v3.26.1
Risk/Return Detail Data - FidelityFlexMidCapIndexFundFidelityFlexSmallCapIndexFund-ComboPRO - USD ($)
 12 Months Ended 60 Months Ended 96 Months Ended 108 Months Ended 120 Months Ended
Jun. 29, 2026
Jun. 29, 2026
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Jun. 29, 2026
Jun. 29, 2026
Jun. 29, 2026
Jun. 29, 2026
FidelityLargeCapValueIndexFund-PRO | Fidelity Large Cap Value Index Fund | Fidelity Large Cap Value Index Fund                                
Prospectus Line Items                                
Management Fees (as a percentage of Assets) 0.035%                              
Distribution and Service (12b-1) Fees 0.00%                              
Other Expenses (as a percentage of Assets): 0.00%                              
Expenses (as a percentage of Assets) 0.035%                              
Expense Example, with Redemption, 1 Year $ 4                              
Expense Example, with Redemption, 3 Years 11                              
Expense Example, with Redemption, 5 Years 20                              
Expense Example, with Redemption, 10 Years $ 45                              
Annual Return, Inception Date Jun. 07, 2016                              
Average Annual Return, Label [Optional Text]   Return Before Taxes                            
Average Annual Return, Percent   15.89%                     11.31%     10.29% [1]
Annual Return [Percent]     15.89% 14.35% 11.50% (7.57%) 25.11% 2.86% 26.51% (8.26%) 13.79%          
FidelityLargeCapValueIndexFund-PRO | Fidelity Large Cap Value Index Fund                                
Prospectus Line Items                                
Risk/Return [Heading] Fund Summary Fund: Fidelity® Large Cap Value Index Fund                              
Objective [Heading] Investment Objective                              
Objective, Primary [Text Block] Fidelity® Large Cap Value Index Fund seeks to provide investment results that correspond to the total return of stocks of large 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 21 % of the average value of its portfolio.                              
Portfolio Turnover, Rate 21.00%                              
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 1000® Value Index. The Russell 1000 ® Value Index is a market capitalization-weighted index designed to measure the performance of the large-cap value segment of the U.S. equity market. It includes those Russell 1000 ® Index companies with lower price-to-book ratios and lower expected growth rates. The stocks of these companies are often called "value" stocks. The Russell 1000 ® 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 Mar. 31, 2026                              
Bar Chart, Year to Date Return 2.08%                              
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                              
Highest Quarterly Return, Date Dec. 31, 2020                              
Highest Quarterly Return 16.29%                              
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                              
Lowest Quarterly Return, Date Mar. 31, 2020                              
Lowest Quarterly Return (26.72%)                              
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.                              
FidelityLargeCapValueIndexFund-PRO | Fidelity Large Cap Value Index Fund | After Taxes on Distributions | Fidelity Large Cap Value Index Fund                                
Prospectus Line Items                                
Average Annual Return, Label [Optional Text]   Return After Taxes on Distributions                            
Average Annual Return, Percent   15.47%                     10.83%     9.69% [1]
FidelityLargeCapValueIndexFund-PRO | Fidelity Large Cap Value Index Fund | After Taxes on Distributions and Sales | Fidelity Large 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   9.69%                     8.93%     8.28% [1]
FidelityLargeCapValueIndexFund-PRO | Fidelity Large Cap Value Index Fund | RS001                                
Prospectus Line Items                                
Average Annual Return, Label [Optional Text]   Russell 1000® Index                            
Average Annual Return, Percent   17.37%                     13.59%     14.77%
FidelityLargeCapValueIndexFund-PRO | Fidelity Large Cap Value Index Fund | RS005                                
Prospectus Line Items                                
Average Annual Return, Label [Optional Text]   Russell 1000® Value Index                            
Average Annual Return, Percent   15.91%                     11.33%     10.30%
FidelityFlexMidCapFocusedandFlexSmallCapFocusedIndexFunds-ComboPRO | Fidelity Flex Mid Cap Focused Index Fund | Fidelity Flex Mid Cap Focused Index Fund                                
Prospectus Line Items                                
Management Fees (as a percentage of Assets) 0.00%                              
Distribution and Service (12b-1) Fees 0.00%                              
Other Expenses (as a percentage of Assets): 0.00%                              
Expenses (as a percentage of Assets) [2] 0.00%                              
Expense Example, with Redemption, 1 Year $ 0                              
Expense Example, with Redemption, 3 Years 0                              
Expense Example, with Redemption, 5 Years 0                              
Expense Example, with Redemption, 10 Years $ 0                              
FidelityFlexMidCapFocusedandFlexSmallCapFocusedIndexFunds-ComboPRO | Fidelity Flex Mid Cap Focused Index Fund                                
Prospectus Line Items                                
Risk/Return [Heading] Fund Summary Fund: Fidelity Flex® Mid Cap Focused Index Fund                              
Objective [Heading] Investment Objective                              
Objective, Primary [Text Block] Fidelity Flex® Mid Cap Focused 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. For the period from July 24, 2025 to April 30, 2026, the fund's portfolio turnover rate was 135 % (annualized) of the average value of its portfolio.                              
Portfolio Turnover, Rate 135.00%                              
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. This example does not include any fees paid at the fee-based account or plan level. 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 of mid-capitalization companies included in the Fidelity U.S. Mid Cap Focused IndexSM. The Fidelity U.S. Mid Cap Focused Index SM is designed to reflect the performance of stocks of mid-capitalization U.S. companies. Mid-capitalization stocks are considered to be stocks whose market capitalization is similar to the market capitalization of companies in the Russell Midcap ® Index or the S&P MidCap 400 ® Index. The Fidelity U.S. Mid Cap Focused Index SM was created by Fidelity Product Services LLC (FPS) using a rules-based proprietary index methodology that includes all U.S. stocks meeting certain market capitalization, liquidity and investability requirements. 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] 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.                              
FidelityFlexMidCapFocusedandFlexSmallCapFocusedIndexFunds-ComboPRO | Fidelity Flex Small Cap Focused Index Fund | Fidelity Flex Small Cap Focused Index Fund                                
Prospectus Line Items                                
Management Fees (as a percentage of Assets) 0.00%                              
Distribution and Service (12b-1) Fees 0.00%                              
Other Expenses (as a percentage of Assets): 0.00%                              
Expenses (as a percentage of Assets) [3] 0.00%                              
Expense Example, with Redemption, 1 Year $ 0                              
Expense Example, with Redemption, 3 Years 0                              
Expense Example, with Redemption, 5 Years 0                              
Expense Example, with Redemption, 10 Years $ 0                              
FidelityFlexMidCapFocusedandFlexSmallCapFocusedIndexFunds-ComboPRO | Fidelity Flex Small Cap Focused Index Fund                                
Prospectus Line Items                                
Risk/Return [Heading] Fund Summary Fund: Fidelity Flex® Small Cap Focused Index Fund                              
Objective [Heading] Investment Objective                              
Objective, Primary [Text Block] Fidelity Flex® Small Cap Focused 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. For the period from July 24, 2025 to April 30, 2026, the fund's portfolio turnover rate was 115 % (annualized) of the average value of its portfolio.                              
Portfolio Turnover, Rate 115.00%                              
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. This example does not include any fees paid at the fee-based account or plan level. 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 of small-capitalization companies included in the Fidelity U.S. Small Cap Focused IndexSM. The Fidelity U.S. Small Cap Focused Index SM is designed to reflect the performance of stocks of small-capitalization companies. Small-capitalization stocks are considered to be stocks whose market capitalization is similar to the market capitalization of companies in the Russell 2000 ® Index or the S&P SmallCap 600 ® Index. The Fidelity U.S. Small Cap Focused Index SM was created by Fidelity Product Services LLC (FPS) using a rules-based proprietary index methodology that includes all U.S. stocks meeting certain market capitalization, liquidity and investability requirements. 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] 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.                              
FidelityMidCapIndexSmallCapIndexFunds-ComboPRO | Fidelity Mid Cap Index Fund | Fidelity Mid Cap Index Fund                                
Prospectus Line Items                                
Management Fees (as a percentage of Assets) 0.025%                              
Distribution and Service (12b-1) Fees 0.00%                              
Other Expenses (as a percentage of Assets): 0.00%                              
Expenses (as a percentage of Assets) 0.025%                              
Expense Example, with Redemption, 1 Year $ 3                              
Expense Example, with Redemption, 3 Years 8                              
Expense Example, with Redemption, 5 Years 14                              
Expense Example, with Redemption, 10 Years $ 32                              
Average Annual Return, Label [Optional Text]   Return Before Taxes                            
Average Annual Return, Percent   10.57%                     8.67%     11.01%
Annual Return [Percent]     10.57% 15.35% 17.21% (17.28%) 22.56% 17.11% 30.51% (9.05%) 18.47% 13.86%        
FidelityMidCapIndexSmallCapIndexFunds-ComboPRO | Fidelity Mid Cap Index Fund                                
Prospectus Line Items                                
Risk/Return [Heading] Fund Summary Fund: Fidelity® Mid Cap Index Fund                              
Objective [Heading] Investment Objective                              
Objective, Primary [Text Block] Fidelity® Mid Cap Index Fund seeks to provide investment results that correspond to the total return of stocks of mid-capitalization United States 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 12 % of the average value of its portfolio.                              
Portfolio Turnover, Rate 12.00%                              
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® Index. The Russell Midcap ® Index is a market capitalization-weighted index of medium-capitalization U.S. company stocks. 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 Mar. 31, 2026                              
Bar Chart, Year to Date Return 1.30%                              
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                              
Highest Quarterly Return, Date Jun. 30, 2020                              
Highest Quarterly Return 24.55%                              
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                              
Lowest Quarterly Return, Date Mar. 31, 2020                              
Lowest Quarterly Return (27.04%)                              
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.                              
FidelityMidCapIndexSmallCapIndexFunds-ComboPRO | Fidelity Mid Cap Index Fund | After Taxes on Distributions | Fidelity Mid Cap Index Fund                                
Prospectus Line Items                                
Average Annual Return, Label [Optional Text]   Return After Taxes on Distributions                            
Average Annual Return, Percent   10.28%                     8.11%     10.36%
FidelityMidCapIndexSmallCapIndexFunds-ComboPRO | Fidelity Mid Cap Index Fund | After Taxes on Distributions and Sales | Fidelity Mid Cap 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   6.46%                     6.73%     8.92%
FidelityMidCapIndexSmallCapIndexFunds-ComboPRO | Fidelity Mid Cap Index Fund | RS006                                
Prospectus Line Items                                
Average Annual Return, Label [Optional Text]   Russell Midcap® Index                            
Average Annual Return, Percent   10.60%                     8.67%     11.01%
FidelityMidCapIndexSmallCapIndexFunds-ComboPRO | Fidelity Mid Cap Index Fund | RS003                                
Prospectus Line Items                                
Average Annual Return, Label [Optional Text]   Russell 3000® Index                            
Average Annual Return, Percent   17.15%                     13.15%     14.29%
FidelityMidCapIndexSmallCapIndexFunds-ComboPRO | Fidelity Small Cap Index Fund | Fidelity Small Cap Index Fund                                
Prospectus Line Items                                
Management Fees (as a percentage of Assets) 0.025%                              
Distribution and Service (12b-1) Fees 0.00%                              
Other Expenses (as a percentage of Assets): 0.00%                              
Expenses (as a percentage of Assets) 0.025%                              
Expense Example, with Redemption, 1 Year $ 3                              
Expense Example, with Redemption, 3 Years 8                              
Expense Example, with Redemption, 5 Years 14                              
Expense Example, with Redemption, 10 Years $ 32                              
Average Annual Return, Label [Optional Text]   Return Before Taxes                            
Average Annual Return, Percent   12.93%                     6.20%     9.76%
Annual Return [Percent]     12.93% 11.69% 17.12% (20.27%) 14.72% 19.99% 25.71% (10.88%) 14.85% 21.63%        
FidelityMidCapIndexSmallCapIndexFunds-ComboPRO | Fidelity Small Cap Index Fund                                
Prospectus Line Items                                
Risk/Return [Heading] Fund Summary Fund: Fidelity® Small Cap Index Fund                              
Objective [Heading] Investment Objective                              
Objective, Primary [Text Block] Fidelity® Small Cap Index Fund seeks to provide investment results that correspond to the total return of stocks of small-capitalization United States 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 20 % of the average value of its portfolio.                              
Portfolio Turnover, Rate 20.00%                              
Expense Example Narrative [Text Block] This example helps compare the cost of investing in the fund with the cost of investing in other funds. Let's say, hypothetically, that the annual return for shares of the fund is 5% and that the fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:                              
Strategy [Heading] Principal Investment Strategies                              
Strategy Narrative [Text Block] Normally investing at least 80% of assets in securities included in the Russell 2000® Index. The Russell 2000 ® Index is a market capitalization-weighted index designed to measure the performance of the small-cap segment of the U.S. equity market. 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 Mar. 31, 2026                              
Bar Chart, Year to Date Return 0.91%                              
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                              
Highest Quarterly Return, Date Dec. 31, 2020                              
Highest Quarterly Return 31.28%                              
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                              
Lowest Quarterly Return, Date Mar. 31, 2020                              
Lowest Quarterly Return (30.62%)                              
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.                              
FidelityMidCapIndexSmallCapIndexFunds-ComboPRO | Fidelity Small Cap Index Fund | After Taxes on Distributions | Fidelity Small Cap Index Fund                                
Prospectus Line Items                                
Average Annual Return, Label [Optional Text]   Return After Taxes on Distributions                            
Average Annual Return, Percent   12.59%                     5.72%     9.05%
FidelityMidCapIndexSmallCapIndexFunds-ComboPRO | Fidelity Small Cap Index Fund | After Taxes on Distributions and Sales | Fidelity Small Cap 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   7.81%                     4.75%     7.78%
FidelityMidCapIndexSmallCapIndexFunds-ComboPRO | Fidelity Small Cap Index Fund | RS003                                
Prospectus Line Items                                
Average Annual Return, Label [Optional Text]   Russell 3000® Index                            
Average Annual Return, Percent   17.15%                     13.15%     14.29%
FidelityMidCapIndexSmallCapIndexFunds-ComboPRO | Fidelity Small Cap Index Fund | RS002                                
Prospectus Line Items                                
Average Annual Return, Label [Optional Text]   Russell 2000® Index                            
Average Annual Return, Percent   12.81%                     6.09%     9.62%
FidelityLargeCapGrowthIndexFund-PRO | Fidelity Large Cap Growth Index Fund | Fidelity Large Cap Growth Index Fund                                
Prospectus Line Items                                
Management Fees (as a percentage of Assets) 0.035%                              
Distribution and Service (12b-1) Fees 0.00%                              
Other Expenses (as a percentage of Assets): 0.00%                              
Expenses (as a percentage of Assets) 0.035%                              
Expense Example, with Redemption, 1 Year $ 4                              
Expense Example, with Redemption, 3 Years 11                              
Expense Example, with Redemption, 5 Years 20                              
Expense Example, with Redemption, 10 Years $ 45                              
Annual Return, Inception Date Jun. 07, 2016                              
Average Annual Return, Label [Optional Text]   Return Before Taxes                            
Average Annual Return, Percent   18.53%                     15.30%     18.66% [4]
Annual Return [Percent]     18.53% 33.26% 42.77% (29.17%) 27.58% 38.43% 36.37% (1.64%) 30.12%          
FidelityLargeCapGrowthIndexFund-PRO | Fidelity Large Cap Growth Index Fund                                
Prospectus Line Items                                
Risk/Return [Heading] Fund Summary Fund: Fidelity® Large Cap Growth Index Fund                              
Objective [Heading] Investment Objective                              
Objective, Primary [Text Block] Fidelity® Large Cap Growth Index Fund seeks to provide investment results that correspond to the total return of stocks of large 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 16 % of the average value of its portfolio.                              
Portfolio Turnover, Rate 16.00%                              
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 1000® Growth Index. The Russell 1000 ® Growth Index is a market capitalization-weighted index designed to measure the performance of the large-cap growth segment of the U.S. equity market. It includes those Russell 1000 ® Index companies with higher price-to-book (P/B) ratios and higher forecasted growth rates. The stocks of these companies are often called "growth" stocks. The Russell 1000® 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. The fund may operate as a non-diversified fund, as defined under the Investment Company Act of 1940 (1940 Act), to the approximate extent the index is non-diversified. The fund may therefore operate as non-diversified solely as a result of a change in relative market capitalization or index weighting of one or more constituents of the index.                              
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 Mar. 31, 2026                              
Bar Chart, Year to Date Return (9.77%)                              
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                              
Highest Quarterly Return, Date Jun. 30, 2020                              
Highest Quarterly Return 27.76%                              
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                              
Lowest Quarterly Return, Date Jun. 30, 2022                              
Lowest Quarterly Return (20.93%)                              
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.                              
FidelityLargeCapGrowthIndexFund-PRO | Fidelity Large Cap Growth Index Fund | After Taxes on Distributions | Fidelity Large Cap Growth Index Fund                                
Prospectus Line Items                                
Average Annual Return, Label [Optional Text]   Return After Taxes on Distributions                            
Average Annual Return, Percent   18.44%                     15.04%     18.33% [4]
FidelityLargeCapGrowthIndexFund-PRO | Fidelity Large Cap Growth Index Fund | After Taxes on Distributions and Sales | Fidelity Large 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   11.04%                     12.27%     15.88% [4]
FidelityLargeCapGrowthIndexFund-PRO | Fidelity Large Cap Growth Index Fund | RS004                                
Prospectus Line Items                                
Average Annual Return, Label [Optional Text]   Russell 1000® Growth Index                            
Average Annual Return, Percent   18.56%                     15.32%     18.71%
FidelityLargeCapGrowthIndexFund-PRO | Fidelity Large Cap Growth Index Fund | RS001                                
Prospectus Line Items                                
Average Annual Return, Label [Optional Text]   Russell 1000® Index                            
Average Annual Return, Percent   17.37%                     13.59%     14.77%
FidelityFlexMidCapIndexFundFidelityFlexSmallCapIndexFund-ComboPRO | Fidelity Flex Mid Cap Index Fund | Fidelity Flex Mid Cap Index Fund                                
Prospectus Line Items                                
Management Fees (as a percentage of Assets) 0.00%                              
Distribution and Service (12b-1) Fees 0.00%                              
Other Expenses (as a percentage of Assets): 0.00%                              
Expenses (as a percentage of Assets) [5] 0.00%                              
Expense Example, with Redemption, 1 Year $ 0                              
Expense Example, with Redemption, 3 Years 0                              
Expense Example, with Redemption, 5 Years 0                              
Expense Example, with Redemption, 10 Years $ 0                              
Annual Return, Inception Date Mar. 09, 2017                              
Average Annual Return, Label [Optional Text]   Return Before Taxes                            
Average Annual Return, Percent   14.33%                     9.41%   10.90% [6]  
Annual Return [Percent]     14.33% 15.28% 17.30% (17.29%) 22.58% 17.29% 30.56% (9.01%)            
FidelityFlexMidCapIndexFundFidelityFlexSmallCapIndexFund-ComboPRO | Fidelity Flex Mid Cap Index Fund                                
Prospectus Line Items                                
Risk/Return [Heading] Fund Summary Fund: Fidelity Flex® Mid Cap Index Fund                              
Objective [Heading] Investment Objective                              
Objective, Primary [Text Block] Fidelity Flex® Mid Cap 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 89 % of the average value of its portfolio.                              
Portfolio Turnover, Rate 89.00%                              
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. This example does not include any fees paid at the fee-based account or plan level. 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 Fidelity U.S. Mid Cap IndexSM.    The Fidelity U.S. Mid Cap Index SM is a float-adjusted market capitalization-weighted index designed to reflect the performance of stocks of U.S. mid-capitalization companies. It is a subset of the Fidelity U.S. Total Investable Market Index SM (a float-adjusted market capitalization-weighted index designed to reflect the performance for the U.S. equity market, including large-, mid-, and small-capitalization stocks). Mid-capitalization stocks are considered to be stocks whose market capitalization is similar to the market capitalization of companies in the Russell Midcap ® Index or the S&P MidCap 400 ® Index.    The Fidelity U.S. Mid Cap Index SM was created by Fidelity Product Services LLC (FPS) using a rules-based proprietary index methodology that includes all U.S. stocks meeting certain market capitalization, liquidity, and investability requirements. 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 additional indexes over various periods of time. The additional indexes have 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. The performance shown does not reflect the impact of any fees paid at the fee-based account or plan level. Prior to August 21, 2025, the fund compared its performance to a different benchmark. The fund's historical performance may not represent its current investment policies.   Visit www.401k.com and log in or 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 additional indexes over various periods of time.                              
Performance Availability Website Address [Text] www.401k.com and log in or 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 Mar. 31, 2026                              
Bar Chart, Year to Date Return 2.52%                              
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                              
Highest Quarterly Return, Date Jun. 30, 2020                              
Highest Quarterly Return 24.64%                              
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                              
Lowest Quarterly Return, Date Mar. 31, 2020                              
Lowest Quarterly Return (26.96%)                              
Performance Table Heading Average Annual Returns                              
Performance Table Market Index Changed Effective August 21, 2025, the fund began comparing its performance to the Fidelity U.S. Mid Cap Index SM rather than the Russell Midcap ® Index because the Fidelity U.S. Mid Cap Index SM conforms more closely to the fund's investment policies .                              
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.                              
FidelityFlexMidCapIndexFundFidelityFlexSmallCapIndexFund-ComboPRO | Fidelity Flex Mid Cap Index Fund | After Taxes on Distributions | Fidelity Flex Mid Cap Index Fund                                
Prospectus Line Items                                
Average Annual Return, Label [Optional Text]   Return After Taxes on Distributions                            
Average Annual Return, Percent   14.33%                     8.97%   10.40% [6]  
FidelityFlexMidCapIndexFundFidelityFlexSmallCapIndexFund-ComboPRO | Fidelity Flex Mid Cap Index Fund | After Taxes on Distributions and Sales | Fidelity Flex Mid Cap 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   8.48%                     7.33%   8.79% [6]  
FidelityFlexMidCapIndexFundFidelityFlexSmallCapIndexFund-ComboPRO | Fidelity Flex Mid Cap Index Fund | RS001                                
Prospectus Line Items                                
Average Annual Return, Label [Optional Text]   Russell 1000® Index                            
Average Annual Return, Percent   17.37%                     13.59%   14.45%  
FidelityFlexMidCapIndexFundFidelityFlexSmallCapIndexFund-ComboPRO | Fidelity Flex Mid Cap Index Fund | RS006                                
Prospectus Line Items                                
Average Annual Return, Label [Optional Text]   Russell Midcap® Index                            
Average Annual Return, Percent   10.60%                     8.67%   10.45%  
FidelityFlexMidCapIndexFundFidelityFlexSmallCapIndexFund-ComboPRO | Fidelity Flex Mid Cap Index Fund | IXRP5                                
Prospectus Line Items                                
Average Annual Return, Label [Optional Text]   Fidelity Flex Mid Cap Linked Index℠                            
Average Annual Return, Percent   14.32%                     9.40%   10.86%  
FidelityFlexMidCapIndexFundFidelityFlexSmallCapIndexFund-ComboPRO | Fidelity Flex Mid Cap Index Fund | IXVWV                                
Prospectus Line Items                                
Average Annual Return, Label [Optional Text]   Fidelity U.S. Mid Cap Index℠                            
Average Annual Return, Percent   11.94%                     [7]   [7]  
FidelityFlexMidCapIndexFundFidelityFlexSmallCapIndexFund-ComboPRO | Fidelity Flex Small Cap Index Fund | Fidelity Flex Small Cap Index Fund                                
Prospectus Line Items                                
Management Fees (as a percentage of Assets) 0.00%                              
Distribution and Service (12b-1) Fees 0.00%                              
Other Expenses (as a percentage of Assets): 0.00%                              
Expenses (as a percentage of Assets) [8] 0.00%                              
Expense Example, with Redemption, 1 Year $ 0                              
Expense Example, with Redemption, 3 Years 0                              
Expense Example, with Redemption, 5 Years 0                              
Expense Example, with Redemption, 10 Years $ 0                              
Annual Return, Inception Date Mar. 09, 2017                              
Average Annual Return, Label [Optional Text]   Return Before Taxes                            
Average Annual Return, Percent   12.02%                     6.05%   8.53% [9]  
Annual Return [Percent]     12.02% 11.66% 17.13% (20.29%) 14.86% 20.05% 25.69% (11.03%)            
FidelityFlexMidCapIndexFundFidelityFlexSmallCapIndexFund-ComboPRO | Fidelity Flex Small Cap Index Fund                                
Prospectus Line Items                                
Risk/Return [Heading] Fund Summary Fund: Fidelity Flex® Small Cap Index Fund                              
Objective [Heading] Investment Objective                              
Objective, Primary [Text Block] Fidelity Flex® Small Cap 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 80 % of the average value of its portfolio.                              
Portfolio Turnover, Rate 80.00%                              
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. This example does not include any fees paid at the fee-based account or plan level. 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 Fidelity U.S. Small Cap IndexSM.    The Fidelity U.S. Small Cap Index SM is a float-adjusted market capitalization-weighted index designed to reflect the performance of stocks of U.S. small-capitalization companies. It is a subset of the Fidelity U.S. Total Investable Market Index SM (a float-adjusted market capitalization-weighted index designed to reflect the performance for the U.S. equity market, including large-, mid-, and small-capitalization stocks). Small-capitalization stocks are considered to be stocks whose market capitalization is similar to the market capitalization of companies in the Russell 2000 ® Index or the S&P SmallCap 600 ® Index.    The Fidelity U.S. Small Cap Index SM was created by Fidelity Product Services LLC (FPS) using a rules-based proprietary index methodology that includes all U.S. stocks meeting certain market capitalization, liquidity, and investability requirements. 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 additional indexes over various periods of time. The additional indexes have 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. The performance shown does not reflect the impact of any fees paid at the fee-based account or plan level. Prior to July 21, 2025, the fund compared its performance to a different benchmark. The fund's historical performance may not represent its current investment policies. Visit www.401k.com and log in or 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 additional indexes over various periods of time.                              
Performance Availability Website Address [Text] www.401k.com and log in or 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 Mar. 31, 2026                              
Bar Chart, Year to Date Return 0.45%                              
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                              
Highest Quarterly Return, Date Dec. 31, 2020                              
Highest Quarterly Return 31.26%                              
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                              
Lowest Quarterly Return, Date Mar. 31, 2020                              
Lowest Quarterly Return (30.54%)                              
Performance Table Heading Average Annual Returns                              
Performance Table Market Index Changed Effective July 21, 2025, the fund began comparing its performance to the Fidelity U.S. Small Cap Index SM rather than the Russell 2000 ® Index because the Fidelity U.S. Small Cap Index SM conforms more closely to the fund's investment policies.                              
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.                              
FidelityFlexMidCapIndexFundFidelityFlexSmallCapIndexFund-ComboPRO | Fidelity Flex Small Cap Index Fund | After Taxes on Distributions | Fidelity Flex Small Cap Index Fund                                
Prospectus Line Items                                
Average Annual Return, Label [Optional Text]   Return After Taxes on Distributions                            
Average Annual Return, Percent   12.02%                     5.65%   8.03% [9]  
FidelityFlexMidCapIndexFundFidelityFlexSmallCapIndexFund-ComboPRO | Fidelity Flex Small Cap Index Fund | After Taxes on Distributions and Sales | Fidelity Flex Small Cap 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   7.11%                     4.63%   6.74% [9]  
FidelityFlexMidCapIndexFundFidelityFlexSmallCapIndexFund-ComboPRO | Fidelity Flex Small Cap Index Fund | RS003                                
Prospectus Line Items                                
Average Annual Return, Label [Optional Text]   Russell 3000® Index                            
Average Annual Return, Percent   17.15%                     13.15%   14.08%  
FidelityFlexMidCapIndexFundFidelityFlexSmallCapIndexFund-ComboPRO | Fidelity Flex Small Cap Index Fund | RS002                                
Prospectus Line Items                                
Average Annual Return, Label [Optional Text]   Russell 2000® Index                            
Average Annual Return, Percent   12.81%                     6.09%   8.52%  
FidelityFlexMidCapIndexFundFidelityFlexSmallCapIndexFund-ComboPRO | Fidelity Flex Small Cap Index Fund | IXRP4                                
Prospectus Line Items                                
Average Annual Return, Label [Optional Text]   Fidelity Flex Small Cap Linked Index℠                            
Average Annual Return, Percent   11.95%                     5.93%   8.42%  
FidelityFlexMidCapIndexFundFidelityFlexSmallCapIndexFund-ComboPRO | Fidelity Flex Small Cap Index Fund | IXVWX                                
Prospectus Line Items                                
Average Annual Return, Label [Optional Text]   Fidelity U.S. Small Cap Index℠                            
Average Annual Return, Percent   13.32%                     [10]   [10]  
FidelitySeriesLargeCapGrowthIndexFund-PRO | Fidelity Series Large Cap Growth Index Fund | Fidelity Series Large Cap Growth Index Fund                                
Prospectus Line Items                                
Management Fees (as a percentage of Assets) 0.00%                              
Distribution and Service (12b-1) Fees 0.00%                              
Other Expenses (as a percentage of Assets): 0.00%                              
Expenses (as a percentage of Assets) 0.00%                              
Expense Example, with Redemption, 1 Year $ 0                              
Expense Example, with Redemption, 3 Years 0                              
Expense Example, with Redemption, 5 Years 0                              
Expense Example, with Redemption, 10 Years $ 0                              
Annual Return, Inception Date Aug. 17, 2018                              
Average Annual Return, Label [Optional Text]   Return Before Taxes                            
Average Annual Return, Percent   18.55%                     15.34% 17.86% [11]    
Annual Return [Percent]     18.55% 33.37% 42.77% (29.14%) 27.64% 38.34% 36.37%              
FidelitySeriesLargeCapGrowthIndexFund-PRO | Fidelity Series Large Cap Growth Index Fund                                
Prospectus Line Items                                
Risk/Return [Heading] Fund Summary Fund: Fidelity® Series Large Cap Growth Index Fund                              
Objective [Heading] Investment Objective                              
Objective, Primary [Text Block] Fidelity® Series Large Cap Growth Index Fund seeks to provide investment results that correspond to the total return of stocks of large 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.                              
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 34 % of the average value of its portfolio.                              
Portfolio Turnover, Rate 34.00%                              
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 of companies with large market capitalizations included in the Russell 1000® Growth Index. The Russell 1000 ® Growth Index is a market capitalization-weighted index designed to measure the performance of the large-cap growth segment of the U.S. equity market. It includes those Russell 1000 ® Index companies with higher price-to-book (P/B) ratios and higher forecasted growth rates. The stocks of these companies are often called "growth" stocks. The Russell 1000® 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. The fund may operate as a non-diversified fund, as defined under the Investment Company Act of 1940 (1940 Act), to the approximate extent the index is non-diversified. The fund may therefore operate as non-diversified solely as a result of a change in relative market capitalization or index weighting of one or more constituents of the index.                              
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. The performance shown does not reflect the impact of any fees paid at the fee-based account or plan level, if applicable. 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 Mar. 31, 2026                              
Bar Chart, Year to Date Return (9.76%)                              
Highest Quarterly Return, Label [Optional Text]    Highest Quarter Return                              
Highest Quarterly Return, Date Jun. 30, 2020                              
Highest Quarterly Return 27.66%                              
Lowest Quarterly Return, Label [Optional Text]    Lowest Quarter Return                              
Lowest Quarterly Return, Date Jun. 30, 2022                              
Lowest Quarterly Return (20.93%)                              
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.                              
FidelitySeriesLargeCapGrowthIndexFund-PRO | Fidelity Series Large Cap Growth Index Fund | After Taxes on Distributions | Fidelity Series Large Cap Growth Index Fund                                
Prospectus Line Items                                
Average Annual Return, Label [Optional Text]   Return After Taxes on Distributions                            
Average Annual Return, Percent   18.27%                     14.92% 17.32% [11]    
FidelitySeriesLargeCapGrowthIndexFund-PRO | Fidelity Series Large Cap Growth Index Fund | After Taxes on Distributions and Sales | Fidelity Series Large 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   11.17%                     12.24% 14.70% [11]    
FidelitySeriesLargeCapGrowthIndexFund-PRO | Fidelity Series Large Cap Growth Index Fund | RS004                                
Prospectus Line Items                                
Average Annual Return, Label [Optional Text]   Russell 1000® Growth Index                            
Average Annual Return, Percent   18.56%                     15.32% 17.86%    
FidelitySeriesLargeCapGrowthIndexFund-PRO | Fidelity Series Large Cap Growth Index Fund | RS001                                
Prospectus Line Items                                
Average Annual Return, Label [Optional Text]   Russell 1000® Index                            
Average Annual Return, Percent   17.37%                     13.59% 14.13%    
Document Type 485BPOS                              
Registrant Name Fidelity Salem Street Trust                              
[1]
A From June 7, 2016 .
[2]
A The fund is available only to certain fee-based accounts and advisory programs offered by Fidelity. Advisory account clients, retirement plans, plan sponsors and/or plan participants typically pay an advisory fee that generally covers investment advisory and administrative services.
[3]
A The fund is available only to certain fee-based accounts and advisory programs offered by Fidelity. Advisory account clients, retirement plans, plan sponsors and/or plan participants typically pay an advisory fee that generally covers investment advisory and administrative services.
[4]
A From June 7, 2016 .
[5]
A The fund is available only to certain fee-based accounts and advisory programs offered by Fidelity. Advisory account clients, retirement plans, plan sponsors and/or plan participants typically pay an advisory fee that generally covers investment advisory and administrative services.
[6]
A From March 9, 2017 .
[7]
B The inception date for the index was after the fund commenced operations, and returns for this time period are not yet available.
[8]
A The fund is available only to certain fee-based accounts and advisory programs offered by Fidelity. Advisory account clients, retirement plans, plan sponsors and/or plan participants typically pay an advisory fee that generally covers investment advisory and administrative services.
[9]
A From March 9, 2017 .
[10]
B The inception date for the index was after the fund commenced operations, and returns for this time period are not yet available.
[11]
A From August 17, 2018 .