| 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.
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| 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.
|
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| 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:
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| 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.
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| 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.
|
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| (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.
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| (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.
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| (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.
|
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| (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.
|
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| (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. | ||||
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