Risk/Return Detail Data - FidelitySAIInternationalIndexFunds-ComboPRO	Dec. 30, 2024 USD ($)
Risk/Return:
Registrant Name	Fidelity Salem Street Trust
FidelitySAIInternationalIndexFunds-ComboPRO | Fidelity SAI International Momentum Index Fund
Risk/Return:
Risk/Return [Heading]	Fund Summary Fund: Fidelity® SAI International Momentum Index Fund
Objective [Heading]	Investment Objective
Objective, Primary [Text Block]	Fidelity® SAI International Momentum Index Fund seeks to provide investment results that correspond to the total return of international stocks with high price momentum.
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
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 62 % of the average value of its portfolio.
Portfolio Turnover, Rate	62.00%
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 Fidelity International Momentum Focus IndexSM and in depositary receipts representing securities included in the index. The Fidelity International Momentum Focus IndexSM is designed to reflect the performance of large and mid-cap developed international stocks that have attractive momentum characteristics. The universe of stocks for consideration in the index, which is intended to reflect the broader developed international equity market, consists of the largest developed international stocks based on float-adjusted market capitalization and certain liquidity and investability requirements. 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, country weightings, and the effect of foreign taxes to attempt to replicate the returns of the Fidelity International Momentum Focus 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 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	Year-to-Date Return
Bar Chart, Year to Date Return, Date	Sep. 30, 2024
Bar Chart, Year to Date Return	17.23%
Highest Quarterly Return, Label	Highest Quarter Return
Highest Quarterly Return, Date	Dec. 31, 2022
Highest Quarterly Return	17.94%
Lowest Quarterly Return, Label	Lowest Quarter Return
Lowest Quarterly Return, Date	Jun. 30, 2022
Lowest Quarterly Return	(18.97%)
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.
FidelitySAIInternationalIndexFunds-ComboPRO | Fidelity SAI International Momentum Index Fund | Fidelity SAI International Momentum Index Fund
Risk/Return:
Management fee	0.15%
Distribution and/or Service (12b-1) fees	none
Other expenses	0.03%
Total annual operating expenses	0.18%
1 year	$ 18
3 years	58
5 years	101
10 years	$ 230
Annual Return, Inception Date	May 12, 2020
2021	12.88%
2022	(21.45%)
2023	21.02%
FidelitySAIInternationalIndexFunds-ComboPRO | Fidelity SAI International Momentum Index Fund | Return Before Taxes | Fidelity SAI International Momentum Index Fund
Risk/Return:
Label	Return Before Taxes
Past 1 year	21.02%
Since Inception	10.18%	[1]
FidelitySAIInternationalIndexFunds-ComboPRO | Fidelity SAI International Momentum Index Fund | After Taxes on Distributions | Fidelity SAI International Momentum Index Fund
Risk/Return:
Label	Return After Taxes on Distributions
Past 1 year	19.99%
Since Inception	9.58%	[1]
FidelitySAIInternationalIndexFunds-ComboPRO | Fidelity SAI International Momentum Index Fund | After Taxes on Distributions and Sales | Fidelity SAI International Momentum Index Fund
Risk/Return:
Label	Return After Taxes on Distributions and Sale of Fund Shares
Past 1 year	13.05%
Since Inception	8.02%	[1]
FidelitySAIInternationalIndexFunds-ComboPRO | Fidelity SAI International Momentum Index Fund | MS112
Risk/Return:
Label	MSCI ACWI (All Country World Index) ex USA Index
Past 1 year	15.82%
Since Inception	10.19%
FidelitySAIInternationalIndexFunds-ComboPRO | Fidelity SAI International Momentum Index Fund | IXXJI
Risk/Return:
Label	Fidelity International Momentum Focus Index℠
Past 1 year	21.64%
Since Inception	10.58%

[1] A From May 12, 2020 .