v3.25.1
Risk/Return Detail Data - FidelitySAILong-TermTreasuryBondIndexFund-PRO
Apr. 29, 2025
USD ($)
Risk/Return:  
Registrant Name Fidelity Salem Street Trust
FidelitySAILong-TermTreasuryBondIndexFund-PRO | Fidelity SAI Long-Term Treasury Bond Index Fund  
Risk/Return:  
Risk/Return [Heading] Fund Summary Fund: Fidelity® SAI Long-Term Treasury Bond Index Fund
Objective [Heading] Investment Objective
Objective, Primary [Text Block] Fidelity® SAI Long-Term Treasury Bond Index Fund seeks a high level of current income.
Expense [Heading] Fee Table
Expense Narrative [Text Block] The following table describes the fees and expenses that may be incurred when you buy, hold, and sell shares of the fund.
Shareholder Fees Caption [Text] Shareholder fees
(fees paid directly from your investment) none
Operating Expenses Caption [Text] Annual Operating Expenses
Fee Waiver or Reimbursement over Assets, Date of Termination Jun. 30, 2026
Portfolio Turnover [Heading] Portfolio Turnover
Portfolio Turnover [Text Block] The fund pays transaction costs, such as commissions, when it buys and sells securities (or "turns over" its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when fund shares are held in a taxable account. These costs, which are not reflected in annual operating expenses or in the example, affect the fund's performance. During the most recent fiscal year, the fund's portfolio turnover rate was 20 % of the average value of its portfolio.
Portfolio Turnover, Rate 20.00%
Expense Example Narrative [Text Block] 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] Principal Investment Strategies
Strategy Narrative [Text Block] Normally investing at least 80% of assets in securities included in the Bloomberg U.S. 20+ Year Treasury Bond Index. The index is a market value-weighted index of investment-grade fixed-rate public obligations of the U.S. Treasury with maturities equal to or greater than 20 years. Effective December 11, 2025, derivative instruments that provide investment exposure to the investments above or exposure to one or more market risk factors associated with such investments are included in the fund's 80% policy, consistent with the fund's investment policies and limitations with respect to investments in derivatives. Normally maintaining a dollar-weighted average maturity that is generally expected to be 20 years or more, consistent with that of the index. Using statistical sampling techniques based on duration, maturity, interest rate sensitivity, security structure, and credit quality to attempt to replicate the returns of the Bloomberg U.S. 20+ Year Treasury Bond Index using a smaller number of securities.
Risk [Heading] Principal Investment Risks
Bar Chart and Performance Table [Heading] Performance
Performance Narrative [Text Block] The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year and compares the performance of the fund's shares to the performance of a securities market index and an additional index over various periods of time. The additional index has characteristics relevant to the fund's investment strategies. Index descriptions appear in the "Additional Index Information" section of the prospectus. Past performance (before and after taxes) is not an indication of future performance. Visit www.fidelity.com for more recent performance information.
Performance Information Illustrates Variability of Returns [Text] The information illustrates the changes in the performance of the fund's shares from year to year and compares the performance of the fund's shares to the performance of a securities market index and an additional index over various periods of time.
Performance Availability Website Address [Text] www.fidelity.com
Performance Past Does Not Indicate Future [Text] Past performance (before and after taxes) is not an indication of future performance.
Bar Chart [Heading] Year-by-Year Returns
Year to Date Return, Label    Year-to-Date Return
Bar Chart, Year to Date Return, Date Mar. 31, 2025
Bar Chart, Year to Date Return 4.87%
Highest Quarterly Return, Label    Highest Quarter Return
Highest Quarterly Return, Date Mar. 31, 2020
Highest Quarterly Return 22.88%
Lowest Quarterly Return, Label    Lowest Quarter Return
Lowest Quarterly Return, Date Mar. 31, 2021
Lowest Quarterly Return (13.80%)
Performance Table Heading Average Annual Returns
Performance Table Uses Highest Federal Rate After-tax returns are calculated using the historical highest individual federal marginal income tax rates, but do not reflect the impact of state or local taxes.
Performance Table Not Relevant to Tax Deferred The after-tax returns shown are not relevant if you hold your shares in a retirement account or in another tax-deferred arrangement, such as an employee benefit plan (profit sharing, 401(k), or 403(b) plan).
Performance Table Explanation after Tax Higher Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.
Performance Table Narrative After-tax returns are calculated using the historical highest individual federal marginal income tax rates, but do not reflect the impact of state or local taxes. Actual after-tax returns may differ depending on your individual circumstances. The after-tax returns shown are not relevant if you hold your shares in a retirement account or in another tax-deferred arrangement, such as an employee benefit plan (profit sharing, 401(k), or 403(b) plan). Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.
FidelitySAILong-TermTreasuryBondIndexFund-PRO | Fidelity SAI Long-Term Treasury Bond Index Fund | Fidelity SAI Long-Term Treasury Bond Index Fund  
Risk/Return:  
Management fee 0.04%
Distribution and/or Service (12b-1) fees none
Other expenses none
Total annual operating expenses 0.04%
Fee waiver and/or expense reimbursement 0.01% [1]
Total annual operating expenses after fee waiver and/or expense reimbursement 0.03%
1 year $ 3
3 years 12
5 years 21
10 years $ 50
Annual Return, Inception Date Oct. 08, 2015
2016 1.23%
2017 9.04%
2018 (1.24%)
2019 14.35%
2020 18.34%
2021 (4.61%)
2022 (31.39%)
2023 2.69%
2024 (7.78%)
FidelitySAILong-TermTreasuryBondIndexFund-PRO | Fidelity SAI Long-Term Treasury Bond Index Fund | Return Before Taxes | Fidelity SAI Long-Term Treasury Bond Index Fund  
Risk/Return:  
Label Return Before Taxes
Past 1 year (7.78%)
Past 5 years (6.01%)
Since Inception (1.04%) [2]
FidelitySAILong-TermTreasuryBondIndexFund-PRO | Fidelity SAI Long-Term Treasury Bond Index Fund | After Taxes on Distributions | Fidelity SAI Long-Term Treasury Bond Index Fund  
Risk/Return:  
Label Return After Taxes on Distributions
Past 1 year (9.04%)
Past 5 years (7.35%)
Since Inception (2.33%) [2]
FidelitySAILong-TermTreasuryBondIndexFund-PRO | Fidelity SAI Long-Term Treasury Bond Index Fund | After Taxes on Distributions and Sales | Fidelity SAI Long-Term Treasury Bond Index Fund  
Risk/Return:  
Label Return After Taxes on Distributions and Sale of Fund Shares
Past 1 year (4.45%)
Past 5 years (4.77%)
Since Inception (1.17%) [2]
FidelitySAILong-TermTreasuryBondIndexFund-PRO | Fidelity SAI Long-Term Treasury Bond Index Fund | LB001  
Risk/Return:  
Label Bloomberg U.S. Aggregate Bond Index
Past 1 year 1.25%
Past 5 years (0.33%)
Since Inception 1.34%
FidelitySAILong-TermTreasuryBondIndexFund-PRO | Fidelity SAI Long-Term Treasury Bond Index Fund | LB145  
Risk/Return:  
Label Bloomberg U.S. 20+ Year Treasury Bond Index
Past 1 year (7.98%)
Past 5 years (5.97%)
Since Inception (0.98%)
[1]
A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse 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 0.03% (the Expense Cap). If at any time during the current fiscal year expenses for 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 June 30, 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.
[2]
A From October 8, 2015 .