Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	FIDELITY SALEM STREET TRUST
12.31 Fidelity Series InflationProtected Bond Index Fund Series PRO-10 | Fidelity Series Inflation-Protected Bond Index Fund
Risk/Return:	rr_RiskReturnAbstract
Risk/Return [Heading]	rr_RiskReturnHeading	Fund Summary Fund:Fidelity® Series Inflation-Protected Bond Index Fund
Objective [Heading]	rr_ObjectiveHeading	Investment Objective
Objective, Primary [Text Block]	rr_ObjectivePrimaryTextBlock	The fund seeks to provide investment results that correspond to the total return of the inflation-protected sector of the United States Treasury market.
Expense [Heading]	rr_ExpenseHeading	Fee Table
Expense Narrative [Text Block]	rr_ExpenseNarrativeTextBlock	The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.
Shareholder Fees Caption [Text]	rr_ShareholderFeesCaption	Shareholder fees
(fees paid directly from your investment)	rr_ShareholderFeeOther	none
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Operating Expenses (expenses that you pay each year as a % of the value of your investment)
Portfolio Turnover [Heading]	rr_PortfolioTurnoverHeading	Portfolio Turnover
Portfolio Turnover [Text Block]	rr_PortfolioTurnoverTextBlock	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 29% of the average value of its portfolio.
Portfolio Turnover, Rate	rr_PortfolioTurnoverRate	29.00%
Expense Example Narrative [Text Block]	rr_ExpenseExampleNarrativeTextBlock	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 your shareholder 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]	rr_StrategyHeading	Principal Investment Strategies
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Normally investing at least 80% of assets in inflation-protected debt securities included in the Bloomberg U.S. Treasury Inflation-Protected Securities (TIPS) 1-10 Year Index. Engaging in transactions that have a leveraging effect on the fund, including investments in derivatives - such as swaps (interest rate, total return, and credit default) and futures contracts - and forward-settling securities, to adjust the fund's risk exposure.
Risk [Heading]	rr_RiskHeading	Principal Investment Risks
Risk Narrative [Text Block]	rr_RiskNarrativeTextBlock	Interest Rate Changes. Interest rate increases can cause the price of a debt security to decrease. Prepayment. The ability of an issuer of a debt security to repay principal prior to a security's maturity can cause greater price volatility if interest rates change. Issuer-Specific Changes. The value of an individual security or particular type of security can be more volatile than, and can perform differently from, the market as a whole. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a security to decrease. Correlation to Index. The performance of the fund and its underlying index may vary somewhat due to factors such as fees and expenses of the fund, transaction costs, sample selection, regulatory restrictions, and timing differences associated with additions to and deletions from the index. Errors in the construction or calculation of the index may occur from time to time and may not be identified and corrected for some period of time, which may have an adverse impact on the fund and its shareholders. Passive Management Risk. The fund is managed with a passive investment strategy, attempting to track the performance of an unmanaged index of securities, regardless of the current or projected performance of the fund’s index or of the actual securities included in the index. This differs from an actively managed fund, which typically seeks to outperform a benchmark index. As a result, the fund’s performance could be lower than actively managed funds that may shift their portfolio assets to take advantage of market opportunities or lessen the impact of a market decline or a decline in the value of one or more issuers. Leverage Risk. Leverage can increase market exposure, magnify investment risks, and cause losses to be realized more quickly. Investing for Inflation Protection. Increases in real interest rates can cause the price of inflation-protected debt securities to decrease. Interest payments on inflation-protected debt securities can be unpredictable. An investment in the fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency. Unlike individual debt securities, which typically pay principal at maturity, the value of an investment in the fund will fluctuate. You could lose money by investing in the fund.
Risk Lose Money [Text]	rr_RiskLoseMoney	You could lose money by investing in the fund.
Risk Not Insured Depository Institution [Text]	rr_RiskNotInsuredDepositoryInstitution	An investment in the fund is not a deposit of a bank and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.
Bar Chart and Performance Table [Heading]	rr_BarChartAndPerformanceTableHeading	Performance
Performance Narrative [Text Block]	rr_PerformanceNarrativeTextBlock	The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year and compares the performance of the fund's shares to the performance of a securities market index over various periods of time. The index description appears in the "Additional Index Information" section of the prospectus. Past performance (before and after taxes) is not an indication of future performance. Visit www.fidelity.com for more recent performance information.
Performance Information Illustrates Variability of Returns [Text]	rr_PerformanceInformationIllustratesVariabilityOfReturns	The information illustrates the changes in the performance of the fund's shares from year to year and compares the performance of the fund's shares to the performance of a securities market index over various periods of time.
Performance Availability Website Address [Text]	rr_PerformanceAvailabilityWebSiteAddress	www.fidelity.com
Performance Past Does Not Indicate Future [Text]	rr_PerformancePastDoesNotIndicateFuture	Past performance (before and after taxes) is not an indication of future performance.
Bar Chart [Heading]	rr_BarChartHeading	Year-by-Year Returns
Performance Table Heading	rr_PerformanceTableHeading	Average Annual Returns
Performance Table Uses Highest Federal Rate	rr_PerformanceTableUsesHighestFederalRate	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	rr_PerformanceTableNotRelevantToTaxDeferred	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	rr_PerformanceTableExplanationAfterTaxHigher	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	rr_PerformanceTableNarrativeTextBlock	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.
12.31 Fidelity Series InflationProtected Bond Index Fund Series PRO-10 | Fidelity Series Inflation-Protected Bond Index Fund | Fidelity Series Inflation-Protected Bond Index Fund
Risk/Return:	rr_RiskReturnAbstract
Management fee	rr_ManagementFeesOverAssets	none
Distribution and/or Service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	none
Total annual operating expenses	rr_ExpensesOverAssets	none
1 year	rr_ExpenseExampleYear01	none
3 years	rr_ExpenseExampleYear03	none
5 years	rr_ExpenseExampleYear05	none
10 years	rr_ExpenseExampleYear10	none
2012	rr_AnnualReturn2012	4.77%
2013	rr_AnnualReturn2013	(5.77%)
2014	rr_AnnualReturn2014	0.80%
2015	rr_AnnualReturn2015	(0.92%)
2016	rr_AnnualReturn2016	3.88%
2017	rr_AnnualReturn2017	1.99%
2018	rr_AnnualReturn2018	(0.42%)
2019	rr_AnnualReturn2019	6.89%
2020	rr_AnnualReturn2020	8.26%
2021	rr_AnnualReturn2021	5.60%
Highest Quarterly Return, Label	rr_HighestQuarterlyReturnLabel	Highest Quarter Return
Highest Quarterly Return, Date	rr_BarChartHighestQuarterlyReturnDate	Jun. 30, 2020
Highest Quarterly Return	rr_BarChartHighestQuarterlyReturn	3.73%
Lowest Quarterly Return, Label	rr_LowestQuarterlyReturnLabel	Lowest Quarter Return
Lowest Quarterly Return, Date	rr_BarChartLowestQuarterlyReturnDate	Jun. 30, 2013
Lowest Quarterly Return	rr_BarChartLowestQuarterlyReturn	(5.62%)
12.31 Fidelity Series InflationProtected Bond Index Fund Series PRO-10 | Fidelity Series Inflation-Protected Bond Index Fund | Return Before Taxes | Fidelity Series Inflation-Protected Bond Index Fund
Risk/Return:	rr_RiskReturnAbstract
Label	rr_AverageAnnualReturnLabel	Return Before Taxes
Past 1 year	rr_AverageAnnualReturnYear01	5.60%
Past 5 years	rr_AverageAnnualReturnYear05	4.41%
Past 10 years	rr_AverageAnnualReturnYear10	2.43%
12.31 Fidelity Series InflationProtected Bond Index Fund Series PRO-10 | Fidelity Series Inflation-Protected Bond Index Fund | After Taxes on Distributions | Fidelity Series Inflation-Protected Bond Index Fund
Risk/Return:	rr_RiskReturnAbstract
Label	rr_AverageAnnualReturnLabel	Return After Taxes on Distributions
Past 1 year	rr_AverageAnnualReturnYear01	2.75%
Past 5 years	rr_AverageAnnualReturnYear05	3.27%
Past 10 years	rr_AverageAnnualReturnYear10	1.29%
12.31 Fidelity Series InflationProtected Bond Index Fund Series PRO-10 | Fidelity Series Inflation-Protected Bond Index Fund | After Taxes on Distributions and Sales | Fidelity Series Inflation-Protected Bond Index Fund
Risk/Return:	rr_RiskReturnAbstract
Label	rr_AverageAnnualReturnLabel	Return After Taxes on Distributions and Sale of Fund Shares
Past 1 year	rr_AverageAnnualReturnYear01	3.30%
Past 5 years	rr_AverageAnnualReturnYear05	2.90%
Past 10 years	rr_AverageAnnualReturnYear10	1.37%
12.31 Fidelity Series InflationProtected Bond Index Fund Series PRO-10 | Fidelity Series Inflation-Protected Bond Index Fund | LB200
Risk/Return:	rr_RiskReturnAbstract
Label	rr_AverageAnnualReturnLabel	Bloomberg U.S. Treasury Inflation-Protected Securities (TIPS) 1-10 Year Index
Past 1 year	rr_AverageAnnualReturnYear01	5.69%
Past 5 years	rr_AverageAnnualReturnYear05	4.46%
Past 10 years	rr_AverageAnnualReturnYear10	2.57%