2] Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse the class of shares of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund’s securities lending program, if applicable), as a percentage of its average net assets, exceed 0.18% (the Expense Cap). If at any time during the current fiscal year expenses for the class of shares of the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through August 31, 2026. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.