v3.26.1
Risk/Return Detail Data - FidelitySAIMunicipalMoneyMarketFund-PRO - USD ($)
 12 Months Ended 60 Months Ended 96 Months Ended
May 30, 2026
May 30, 2026
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
May 30, 2026
May 30, 2026
 [2]
FidelitySAIMunicipalMoneyMarketFund-PRO | Fidelity SAI Municipal Money Market Fund | Fidelity SAI Municipal Money Market Fund                      
Prospectus Line Items                      
Management Fees (as a percentage of Assets) 0.14%                    
Distribution and Service (12b-1) Fees 0.00%                    
Other Expenses (as a percentage of Assets): 0.15%                    
Expenses (as a percentage of Assets) 0.29%                    
Fee Waiver or Reimbursement [1] (0.22%)                    
Net Expenses (as a percentage of Assets) 0.07%                    
Expense Example, with Redemption, 1 Year $ 7                    
Expense Example, with Redemption, 3 Years 63                    
Expense Example, with Redemption, 5 Years 133                    
Expense Example, with Redemption, 10 Years $ 339                    
Annual Return, Inception Date Jan. 11, 2018                    
Average Annual Return, Label [Optional Text]   Fidelity® SAI Municipal Money Market Fund                  
Average Annual Return, Percent   2.66%               2.10% 1.73%
Annual Return [Percent]     2.66% 3.38% 3.40% 1.10% 0.02% 0.53% 1.44%    
FidelitySAIMunicipalMoneyMarketFund-PRO | Fidelity SAI Municipal Money Market Fund                      
Prospectus Line Items                      
Risk/Return [Heading] Fund Summary Fund: Fidelity® SAI Municipal Money Market Fund                    
Objective [Heading] Investment Objective                    
Objective, Primary [Text Block] Fidelity® SAI Municipal Money Market Fund seeks as high a level of interest income exempt from federal income tax as is consistent with liquidity and stability of principal.                    
Expense Heading [Optional Text] 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 [Optional Text] Shareholder fees                    
Shareholder Fee, Other $ 0                    
Operating Expenses Caption [Optional Text] Annual Operating Expenses                    
Fee Waiver or Reimbursement over Assets, Date of Termination Jul. 31, 2027                    
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 in municipal money market securities. Normally investing at least 80% of assets in municipal securities whose interest is exempt from federal income tax (but may be subject to the federal alternative minimum tax). Municipal money market securities are high-quality, short-term debt securities that pay a fixed, variable, or floating interest rate, and include variable rate demand notes, commercial paper, and municipal notes. Municipal securities are issued to raise money for a variety of public and private purposes, including general financing for state and local governments, financing for eligible non-profit organizations, or financing for a specific project or public facility. Potentially investing up to 20% of assets in securities subject to state and/or federal income tax. Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects. Investing in compliance with industry-standard regulatory requirements for money market funds for the quality, maturity, liquidity, and diversification of investments. Investing in Fidelity's Central funds (specialized investment vehicles used by Fidelity® funds to invest in particular security types or investment disciplines). Although the fund is a money market fund, it will price and transact at a "floating" net asset value per share (NAV) that will fluctuate based on changes in the value of the fund's investments.                    
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.   Past performance 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.                    
Performance Availability Website Address [Text] www.fidelity.com                    
Performance Past Does Not Indicate Future [Text] Past performance is not an indication of future performance.                    
Bar Chart [Heading] Year-by-Year Returns                    
Year to Date Return, Label [Optional Text]    Year-to-Date Return                    
Bar Chart, Year to Date Return, Date Mar. 31, 2026                    
Bar Chart, Year to Date Return 0.46%                    
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                    
Highest Quarterly Return, Date Jun. 30, 2024                    
Highest Quarterly Return 0.95%                    
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                    
Lowest Quarterly Return, Date Mar. 31, 2022                    
Lowest Quarterly Return (0.02%)                    
Performance Table Heading Average Annual Returns                    
Document Type 485BPOS                    
Registrant Name Fidelity Salem Street Trust                    
[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 0.07% (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 July 31, 2027 . 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 January 11, 2018 .