Risk/Return Detail Data - FidelitySAIEmergingMarketsIndexFund-PRO	Dec. 30, 2024 USD ($)
Risk/Return:
Registrant Name	Fidelity Salem Street Trust
FidelitySAIEmergingMarketsIndexFund-PRO | Fidelity SAI Emerging Markets Index Fund
Risk/Return:
Risk/Return [Heading]	Fund Summary Fund: Fidelity® SAI Emerging Markets Index Fund
Objective [Heading]	Investment Objective
Objective, Primary [Text Block]	Fidelity® SAI Emerging Markets Index Fund seeks to provide investment results that correspond to the total return of emerging stock markets.
Expense [Heading]	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 [Text]	Shareholder fees
(fees paid directly from your investment)	none
Operating Expenses Caption [Text]	Annual Operating Expenses
Fee Waiver or Reimbursement over Assets, Date of Termination	Feb. 28, 2026
Portfolio Turnover [Heading]	Portfolio Turnover
Portfolio Turnover [Text Block]	The fund pays transaction costs, such as commissions, when it buys and sells securities (or "turns over" its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when fund shares are held in a taxable account. These costs, which are not reflected in annual operating expenses or in the example, affect the fund's performance. During the most recent fiscal year, the fund's portfolio turnover rate was 29 % of the average value of its portfolio.
Portfolio Turnover, Rate	29.00%
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 MSCI Emerging Markets Index and in depositary receipts representing securities included in the index. The MSCI Emerging Markets Index is a market capitalization-weighted index designed to measure the performance of large and medium-capitalization companies domiciled in emerging markets countries across the world. Effective December 11, 2025, derivative instruments that provide investment exposure to the investments above or exposure to one or more market risk factors associated with such investments are included in the fund's 80% policy, consistent with the fund's investment policies and limitations with respect to investments in derivatives. Using statistical sampling techniques based on such factors as capitalization, industry exposures, dividend yield, price -to-earnings (P/E) ratio, price -to-book (P/B) ratio, earnings growth, and country weightings to attempt to replicate the returns of the MSCI Emerging Markets Index. Lending securities to earn income for the fund.
Risk [Heading]	Principal Investment Risks
Bar Chart and Performance Table [Heading]	Performance
Performance Narrative [Text Block]	The following information is intended to help you understand the risks of investing in the fund. The information illustrates the changes in the performance of the fund's shares from year to year and compares the performance of the fund's shares to the performance of a securities market index over various periods of time. The index description appears in the "Additional Index Information" section of the prospectus. Past performance (before and after taxes) is not an indication of future performance. Visit www.fidelity.com for more recent performance information.
Performance Information Illustrates Variability of Returns [Text]	The information illustrates the changes in the performance of the fund's shares from year to year and compares the performance of the fund's shares to the performance of a securities market index over various periods of time.
Performance Availability Website Address [Text]	www.fidelity.com
Performance Past Does Not Indicate Future [Text]	Past performance (before and after taxes) is not an indication of future performance.
Bar Chart [Heading]	Year-by-Year Returns
Year to Date Return, Label	Year-to-Date Return
Bar Chart, Year to Date Return, Date	Sep. 30, 2024
Bar Chart, Year to Date Return	15.18%
Highest Quarterly Return, Label	Highest Quarter Return
Highest Quarterly Return, Date	Jun. 30, 2020
Highest Quarterly Return	18.79%
Lowest Quarterly Return, Label	Lowest Quarter Return
Lowest Quarterly Return, Date	Mar. 31, 2020
Lowest Quarterly Return	(24.08%)
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.
FidelitySAIEmergingMarketsIndexFund-PRO | Fidelity SAI Emerging Markets Index Fund | Fidelity SAI Emerging Markets Index Fund
Risk/Return:
Management fee	0.075%
Distribution and/or Service (12b-1) fees	none
Other expenses	0.069%
Total annual operating expenses	0.144%
Fee waiver and/or expense reimbursement	0.069%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.075%
1 year	$ 8
3 years	37
5 years	72
10 years	$ 175
Annual Return, Inception Date	Jan. 05, 2016
2017	37.60%
2018	(14.48%)
2019	18.20%
2020	17.44%
2021	(3.00%)
2022	(20.20%)
2023	9.40%
FidelitySAIEmergingMarketsIndexFund-PRO | Fidelity SAI Emerging Markets Index Fund | Return Before Taxes | Fidelity SAI Emerging Markets Index Fund
Risk/Return:
Label	Return Before Taxes
Past 1 year	9.40%
Past 5 years	3.29%
Since Inception	5.81%	[2]
FidelitySAIEmergingMarketsIndexFund-PRO | Fidelity SAI Emerging Markets Index Fund | After Taxes on Distributions | Fidelity SAI Emerging Markets Index Fund
Risk/Return:
Label	Return After Taxes on Distributions
Past 1 year	8.72%
Past 5 years	2.75%
Since Inception	5.24%	[2]
FidelitySAIEmergingMarketsIndexFund-PRO | Fidelity SAI Emerging Markets Index Fund | After Taxes on Distributions and Sales | Fidelity SAI Emerging Markets Index Fund
Risk/Return:
Label	Return After Taxes on Distributions and Sale of Fund Shares
Past 1 year	6.05%
Past 5 years	2.60%
Since Inception	4.63%	[2]
FidelitySAIEmergingMarketsIndexFund-PRO | Fidelity SAI Emerging Markets Index Fund | MC041
Risk/Return:
Label	MSCI Emerging Markets Index
Past 1 year	9.86%
Past 5 years	3.71%
Since Inception	6.21%

[1] A Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund's securities lending program, if applicable), as a percentage of its average net assets, exceed 0.075% (the Expense Cap). If at any time during the current fiscal year expenses for the fund fall below the Expense Cap, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Cap. This arrangement will remain in effect through February 28, 2026 . FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees and may extend it in its discretion after that date.

[2] A From January 5, 2016 .