Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Dec. 30, 2014

Inst Adv Inst | Spartan Emerging Markets Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund/Class:
Spartan^® Emerging Markets Index Fund/Institutional, Fidelity Advantage^® Institutional

Objective [Heading]

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Investment Objective

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks to provide investment results that correspond to the total return of emerging stock markets.

Expense [Heading]

rr_ExpenseHeading

Fee Table

Expense Narrative [Text Block]

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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)

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

Annual operating expenses
(expenses that you pay each year as a % of the value of your investment)

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

December 31, 2015

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 8% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

8.00%

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 securities included in the FTSE^® Emerging Index and in depository receipts representing securities included in the index.
• Using statistical sampling techniques based on such factors as capitalization, industry exposures, dividend yield, price/earnings (P/E) ratio, price/book (P/B) ratio, earnings growth, country weightings, and the effect of foreign taxes to attempt to replicate the returns of the FTSE^® Emerging Index.
• Lending securities to earn income for the fund.

Risk [Heading]

rr_RiskHeading

Principal Investment Risks

Risk Narrative [Text Block]

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• Stock Market Volatility. Stock markets are volatile and can decline significantly in response to adverse issuer, political, regulatory, market, or economic developments. Different parts of the market, including different market sectors, and different types of securities can react differently to these developments.
• Foreign and Emerging Market Risk. Foreign markets, particularly emerging markets, can be more volatile than the U.S. market due to increased risks of adverse issuer, political, regulatory, market, or economic developments and can perform differently from the U.S. market. Emerging markets can be subject to greater social, economic, regulatory, and political uncertainties and can be extremely volatile. Foreign exchange rates also can be extremely volatile.
• 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.
• Correlation to Index. The performance of the fund and its index may vary somewhat due to factors such as transaction costs, sample selection, and timing differences associated with additions to and deletions from its index.

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. 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]

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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]

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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 is not an indication of future performance.

Visit www.401k.com and log in for updated return 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.401k.com and log in for updated return information

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

Past performance is not an indication of future performance.

Bar Chart [Heading]

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Year-by-Year Returns
Calendar Years

Bar Chart Closing [Text Block]

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During the periods shown in the chart for Institutional Class:	Returns	Quarter ended
Highest Quarter Return	13.15%	March 31, 2012
Lowest Quarter Return	-8.64%	June 30, 2012
Year-to-Date Return	4.28%	September30, 2014

Performance Table Heading

rr_PerformanceTableHeading

Average Annual Returns
For the periods ended
December 31, 2013

Inst Adv Inst | Spartan Emerging Markets Index Fund | Institutional Class

Risk/Return:

rr_RiskReturnAbstract

Redemption fee on shares held less than 90 days (as a % of amount redeemed)

rr_RedemptionFeeOverRedemption

1.50%

Management fee

rr_ManagementFeesOverAssets

0.25%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.03%

Total annual operating expenses

rr_ExpensesOverAssets

0.28%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.15%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.13%

1 year

rr_ExpenseExampleYear01

13

3 years

rr_ExpenseExampleYear03

72

5 years

rr_ExpenseExampleYear05

140

10 years

rr_ExpenseExampleYear10

338

2012

rr_AnnualReturn2012

17.58%

2013

rr_AnnualReturn2013

(4.62%)

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Sep. 30, 2014

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

4.28%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Mar. 31, 2012

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

13.15%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Jun. 30, 2012

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(8.64%)

Label

rr_AverageAnnualReturnLabel

Institutional Class

Past 1 year

rr_AverageAnnualReturnYear01

(4.62%)

Life of class

rr_AverageAnnualReturnSinceInception

0.07%

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

Inst Adv Inst | Spartan Emerging Markets Index Fund | Fidelity Advantage Institutional Class

Risk/Return:

rr_RiskReturnAbstract

Redemption fee on shares held less than 90 days (as a % of amount redeemed)

rr_RedemptionFeeOverRedemption

1.50%

Management fee

rr_ManagementFeesOverAssets

0.25%

Distribution and/or Service (12b-1) fees

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

none

Total annual operating expenses

rr_ExpensesOverAssets

0.25%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.15%

^[1]

Total annual operating expenses after fee waiver and/or expense reimbursement

rr_NetExpensesOverAssets

0.10%

1 year

rr_ExpenseExampleYear01

10

3 years

rr_ExpenseExampleYear03

63

5 years

rr_ExpenseExampleYear05

123

10 years

rr_ExpenseExampleYear10

301

Label

rr_AverageAnnualReturnLabel

Fidelity Advantage Institutional Class

Past 1 year

rr_AverageAnnualReturnYear01

(4.69%)

Life of class

rr_AverageAnnualReturnSinceInception

0.06%

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

Inst Adv Inst | Spartan Emerging Markets Index Fund | FTSE Emerging Index (reflects no deduction for fees, expenses, or taxes)

Risk/Return:

rr_RiskReturnAbstract

Label

rr_AverageAnnualReturnLabel

FTSE Emerging Index

Past 1 year

rr_AverageAnnualReturnYear01

(3.50%)

Life of class

rr_AverageAnnualReturnSinceInception

1.23%

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse Institutional Class and Fidelity Advantage Institutional Class of the fund to the extent that total operating expenses (excluding interest, certain taxes, certain securities lending costs, brokerage commissions, extraordinary expenses, and acquired fund fees and expenses, if any), as a percentage of their respective average net assets, exceed 0.13% and 0.10%. These arrangements will remain in effect through December 31, 2015. FMR may not terminate these arrangements before the expiration date without the approval of the Board of Trustees.

[2] From September 8, 2011.