v3.25.2
Risk/Return Detail Data - FidelityMidCapGrowthMidCapValueSmallCapGrowthSmallCapValueIndexFunds-ComboPRO - USD ($)
 12 Months Ended 60 Months Ended 72 Months Ended
Aug. 29, 2025
Aug. 29, 2025
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Aug. 29, 2025
Aug. 29, 2025
FidelitySAIMunicipalBondIndexFund-PRO | Fidelity SAI Municipal Bond Index Fund | Fidelity SAI Municipal Bond Index Fund                  
Prospectus Line Items                  
Management Fees (as a percentage of Assets) 0.05%                
Distribution and Service (12b-1) Fees 0.00%                
Other Expenses (as a percentage of Assets): 0.13%                
Expenses (as a percentage of Assets) 0.18%                
Fee Waiver or Reimbursement [1] (0.11%)                
Net Expenses (as a percentage of Assets) 0.07%                
Expense Example, with Redemption, 1 Year $ 7                
Expense Example, with Redemption, 3 Years 43                
Expense Example, with Redemption, 5 Years 86                
Expense Example, with Redemption, 10 Years $ 215                
Annual Return, Inception Date Jul. 11, 2019                
Average Annual Return, Label [Optional Text]   Return Before Taxes              
Average Annual Return, Percent   1.26%           0.44% 0.60% [2]
Annual Return [Percent]     1.26% 5.82% (9.90%) 1.35% 4.45%    
FidelitySAIMunicipalBondIndexFund-PRO | Fidelity SAI Municipal Bond Index Fund                  
Prospectus Line Items                  
Risk/Return [Heading] Fund Summary Fund: Fidelity® SAI Municipal Bond Index Fund                
Objective [Heading] Investment Objective                
Objective, Primary [Text Block] Fidelity® SAI Municipal Bond Index Fund seeks to provide a high current yield exempt from federal income tax.                
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 Oct. 31, 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 25 % of the average value of its portfolio.                
Portfolio Turnover, Rate 25.00%                
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 municipal securities whose interest is exempt from federal income tax (but may be subject to the federal alternative minimum tax). Normally investing at least 80% of assets in securities included in the Bloomberg Municipal Bond Index . The Bloomberg Municipal Bond Index is a market value-weighted index of investment-grade municipal bonds with maturities of one year or more. 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.   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 Municipal Bond Index using a smaller number of securities.                
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 over various periods of time. The index description appears 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 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 [Optional Text]    Year-to-Date Return                
Bar Chart, Year to Date Return, Date Jun. 30, 2025                
Bar Chart, Year to Date Return (0.67%)                
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                
Highest Quarterly Return, Date Dec. 31, 2023                
Highest Quarterly Return 7.64%                
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                
Lowest Quarterly Return, Date Mar. 31, 2022                
Lowest Quarterly Return (6.58%)                
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 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. 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.                
FidelitySAIMunicipalBondIndexFund-PRO | Fidelity SAI Municipal Bond Index Fund | After Taxes on Distributions | Fidelity SAI Municipal Bond Index Fund                  
Prospectus Line Items                  
Average Annual Return, Label [Optional Text]   Return After Taxes on Distributions              
Average Annual Return, Percent   1.26%           0.44% 0.60% [2]
FidelitySAIMunicipalBondIndexFund-PRO | Fidelity SAI Municipal Bond Index Fund | After Taxes on Distributions and Sales | Fidelity SAI Municipal Bond Index Fund                  
Prospectus Line Items                  
Average Annual Return, Label [Optional Text]   Return After Taxes on Distributions and Sale of Fund Shares              
Average Annual Return, Percent   1.88%           0.83% 0.95% [2]
FidelitySAIMunicipalBondIndexFund-PRO | Fidelity SAI Municipal Bond Index Fund | LB015                  
Prospectus Line Items                  
Average Annual Return, Label [Optional Text]   Bloomberg Municipal Bond Index              
Average Annual Return, Percent   1.05%           0.99% 1.26%
FidelitySAIEnhancedMunicipalIncomeFund-PRO | Fidelity SAI Enhanced Municipal Income Fund | Fidelity SAI Enhanced Municipal Income Fund                  
Prospectus Line Items                  
Management Fees (as a percentage of Assets) 0.20%                
Distribution and Service (12b-1) Fees 0.00%                
Other Expenses (as a percentage of Assets): [3] 0.10%                
Expenses (as a percentage of Assets) 0.30%                
Fee Waiver or Reimbursement [4] (0.15%)                
Net Expenses (as a percentage of Assets) 0.15%                
Expense Example, with Redemption, 1 Year $ 15                
Expense Example, with Redemption, 3 Years $ 76                
FidelitySAIEnhancedMunicipalIncomeFund-PRO | Fidelity SAI Enhanced Municipal Income Fund                  
Prospectus Line Items                  
Risk/Return [Heading] Fund Summary Fund: Fidelity® SAI Enhanced Municipal Income Fund                
Objective [Heading] Investment Objective                
Objective, Primary [Text Block] Fidelity® SAI Enhanced Municipal Income Fund seeks to provide a high current yield exempt from federal income tax.                
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 Oct. 31, 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. For the period from March 18, 2025 to June 30, 2025, the fund's portfolio turnover rate was 21 % of the average value of its portfolio.                
Portfolio Turnover, Rate 21.00%                
Other Expenses, New Fund, Based on Estimates [Text] Based on estimated amounts for the current fiscal year.                
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 municipal securities whose interest is exempt from federal income tax. The municipal securities in which the fund invests are normally investment-grade (those of medium and high quality). Municipal debt 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. Municipal debt securities in which the fund invests include securities issued by U.S. territories and possessions, general obligation bonds of municipalities, local or state governments, project or revenue-specific bonds, or pre-refunded or escrowed bonds, municipal money market securities, and synthetic securities. Municipal securities usually pay a fixed, variable, or floating rate of interest, and must repay the amount borrowed, usually at the maturity of the security. Investment grade securities in which the fund invests include securities or issuers rated at least BBB- or its equivalent by at least one nationally recognized statistical ratings organization (NRSRO), or, if unrated, determined by the Adviser to be of comparable quality. 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. Using the Bloomberg Municipal Bond Index as a guide in structuring the fund and selecting its investments as it relates to credit quality distribution and risk characteristics. Potentially investing more than 25% of total assets in municipal securities that finance similar projects. Generally utilizing a research-driven approach identifying long-term drivers of municipal debt security returns that may include, but are not limited to, valuation, quality, momentum, and other factors. The research is systematically applied alongside a proprietary portfolio construction and risk management framework to select a broadly diversified group of debt securities that may have the potential to provide a higher total return than that of the Bloomberg Municipal Bond Index.                
Bar Chart and Performance Table [Heading] Performance                
Performance Narrative [Text Block] Performance history will be available for the fund after the fund has been in operation for one calendar year.                
Performance One Year or Less [Text] Performance history will be available for the fund after the fund has been in operation for one calendar year.                
FidelityMidCapGrowthMidCapValueSmallCapGrowthSmallCapValueIndexFunds-ComboPRO | Fidelity Mid Cap Growth Index Fund | Fidelity Mid Cap Growth Index Fund                  
Prospectus Line Items                  
Management Fees (as a percentage of Assets) 0.05%                
Distribution and Service (12b-1) Fees 0.00%                
Other Expenses (as a percentage of Assets): 0.00%                
Expenses (as a percentage of Assets) 0.05%                
Expense Example, with Redemption, 1 Year $ 5                
Expense Example, with Redemption, 3 Years 16                
Expense Example, with Redemption, 5 Years 28                
Expense Example, with Redemption, 10 Years $ 64                
Annual Return, Inception Date Jul. 11, 2019                
Average Annual Return, Label [Optional Text]   Return Before Taxes              
Average Annual Return, Percent   22.02%           11.34% 11.28% [5]
Annual Return [Percent]     22.02% 25.79% (26.67%) 12.71% 34.84%    
FidelityMidCapGrowthMidCapValueSmallCapGrowthSmallCapValueIndexFunds-ComboPRO | Fidelity Mid Cap Growth Index Fund                  
Prospectus Line Items                  
Risk/Return [Heading] Fund Summary Fund: Fidelity® Mid Cap Growth Index Fund                
Objective [Heading] Investment Objective                
Objective, Primary [Text Block] Fidelity® Mid Cap Growth Index Fund seeks to provide investment results that correspond to the total return of stocks of mid-capitalization U.S. companies.                
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. You may pay other fees, such as brokerage commissions , which are not reflected in the tables and examples below.                
Shareholder Fees Caption [Optional Text] Shareholder fees                
Shareholder Fee, Other $ 0                
Operating Expenses Caption [Optional Text] Annual Operating Expenses                
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 53 % of the average value of its portfolio.                
Portfolio Turnover, Rate 53.00%                
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 Russell Midcap® Growth Index. The Russell Midcap ® Growth Index is a market capitalization-weighted index designed to measure the performance of the mid-cap growth segment of the U.S. equity market. It includes those Russell MidCap ® Index companies with higher price-to-book ratios and higher forecasted growth values. Effective March 24, 2025, the Russell Midcap ® Growth Index applies a capping methodology. Index constituents are capped quarterly so that no more than 22.5% of the index weight may be allocated to a single constituent, and the sum of the weights of all constituents representing more than 4.5% of the index should not exceed 45% of the total index weight. For periods prior to March 24, 2025, the index was uncapped. 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.   Using statistical sampling techniques based on such factors as capitalization, industry exposures, dividend yield, price-to-earnings (P/E) ratio, price-to-book (P/B) ratio, and earnings growth to attempt to replicate the returns of the Russell Midcap® Growth Index. Lending securities to earn income for the fund.                
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 [Optional Text]    Year-to-Date Return                
Bar Chart, Year to Date Return, Date Jun. 30, 2025                
Bar Chart, Year to Date Return 9.73%                
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                
Highest Quarterly Return, Date Jun. 30, 2020                
Highest Quarterly Return 30.08%                
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                
Lowest Quarterly Return, Date Jun. 30, 2022                
Lowest Quarterly Return (21.04%)                
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.                
FidelityMidCapGrowthMidCapValueSmallCapGrowthSmallCapValueIndexFunds-ComboPRO | Fidelity Mid Cap Growth Index Fund | After Taxes on Distributions | Fidelity Mid Cap Growth Index Fund                  
Prospectus Line Items                  
Average Annual Return, Label [Optional Text]   Return After Taxes on Distributions              
Average Annual Return, Percent   21.88%           10.77% 10.75% [5]
FidelityMidCapGrowthMidCapValueSmallCapGrowthSmallCapValueIndexFunds-ComboPRO | Fidelity Mid Cap Growth Index Fund | After Taxes on Distributions and Sales | Fidelity Mid Cap Growth Index Fund                  
Prospectus Line Items                  
Average Annual Return, Label [Optional Text]   Return After Taxes on Distributions and Sale of Fund Shares              
Average Annual Return, Percent   13.13%           8.86% 8.87% [5]
FidelityMidCapGrowthMidCapValueSmallCapGrowthSmallCapValueIndexFunds-ComboPRO | Fidelity Mid Cap Growth Index Fund | RS012                  
Prospectus Line Items                  
Average Annual Return, Label [Optional Text]   Russell Midcap® Growth Index              
Average Annual Return, Percent   22.10%           11.47% 11.43%
FidelityMidCapGrowthMidCapValueSmallCapGrowthSmallCapValueIndexFunds-ComboPRO | Fidelity Mid Cap Growth Index Fund | RS003                  
Prospectus Line Items                  
Average Annual Return, Label [Optional Text]   Russell 3000® Index              
Average Annual Return, Percent   23.81%           13.86% 14.24%
FidelityMidCapGrowthMidCapValueSmallCapGrowthSmallCapValueIndexFunds-ComboPRO | Fidelity Mid Cap Value Index Fund | Fidelity Mid Cap Value Index Fund                  
Prospectus Line Items                  
Management Fees (as a percentage of Assets) 0.05%                
Distribution and Service (12b-1) Fees 0.00%                
Other Expenses (as a percentage of Assets): 0.00%                
Expenses (as a percentage of Assets) 0.05%                
Expense Example, with Redemption, 1 Year $ 5                
Expense Example, with Redemption, 3 Years 16                
Expense Example, with Redemption, 5 Years 28                
Expense Example, with Redemption, 10 Years $ 64                
Annual Return, Inception Date Jul. 11, 2019                
Average Annual Return, Label [Optional Text]   Return Before Taxes              
Average Annual Return, Percent   13.00%           8.51% 9.03% [6]
Annual Return [Percent]     13.00% 12.76% (12.07%) 28.19% 4.74%    
FidelityMidCapGrowthMidCapValueSmallCapGrowthSmallCapValueIndexFunds-ComboPRO | Fidelity Mid Cap Value Index Fund                  
Prospectus Line Items                  
Risk/Return [Heading] Fund Summary Fund: Fidelity® Mid Cap Value Index Fund                
Objective [Heading] Investment Objective                
Objective, Primary [Text Block] Fidelity® Mid Cap Value Index Fund seeks to provide investment results that correspond to the total return of stocks of mid-capitalization U.S. companies.                
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. You may pay other fees, such as brokerage commissions , which are not reflected in the tables and examples below.                
Shareholder Fees Caption [Optional Text] Shareholder fees                
Shareholder Fee, Other $ 0                
Operating Expenses Caption [Optional Text] Annual Operating Expenses                
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 32 % of the average value of its portfolio.                
Portfolio Turnover, Rate 32.00%                
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 Russell Midcap® Value Index. The Russell Midcap ® Value Index is a market capitalization-weighted index designed to measure the performance of the mid-cap value segment of the U.S. equity market. It includes those Russell MidCap ® Index companies with lower price-to-book ratios and lower forecasted growth values. Effective March 24, 2025, the Russell Midcap ® Value Index applies a capping methodology. Index constituents are capped quarterly so that no more than 22.5% of the index weight may be allocated to a single constituent, and the sum of the weights of all constituents representing more than 4.5% of the index should not exceed 45% of the total index weight. For periods prior to March 24, 2025, the index was uncapped. 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.   Using statistical sampling techniques based on such factors as capitalization, industry exposures, dividend yield, price-to-earnings (P/E) ratio, price-to-book (P/B) ratio, and earnings growth to attempt to replicate the returns of the Russell Midcap® Value Index. Lending securities to earn income for the fund.                
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 [Optional Text]    Year-to-Date Return                
Bar Chart, Year to Date Return, Date Jun. 30, 2025                
Bar Chart, Year to Date Return 3.09%                
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                
Highest Quarterly Return, Date Dec. 31, 2020                
Highest Quarterly Return 20.43%                
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                
Lowest Quarterly Return, Date Mar. 31, 2020                
Lowest Quarterly Return (31.67%)                
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.                
FidelityMidCapGrowthMidCapValueSmallCapGrowthSmallCapValueIndexFunds-ComboPRO | Fidelity Mid Cap Value Index Fund | After Taxes on Distributions | Fidelity Mid Cap Value Index Fund                  
Prospectus Line Items                  
Average Annual Return, Label [Optional Text]   Return After Taxes on Distributions              
Average Annual Return, Percent   11.74%           7.55% 8.12% [6]
FidelityMidCapGrowthMidCapValueSmallCapGrowthSmallCapValueIndexFunds-ComboPRO | Fidelity Mid Cap Value Index Fund | After Taxes on Distributions and Sales | Fidelity Mid Cap Value Index Fund                  
Prospectus Line Items                  
Average Annual Return, Label [Optional Text]   Return After Taxes on Distributions and Sale of Fund Shares              
Average Annual Return, Percent   8.42%           6.48% 6.94% [6]
FidelityMidCapGrowthMidCapValueSmallCapGrowthSmallCapValueIndexFunds-ComboPRO | Fidelity Mid Cap Value Index Fund | RS003                  
Prospectus Line Items                  
Average Annual Return, Label [Optional Text]   Russell 3000® Index              
Average Annual Return, Percent   23.81%           13.86% 14.24%
FidelityMidCapGrowthMidCapValueSmallCapGrowthSmallCapValueIndexFunds-ComboPRO | Fidelity Mid Cap Value Index Fund | RS013                  
Prospectus Line Items                  
Average Annual Return, Label [Optional Text]   Russell Midcap® Value Index              
Average Annual Return, Percent   13.07%           8.59% 9.13%
FidelityMidCapGrowthMidCapValueSmallCapGrowthSmallCapValueIndexFunds-ComboPRO | Fidelity Small Cap Growth Index Fund | Fidelity Small Cap Growth Index Fund                  
Prospectus Line Items                  
Management Fees (as a percentage of Assets) 0.05%                
Distribution and Service (12b-1) Fees 0.00%                
Other Expenses (as a percentage of Assets): 0.00%                
Expenses (as a percentage of Assets) 0.05%                
Expense Example, with Redemption, 1 Year $ 5                
Expense Example, with Redemption, 3 Years 16                
Expense Example, with Redemption, 5 Years 28                
Expense Example, with Redemption, 10 Years $ 64                
Annual Return, Inception Date Jul. 11, 2019                
Average Annual Return, Label [Optional Text]   Return Before Taxes              
Average Annual Return, Percent   15.25%           6.93% 7.63% [7]
Annual Return [Percent]     15.25% 18.90% (26.17%) 2.82% 34.40%    
FidelityMidCapGrowthMidCapValueSmallCapGrowthSmallCapValueIndexFunds-ComboPRO | Fidelity Small Cap Growth Index Fund                  
Prospectus Line Items                  
Risk/Return [Heading] Fund Summary Fund: Fidelity® Small Cap Growth Index Fund                
Objective [Heading] Investment Objective                
Objective, Primary [Text Block] Fidelity® Small Cap Growth Index Fund seeks to provide investment results that correspond to the total return of stocks of small-capitalization U.S. companies.                
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. You may pay other fees, such as brokerage commissions , which are not reflected in the tables and examples below.                
Shareholder Fees Caption [Optional Text] Shareholder fees                
Shareholder Fee, Other $ 0                
Operating Expenses Caption [Optional Text] Annual Operating Expenses                
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 36 % of the average value of its portfolio.                
Portfolio Turnover, Rate 36.00%                
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 Russell 2000® Growth Index. The Russell 2000 ® Growth Index  is a market capitalization-weighted index designed to measure the performance of the small-cap growth segment of the U.S. equity market. It includes those Russell 2000 ® Index companies with higher price-to-book ratios and higher forecasted growth rates. Effective March 24, 2025, the Russell 2000 ® Growth Index applies a capping methodology. Index constituents are capped quarterly so that no more than 22.5% of the index weight may be allocated to a single constituent, and the sum of the weights of all constituents representing more than 4.5% of the index should not exceed 45% of the total index weight. For periods prior to March 24, 2025, the index was uncapped. 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.   Using statistical sampling techniques based on such factors as capitalization, industry exposures, dividend yield, price-to-earnings (P/E) ratio, price-to-book (P/B) ratio, and earnings growth to attempt to replicate the returns of the Russell 2000® Growth Index. Lending securities to earn income for the fund.                
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 [Optional Text]    Year-to-Date Return                
Bar Chart, Year to Date Return, Date Jun. 30, 2025                
Bar Chart, Year to Date Return (0.47%)                
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                
Highest Quarterly Return, Date Jun. 30, 2020                
Highest Quarterly Return 30.43%                
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                
Lowest Quarterly Return, Date Mar. 31, 2020                
Lowest Quarterly Return (25.77%)                
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.                
FidelityMidCapGrowthMidCapValueSmallCapGrowthSmallCapValueIndexFunds-ComboPRO | Fidelity Small Cap Growth Index Fund | After Taxes on Distributions | Fidelity Small Cap Growth Index Fund                  
Prospectus Line Items                  
Average Annual Return, Label [Optional Text]   Return After Taxes on Distributions              
Average Annual Return, Percent   14.74%           6.45% 7.17% [7]
FidelityMidCapGrowthMidCapValueSmallCapGrowthSmallCapValueIndexFunds-ComboPRO | Fidelity Small Cap Growth Index Fund | After Taxes on Distributions and Sales | Fidelity Small Cap Growth Index Fund                  
Prospectus Line Items                  
Average Annual Return, Label [Optional Text]   Return After Taxes on Distributions and Sale of Fund Shares              
Average Annual Return, Percent   9.11%           5.30% 5.89% [7]
FidelityMidCapGrowthMidCapValueSmallCapGrowthSmallCapValueIndexFunds-ComboPRO | Fidelity Small Cap Growth Index Fund | RS003                  
Prospectus Line Items                  
Average Annual Return, Label [Optional Text]   Russell 3000® Index              
Average Annual Return, Percent   23.81%           13.86% 14.24%
FidelityMidCapGrowthMidCapValueSmallCapGrowthSmallCapValueIndexFunds-ComboPRO | Fidelity Small Cap Growth Index Fund | RS014                  
Prospectus Line Items                  
Average Annual Return, Label [Optional Text]   Russell 2000® Growth Index              
Average Annual Return, Percent   15.15%           6.86% 7.55%
FidelityMidCapGrowthMidCapValueSmallCapGrowthSmallCapValueIndexFunds-ComboPRO | Fidelity Small Cap Value Index Fund | Fidelity Small Cap Value Index Fund                  
Prospectus Line Items                  
Management Fees (as a percentage of Assets) 0.05%                
Distribution and Service (12b-1) Fees 0.00%                
Other Expenses (as a percentage of Assets): 0.00%                
Expenses (as a percentage of Assets) 0.05%                
Expense Example, with Redemption, 1 Year $ 5                
Expense Example, with Redemption, 3 Years 16                
Expense Example, with Redemption, 5 Years 28                
Expense Example, with Redemption, 10 Years $ 64                
Annual Return, Inception Date Jul. 11, 2019                
Average Annual Return, Label [Optional Text]   Return Before Taxes              
Average Annual Return, Percent   8.16%           7.29% 8.27% [8]
Annual Return [Percent]     8.16% 14.73% (14.43%) 28.14% 4.48%    
FidelityMidCapGrowthMidCapValueSmallCapGrowthSmallCapValueIndexFunds-ComboPRO | Fidelity Small Cap Value Index Fund                  
Prospectus Line Items                  
Risk/Return [Heading] Fund Summary Fund: Fidelity® Small Cap Value Index Fund                
Objective [Heading] Investment Objective                
Objective, Primary [Text Block] Fidelity® Small Cap Value Index Fund seeks to provide investment results that correspond to the total return of stocks of small-capitalization U.S. companies.                
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. You may pay other fees, such as brokerage commissions , which are not reflected in the tables and examples below.                
Shareholder Fees Caption [Optional Text] Shareholder fees                
Shareholder Fee, Other $ 0                
Operating Expenses Caption [Optional Text] Annual Operating Expenses                
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 31 % of the average value of its portfolio.                
Portfolio Turnover, Rate 31.00%                
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 Russell 2000® Value Index. The Russell 2000 ® Value Index is a market capitalization-weighted index designed to measure the performance of the small-cap value segment of the U.S. equity market. It includes those Russell 2000 ® Index companies with lower price-to-book ratios and lower forecasted growth rates. Effective March 24, 2025, the Russell 2000 ® Value Index applies a capping methodology. Index constituents are capped quarterly so that no more than 22.5% of the index weight may be allocated to a single constituent, and the sum of the weights of all constituents representing more than 4.5% of the index should not exceed 45% of the total index weight. For periods prior to March 24, 2025, the index was uncapped. 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.   Using statistical sampling techniques based on such factors as capitalization, industry exposures, dividend yield, price-to-earnings (P/E) ratio, price-to-book (P/B) ratio, and earnings growth to attempt to replicate the returns of the Russell 2000® Value Index. Lending securities to earn income for the fund.                
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 [Optional Text]    Year-to-Date Return                
Bar Chart, Year to Date Return, Date Jun. 30, 2025                
Bar Chart, Year to Date Return (3.09%)                
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                
Highest Quarterly Return, Date Dec. 31, 2020                
Highest Quarterly Return 33.20%                
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                
Lowest Quarterly Return, Date Mar. 31, 2020                
Lowest Quarterly Return (35.72%)                
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.                
FidelityMidCapGrowthMidCapValueSmallCapGrowthSmallCapValueIndexFunds-ComboPRO | Fidelity Small Cap Value Index Fund | After Taxes on Distributions | Fidelity Small Cap Value Index Fund                  
Prospectus Line Items                  
Average Annual Return, Label [Optional Text]   Return After Taxes on Distributions              
Average Annual Return, Percent   7.68%           6.15% 7.19% [8]
FidelityMidCapGrowthMidCapValueSmallCapGrowthSmallCapValueIndexFunds-ComboPRO | Fidelity Small Cap Value Index Fund | After Taxes on Distributions and Sales | Fidelity Small Cap Value Index Fund                  
Prospectus Line Items                  
Average Annual Return, Label [Optional Text]   Return After Taxes on Distributions and Sale of Fund Shares              
Average Annual Return, Percent   5.10%           5.41% 6.24% [8]
FidelityMidCapGrowthMidCapValueSmallCapGrowthSmallCapValueIndexFunds-ComboPRO | Fidelity Small Cap Value Index Fund | RS003                  
Prospectus Line Items                  
Average Annual Return, Label [Optional Text]   Russell 3000® Index              
Average Annual Return, Percent   23.81%           13.86% 14.24%
FidelityMidCapGrowthMidCapValueSmallCapGrowthSmallCapValueIndexFunds-ComboPRO | Fidelity Small Cap Value Index Fund | RS015                  
Prospectus Line Items                  
Average Annual Return, Label [Optional Text]   Russell 2000® Value Index              
Average Annual Return, Percent   8.05%           7.29% 8.29%
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 October 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.
[2]
A From July 11, 2019 .
[3]
A Based on estimated amounts for the current fiscal year.
[4]
B 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 0.15% (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 October 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.
[5]
A From July 11, 2019 .
[6]
A From July 11, 2019 .
[7]
A From July 11, 2019 .
[8]
A From July 11, 2019 .