v3.26.1
Risk/Return Detail Data - FidelityMidCapGrowthMidCapValueSmallCapGrowthSmallCapValueIndexFunds-ComboPRO - USD ($)
 12 Months Ended 60 Months Ended 84 Months Ended
Aug. 29, 2026
Aug. 29, 2026
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Aug. 29, 2026
Aug. 29, 2026
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): 0.05%                  
Expenses (as a percentage of Assets) 0.25%                  
Fee Waiver or Reimbursement [1] (0.10%)                  
Net Expenses (as a percentage of Assets) 0.15%                  
Expense Example, with Redemption, 1 Year $ 15                  
Expense Example, with Redemption, 3 Years 67                  
Expense Example, with Redemption, 5 Years 127                  
Expense Example, with Redemption, 10 Years $ 305                  
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, 2027                  
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 23 % of the average value of its portfolio.                  
Portfolio Turnover, Rate 23.00%                  
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. 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. 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   8.62%             6.62% 10.87% [2]
Annual Return [Percent]     8.62% 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 61 % of the average value of its portfolio.                  
Portfolio Turnover, Rate 61.00%                  
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. 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. 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. Primarily using replication; however, the fund may or may not hold all of the securities in the 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, 2026                  
Bar Chart, Year to Date Return 7.24%                  
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   8.13%             6.01% 10.34% [2]
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   5.42%             5.01% 8.61% [2]
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   8.66%             6.65% 10.99%
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   17.15%             13.15% 14.68%
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   11.00%             9.78% 9.33% [3]
Annual Return [Percent]     11.00% 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 43 % of the average value of its portfolio.                  
Portfolio Turnover, Rate 43.00%                  
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. 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. 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. Primarily using replication; however, the fund may or may not hold all of the securities in the 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, 2026                  
Bar Chart, Year to Date Return 17.56%                  
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   10.25%             8.73% 8.44% [3]
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   6.87%             7.47% 7.22% [3]
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   17.15%             13.15% 14.68%
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   11.05%             9.83% 9.43%
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   13.03%             3.29% 8.45% [4]
Annual Return [Percent]     13.03% 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 63 % of the average value of its portfolio.                  
Portfolio Turnover, Rate 63.00%                  
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. 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. 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. Primarily using replication; however, the fund may or may not hold all of the securities in the 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, 2026                  
Bar Chart, Year to Date Return 22.25%                  
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   12.84%             2.88% 8.03% [4]
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   7.78%             2.42% 6.60% [4]
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   17.15%             13.15% 14.68%
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   13.01%             3.18% 8.38%
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   12.70%             8.93% 8.94% [5]
Annual Return [Percent]     12.70% 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 41 % of the average value of its portfolio.                  
Portfolio Turnover, Rate 41.00%                  
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. 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. 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. Primarily using replication; however, the fund may or may not hold all of the securities in the 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, 2026                  
Bar Chart, Year to Date Return 22.91%                  
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   11.95%             7.72% 7.91% [5]
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   7.77%             6.67% 6.81% [5]
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   17.15%             13.15% 14.68%
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   12.59%             8.88% 8.94%
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 0.15% (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, 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 July 11, 2019 .
[3]
A From July 11, 2019 .
[4]
A From July 11, 2019 .
[5]
A From July 11, 2019 .