485BPOS

Securities Act of 1933 Registration No. 002-41839

Investment Company Act of 1940 Registration No. 811-02105

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM N-1A

REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933 x

o Pre-Effective Amendment No.  ______

x Post-Effective Amendment No.  401

and

REGISTRATION STATEMENT UNDER THE INVESTMENT COMPANY ACT OF 1940 x

x Amendment No.  401

Fidelity Salem Street Trust

 (Exact Name of Registrant as Specified in Charter)

245 Summer Street, Boston, Massachusetts 02210

(Address of Principal Executive Offices)(Zip Code)

Registrant’s Telephone Number: 617-563-7000

Marc Bryant, Secretary

245 Summer Street

Boston, Massachusetts 02210

(Name and Address of Agent for Service)

It is proposed that this filing will become effective immediately upon filing pursuant to paragraph (b).

EXHIBIT INDEX

Exhibit	Exhibit No.
XBRL Instance Document	EX-101.INS
XBRL Taxonomy Extension Schema Document	EX-101.SCH
XBRL Taxonomy Extension Calculation Linkbase Document	EX-101.CAL
XBRL Taxonomy Extension Definition Linkbase Document	EX-101.DEF
XBRL Taxonomy Extension Labels Linkbase Document	EX-101.LAB
XBRL Taxonomy Extension Presentation Linkbase Document	EX-101.PRE

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933 and the Investment Company Act of 1940, the Registrant certifies that it meets all of the requirements for the effectiveness of this Registration Statement pursuant to Rule 485(b) under the Securities Act of 1933 and has duly caused this Post-Effective Amendment No.401to the Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of Boston, and Commonwealth of Massachusetts, on the 16th day of January 2018.

 Fidelity Salem Street Trust

	By	/s/Stephanie J. Dorsey
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||		Stephanie J. Dorsey, President

Pursuant to the requirements of the Securities Act of 1933, this Registration Statement has been signed below by the following persons in the capacities and on the dates indicated.

(Signature)		(Title)	(Date)
/s/Stephanie J. Dorsey		President and Treasurer	January 16, 2018
Stephanie J. Dorsey		(Principal Executive Officer)
/s/Howard J. Galligan III		Chief Financial Officer	January 16, 2018
Howard J. Galligan III		(Principal Financial Officer)
/s/Abigail P. Johnson	†	Trustee	January 16, 2018
Abigail P. Johnson
/s/Elizabeth S. Acton	*	Trustee	January 16, 2018
Elizabeth S. Acton
/s/Jonathan Chiel	*	Trustee	January 16, 2018
Jonathan Chiel
/s/John Engler	*	Trustee	January 16, 2018
John Engler
/s/Albert R. Gamper, Jr.	*	Trustee	January 16, 2018
Albert R. Gamper, Jr.
/s/Robert F. Gartland	*	Trustee	January 16, 2018
Robert F. Gartland
/s/Arthur E. Johnson	*	Trustee	January 16, 2018
Arthur E. Johnson
/s/Michael E. Kenneally	*	Trustee	January 16, 2018
Michael E. Kenneally
/s/Marie L. Knowles	*	Trustee	January 16, 2018
Marie L. Knowles
/s/Mark A. Murray	*	Trustee	January 16, 2018
Mark A. Murray
/s/Jennifer Toolin McAuliffe	*	Trustee	January 16, 2018
Jennifer Toolin McAuliffe

†	By:	/s/Marc R. Bryant
		Marc R. Bryant, pursuant to a power of attorney dated December 16, 2015 and filed herewith.
*	By:	/s/Megan C. Johnson
		Megan C. Johnson, pursuant to powers of attorney dated October 5, 2016 and September 1, 2017 and filed herewith.

POWER OF ATTORNEY

We, the undersigned Directors or Trustees, as the case may be, of the following investment companies:

Fidelity Aberdeen Street Trust Fidelity Advisor Series II Fidelity Advisor Series IV Fidelity Boylston Street Trust Fidelity California Municipal Trust Fidelity California Municipal Trust II Fidelity Central Investment Portfolios II LLC Fidelity Charles Street Trust Fidelity Colchester Street Trust Fidelity Court Street Trust Fidelity Court Street Trust II Fidelity Garrison Street Trust Fidelity Hereford Street Trust Fidelity Income Fund Fidelity Massachusetts Municipal Trust Fidelity Merrimack Street Trust Fidelity Money Market Trust Fidelity Municipal Trust Fidelity Municipal Trust II Fidelity Newbury Street Trust Fidelity New York Municipal Trust Fidelity New York Municipal Trust II Fidelity Oxford Street Trust Fidelity Oxford Street Trust II Fidelity Phillips Street Trust Fidelity Revere Street Trust Fidelity Salem Street Trust Fidelity School Street Trust Fidelity Union Street Trust Fidelity Union Street Trust II Variable Insurance Products Fund V

in addition to any other Fidelity Fund for which the undersigned individuals serve as Directors or Trustees (collectively, the “Funds”), hereby revoke all previous powers of attorney we have given to sign and otherwise act in our names and behalf in matters involving any investment company for which FMR or an affiliate acts as investment adviser and hereby constitute and appoint Thomas C. Bogle, John V. O’Hanlon, Robert W. Helm, Megan C. Johnson, and Anthony H. Zacharski, each of them singly, our true and lawful attorneys-in-fact, with full power of substitution, and with full power to each of them, to sign for us and in our names in the appropriate capacities, all Registration Statements of the Funds on Form N-1A, or any successors thereto, any and all subsequent Amendments, Pre-Effective Amendments, or Post-Effective Amendments to said Registration Statements or any successors thereto, and any supplements or other instruments in connection therewith, and generally to do all such things in our names and behalf in connection therewith as said attorneys-in-fact deem necessary or appropriate, to comply with the provisions of the Securities Act of 1933 and the Investment Company Act of 1940, and all related requirements of the Securities and Exchange Commission.  We hereby ratify and confirm all that said attorneys-in-fact or their substitutes may do or cause to be done by virtue hereof.  This power of attorney is effective for all documents filed on or after September 1, 2017.

WITNESS our hands on this first day of September 2017.

/s/Elizabeth S. Acton	/s/Michael E. Kenneally
Elizabeth S. Acton	Michael E. Kenneally
/s/John Engler	/s/Marie L. Knowles
John Engler	Marie L. Knowles
/s/Albert R. Gamper, Jr.	/s/Jennifer Toolin McAuliffe
Albert R. Gamper, Jr.	Jennifer Toolin McAuliffe
/s/Robert F. Gartland	/s/Mark A. Murray
Robert F. Gartland	Mark A. Murray
/s/Arthur E. Johnson
Arthur E. Johnson

POWER OF ATTORNEY

I, the undersigned Trustee of the following investment companies:

Fidelity Aberdeen Street Trust Fidelity Advisor Series II Fidelity Advisor Series IV Fidelity Boylston Street Trust Fidelity California Municipal Trust Fidelity California Municipal Trust II Fidelity Central Investment Portfolios II LLC Fidelity Charles Street Trust Fidelity Colchester Street Trust Fidelity Commonwealth Trust II Fidelity Court Street Trust Fidelity Court Street Trust II Fidelity Garrison Street Trust Fidelity Hereford Street Trust Fidelity Income Fund Fidelity Massachusetts Municipal Trust
Fidelity Merrimack Street Trust Fidelity Money Market Trust Fidelity Municipal Trust Fidelity Municipal Trust II Fidelity Newbury Street Trust Fidelity New York Municipal Trust Fidelity New York Municipal Trust II Fidelity Oxford Street Trust Fidelity Oxford Street Trust II Fidelity Phillips Street Trust Fidelity Revere Street Trust Fidelity Salem Street Trust Fidelity School Street Trust Fidelity Union Street Trust Fidelity Union Street Trust II Variable Insurance Products Fund V

in addition to any other Fidelity Fund for which the undersigned individual serves as Trustee (collectively, the “Funds”), hereby constitute and appoint Marc R. Bryant, my true and lawful attorney-in-fact, with full power of substitution, and with full power to sign for me and in my name in the appropriate capacity, all Registration Statements of the Funds on Form N-1A, or any successors thereto, any and all subsequent Amendments, Pre-Effective Amendments, or Post-Effective Amendments to said Registration Statements or any successors thereto, and any supplements or other instruments in connection therewith, and generally to do all such things in my name and behalf in connection therewith as said attorney-in-fact deems necessary or appropriate, to comply with the provisions of the Securities Act of 1933 and the Investment Company Act of 1940, and all related requirements of the Securities and Exchange Commission.  I hereby ratify and confirm all that said attorney-in-fact or his substitutes may do or cause to be done by virtue hereof.  This power of attorney is effective for all documents filed on or after December 16^th, 2015.

WITNESS my hand on this 16th day of December, 2015.

/s/Abigail P. Johnson

Abigail P. Johnson

POWER OF ATTORNEY

I, the undersigned Trustee of the following investment company:

Fidelity Salem Street Trust

in addition to any other Fidelity Fund for which the undersigned individual serves as Director or Trustee (collectively, the “Funds”), hereby constitute and appoint Thomas C. Bogle, Marc R. Bryant, John V. O’Hanlon, Robert W. Helm, Megan C. Johnson, and Anthony H. Zacharski, each of them singly, my true and lawful attorneys-in-fact, with full power of substitution, and with full power to each of them, to sign for me and in my name in the appropriate capacity, all Registration Statements of the Funds on Form N-1A, or any successors thereto, any and all subsequent Amendments, Pre-Effective Amendments, or Post-Effective Amendments to said Registration Statements or any successors thereto, and any supplements or other instruments in connection therewith, and generally to do all such things in my name and behalf in connection therewith as said attorneys-in-fact deem necessary or appropriate, to comply with the provisions of the Securities Act of 1933 and the Investment Company Act of 1940, and all related requirements of the Securities and Exchange Commission.  I hereby ratify and confirm all that said attorneys-in-fact or their substitutes may do or cause to be done by virtue hereof. This power of attorney is effective for all documents filed on or after October 5, 2016.

WITNESS my hand on this 5^th day of October, 2016.

/s/Jonathan Chiel
          Jonathan Chiel

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Document Type	dei_DocumentType	485BPOS
Document Period End Date	dei_DocumentPeriodEndDate	Oct. 31, 2017
Registrant Name	dei_EntityRegistrantName	FIDELITY SALEM STREET TRUST
Central Index Key	dei_EntityCentralIndexKey	0000035315
Amendment Flag	dei_AmendmentFlag	false
Document Creation Date	dei_DocumentCreationDate	Dec. 28, 2017
Document Effective Date	dei_DocumentEffectiveDate	Dec. 30, 2017
Prospectus Date	rr_ProspectusDate	Dec. 30, 2017

XML

10.31 Fidelity SAI International Index Fund PRO-03 | Fidelity® SAI International Index Fund

Fund Summary

Fund:
Fidelity® SAI International Index Fund

Investment Objective

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

Fee Table

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder fees

Shareholder Fees {- Fidelity® SAI International Index Fund}	10.31 Fidelity SAI International Index Fund PRO-03 Fidelity® SAI International Index Fund USD ($)
(fees paid directly from your investment)	none

Annual Operating Expenses

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

Annual Operating Expenses {- Fidelity® SAI International Index Fund}	10.31 Fidelity SAI International Index Fund PRO-03 Fidelity® SAI International Index Fund Fidelity SAI International Index Fund-Default
Management fee	0.05%	[1]
Distribution and/or Service (12b-1) fees	none
Other expenses	0.075%	[1]
Total annual operating expenses	0.125%
Fee waiver and/or expense reimbursement	0.08%	[2]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.045%

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:

Sell All Shares

Expense Example {- Fidelity® SAI International Index Fund}	10.31 Fidelity SAI International Index Fund PRO-03 Fidelity® SAI International Index Fund Fidelity SAI International Index Fund-Default USD ($)
1 year	$ 5
3 years	31
5 years	61
10 years	$ 151

Portfolio Turnover

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 2% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in securities included in the MSCI EAFE 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, and country weightings to attempt to replicate the returns of the MSCI EAFE Index.

• Lending securities to earn income for the fund.

Principal Investment Risks

• 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 Exposure. Foreign 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. 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

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.

Performance

Performance history will be available for the fund after the fund has been in operation for one calendar year.

[1] Adjusted to reflect current fees.

[2] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse the fund to the extent that total operating expenses (excluding interest, certain taxes, certain securities lending costs, brokerage commissions, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any), as a percentage of its average net assets, exceed 0.045%. This arrangement will remain in effect through December 31, 2018. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Dec. 30, 2017

10.31 Fidelity SAI International Index Fund PRO-03 | Fidelity® SAI International Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund:
Fidelity® SAI International 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 foreign stock markets.

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)

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

December 31, 2018

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 2% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

2.00%

Expenses Restated to Reflect Current [Text]

rr_ExpensesRestatedToReflectCurrent

Adjusted to reflect current fees.

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:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

Sell All Shares

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 MSCI EAFE 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, and country weightings to attempt to replicate the returns of the MSCI EAFE Index.

• Lending securities to earn income for the fund.

Risk [Heading]

rr_RiskHeading

Principal Investment Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• 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 Exposure. Foreign 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. 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

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]

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

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]

rr_PerformanceOneYearOrLess

Performance history will be available for the fund after the fund has been in operation for one calendar year.

10.31 Fidelity SAI International Index Fund PRO-03 | Fidelity® SAI International Index Fund | Fidelity SAI International Index Fund-Default

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.05%

^[1]

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.075%

^[1]

Total annual operating expenses

rr_ExpensesOverAssets

0.125%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.08%

^[2]

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

rr_NetExpensesOverAssets

0.045%

1 year

rr_ExpenseExampleYear01

$ 5

3 years

rr_ExpenseExampleYear03

31

5 years

rr_ExpenseExampleYear05

61

10 years

rr_ExpenseExampleYear10

$ 151

[1] Adjusted to reflect current fees.

[2] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse the fund to the extent that total operating expenses (excluding interest, certain taxes, certain securities lending costs, brokerage commissions, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any), as a percentage of its average net assets, exceed 0.045%. This arrangement will remain in effect through December 31, 2018. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees.

XML

10.31 Fidelity Flex International Index Fund PRO-03 | Fidelity Flex℠ International Index Fund

Fund Summary

Fund:
Fidelity Flex℠ International Index Fund

Investment Objective

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

Fee Table

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder fees

Shareholder Fees {- Fidelity Flex℠ International Index Fund}	10.31 Fidelity Flex International Index Fund PRO-03 Fidelity Flex℠ International Index Fund USD ($)
(fees paid directly from your investment)	none

Annual Operating Expenses

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

Annual Operating Expenses {- Fidelity Flex℠ International Index Fund}	10.31 Fidelity Flex International Index Fund PRO-03 Fidelity Flex℠ International Index Fund Fidelity Flex International Index Fund-Default
Management fee	none
Distribution and/or Service (12b-1) fees	none
Other expenses	none
Acquired fund fees and expenses	0.01%
Total annual operating expenses	0.01%	[1],[2]

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. 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:

Sell All Shares

Expense Example {- Fidelity Flex℠ International Index Fund}	10.31 Fidelity Flex International Index Fund PRO-03 Fidelity Flex℠ International Index Fund Fidelity Flex International Index Fund-Default USD ($)
1 year	$ 1
3 years	3
5 years	6
10 years	$ 13

Portfolio Turnover

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 March 9, 2017 to October 31, 2017, the fund's portfolio turnover rate was 18% (annualized) of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in securities included in the MSCI ACWI (All Country World Index) ex USA Index and in depository receipts representing securities included in the index. The MSCI ACWI (All Country World Index) ex USA Index is a market capitalization-weighted index designed to measure the investable equity market performance for global investors of developed and emerging markets, excluding the United States.

• 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 MSCI ACWI (All Country World Index) ex USA Index.

• Lending securities to earn income for the fund.

Principal Investment Risks

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

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.

Performance

Performance history will be available for the fund after the fund has been in operation for one calendar year.

[1] Differs from the ratios of expenses to average net assets in the Financial Highlights section of the prospectus because of acquired fund fees and expenses.

[2] The fund is available only to certain fee-based accounts offered by Fidelity. Managed account clients, retirement plans, plan sponsors and/or plan participants pay a wrap fee that covers investment advisory and administrative services.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Dec. 30, 2017

10.31 Fidelity Flex International Index Fund PRO-03 | Fidelity Flex℠ International Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund:
Fidelity Flex℠ International 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 foreign developed and emerging stock markets.

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. For the period from March 9, 2017 to October 31, 2017, the fund's portfolio turnover rate was 18% (annualized) of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

18.00%

Expenses Not Correlated to Ratio Due to Acquired Fund Fees [Text]

rr_ExpensesNotCorrelatedToRatioDueToAcquiredFundFees

Differs from the ratios of expenses to average net assets in the Financial Highlights section of the prospectus because of acquired fund fees and expenses.

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. 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:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

Sell All Shares

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 MSCI ACWI (All Country World Index) ex USA Index and in depository receipts representing securities included in the index. The MSCI ACWI (All Country World Index) ex USA Index is a market capitalization-weighted index designed to measure the investable equity market performance for global investors of developed and emerging markets, excluding the United States.

• 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 MSCI ACWI (All Country World Index) ex USA Index.

• Lending securities to earn income for the fund.

Risk [Heading]

rr_RiskHeading

Principal Investment Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

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]

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

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]

rr_PerformanceOneYearOrLess

Performance history will be available for the fund after the fund has been in operation for one calendar year.

10.31 Fidelity Flex International Index Fund PRO-03 | Fidelity Flex℠ International Index Fund | Fidelity Flex International Index Fund-Default

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

none

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

none

Acquired fund fees and expenses

rr_AcquiredFundFeesAndExpensesOverAssets

0.01%

Total annual operating expenses

rr_ExpensesOverAssets

0.01%

^[1],[2]

1 year

rr_ExpenseExampleYear01

$ 1

3 years

rr_ExpenseExampleYear03

3

5 years

rr_ExpenseExampleYear05

6

10 years

rr_ExpenseExampleYear10

$ 13

[1] Differs from the ratios of expenses to average net assets in the Financial Highlights section of the prospectus because of acquired fund fees and expenses.

[2] The fund is available only to certain fee-based accounts offered by Fidelity. Managed account clients, retirement plans, plan sponsors and/or plan participants pay a wrap fee that covers investment advisory and administrative services.

XML

10.31 Fidelity Sustainability Index Funds Combo Institutional PRO-02 | Fidelity® International Sustainability Index Fund

Fund Summary

Fund/Class:
Fidelity® International Sustainability Index Fund/Institutional

Investment Objective

The fund seeks to provide investment results that correspond to the total return of the MSCI ACWI (All Country World Index) ex USA ESG Leaders Index.

Fee Table

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder fees

Shareholder Fees {- Fidelity® International Sustainability Index Fund}	10.31 Fidelity Sustainability Index Funds Combo Institutional PRO-02 Fidelity® International Sustainability Index Fund USD ($)
(fees paid directly from your investment)	none

Annual Operating Expenses

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

Annual Operating Expenses {- Fidelity® International Sustainability Index Fund}	10.31 Fidelity Sustainability Index Funds Combo Institutional PRO-02 Fidelity® International Sustainability Index Fund Fidelity International Sustainability Index Fund-Institutional Class
Management fee	0.17%
Distribution and/or Service (12b-1) fees	none
Other expenses	0.03%	[1]
Total annual operating expenses	0.20%

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:

Sell All Shares

Expense Example {- Fidelity® International Sustainability Index Fund}	10.31 Fidelity Sustainability Index Funds Combo Institutional PRO-02 Fidelity® International Sustainability Index Fund Fidelity International Sustainability Index Fund-Institutional Class USD ($)
1 year	$ 20
3 years	$ 64

Portfolio Turnover

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 May 9, 2017 to October 31, 2017, the fund's portfolio turnover rate was 4% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in securities included in the MSCI ACWI (All Country World Index) ex USA ESG Leaders Index and in depository receipts representing securities included in the index. The MSCI ACWI (All Country World Index) ex USA ESG Leaders Index is a capitalization-weighted index that provides exposure to companies with high environmental, social, and governance (ESG) performance relative to their sector peers, as rated by MSCI ESG Research. The MSCI ACWI (All Country World Index) ex USA ESG Leaders Index consists of large- and mid-cap companies across developed and emerging markets, excluding the United States.

• 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 MSCI ACWI (All Country World Index) ex USA ESG Leaders Index.

• Lending securities to earn income for the fund.

Principal Investment Risks

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

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.

Performance

Performance history will be available for the fund after the fund has been in operation for one calendar year.

[1] Based on estimated amounts for the current fiscal year.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Dec. 30, 2017

10.31 Fidelity Sustainability Index Funds Combo Institutional PRO-02 | Fidelity® International Sustainability Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® International Sustainability Index Fund/Institutional

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 MSCI ACWI (All Country World Index) ex USA ESG Leaders Index.

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. For the period from May 9, 2017 to October 31, 2017, the fund's portfolio turnover rate was 4% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

4.00%

Other Expenses, New Fund, Based on Estimates [Text]

rr_OtherExpensesNewFundBasedOnEstimates

Based on estimated amounts for the current fiscal year.

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:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

Sell All Shares

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 MSCI ACWI (All Country World Index) ex USA ESG Leaders Index and in depository receipts representing securities included in the index. The MSCI ACWI (All Country World Index) ex USA ESG Leaders Index is a capitalization-weighted index that provides exposure to companies with high environmental, social, and governance (ESG) performance relative to their sector peers, as rated by MSCI ESG Research. The MSCI ACWI (All Country World Index) ex USA ESG Leaders Index consists of large- and mid-cap companies across developed and emerging markets, excluding the United States.

• 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 MSCI ACWI (All Country World Index) ex USA ESG Leaders Index.

• Lending securities to earn income for the fund.

Risk [Heading]

rr_RiskHeading

Principal Investment Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

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]

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

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]

rr_PerformanceOneYearOrLess

Performance history will be available for the fund after the fund has been in operation for one calendar year.

10.31 Fidelity Sustainability Index Funds Combo Institutional PRO-02 | Fidelity® International Sustainability Index Fund | Fidelity International Sustainability Index Fund-Institutional Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.17%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.03%

^[1]

Total annual operating expenses

rr_ExpensesOverAssets

0.20%

1 year

rr_ExpenseExampleYear01

$ 20

3 years

rr_ExpenseExampleYear03

$ 64

[1] Based on estimated amounts for the current fiscal year.

XML

10.31 Fidelity Sustainability Index Funds Combo Institutional PRO-02 | Fidelity® U.S. Sustainability Index Fund

Fund Summary

Fund/Class:
Fidelity® U.S. Sustainability Index Fund/Institutional

Investment Objective

The fund seeks to provide investment results that correspond to the total return of the MSCI USA ESG Leaders Index.

Fee Table

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder fees

Shareholder Fees {- Fidelity® U.S. Sustainability Index Fund}	10.31 Fidelity Sustainability Index Funds Combo Institutional PRO-02 Fidelity® U.S. Sustainability Index Fund USD ($)
(fees paid directly from your investment)	none

Annual Operating Expenses

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

Annual Operating Expenses {- Fidelity® U.S. Sustainability Index Fund}	10.31 Fidelity Sustainability Index Funds Combo Institutional PRO-02 Fidelity® U.S. Sustainability Index Fund Fidelity U.S. Sustainability Index Fund-Institutional Class
Management fee	0.08%
Distribution and/or Service (12b-1) fees	none
Other expenses	0.03%	[1]
Total annual operating expenses	0.11%

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:

Sell All Shares

Expense Example {- Fidelity® U.S. Sustainability Index Fund}	10.31 Fidelity Sustainability Index Funds Combo Institutional PRO-02 Fidelity® U.S. Sustainability Index Fund Fidelity U.S. Sustainability Index Fund-Institutional Class USD ($)
1 year	$ 11
3 years	$ 35

Portfolio Turnover

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 May 9, 2017 to October 31, 2017, the fund's portfolio turnover rate was 3% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in securities included in the MSCI USA ESG Leaders Index, which represents the performance of stocks of large- to mid-capitalization U.S. companies with high environmental, social, and governance (ESG) performance relative to their sector peers, as rated by MSCI ESG Research.

• Using statistical sampling techniques based on such factors as capitalization, industry exposures, dividend yield, price/earnings (P/E) ratio, price/book (P/B) ratio, and earnings growth to attempt to replicate the returns of the MSCI USA ESG Leaders Index using a smaller number of securities.

• Lending securities to earn income for the fund.

Principal Investment Risks

• 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.

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

• Mid Cap Investing. The value of securities of medium size, less well-known issuers can perform differently from the market as a whole and other types of stocks and can be more volatile than that of larger issuers.

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.

Performance

Performance history will be available for the fund after the fund has been in operation for one calendar year.

[1] Based on estimated amounts for the current fiscal year.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Dec. 30, 2017

10.31 Fidelity Sustainability Index Funds Combo Institutional PRO-02 | Fidelity® U.S. Sustainability Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® U.S. Sustainability Index Fund/Institutional

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 MSCI USA ESG Leaders Index.

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. For the period from May 9, 2017 to October 31, 2017, the fund's portfolio turnover rate was 3% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

3.00%

Other Expenses, New Fund, Based on Estimates [Text]

rr_OtherExpensesNewFundBasedOnEstimates

Based on estimated amounts for the current fiscal year.

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:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

Sell All Shares

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 MSCI USA ESG Leaders Index, which represents the performance of stocks of large- to mid-capitalization U.S. companies with high environmental, social, and governance (ESG) performance relative to their sector peers, as rated by MSCI ESG Research.

• Using statistical sampling techniques based on such factors as capitalization, industry exposures, dividend yield, price/earnings (P/E) ratio, price/book (P/B) ratio, and earnings growth to attempt to replicate the returns of the MSCI USA ESG Leaders Index using a smaller number of securities.

• Lending securities to earn income for the fund.

Risk [Heading]

rr_RiskHeading

Principal Investment Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• 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.

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

• Mid Cap Investing. The value of securities of medium size, less well-known issuers can perform differently from the market as a whole and other types of stocks and can be more volatile than that of larger issuers.

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]

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

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]

rr_PerformanceOneYearOrLess

Performance history will be available for the fund after the fund has been in operation for one calendar year.

10.31 Fidelity Sustainability Index Funds Combo Institutional PRO-02 | Fidelity® U.S. Sustainability Index Fund | Fidelity U.S. Sustainability Index Fund-Institutional Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.08%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.03%

^[1]

Total annual operating expenses

rr_ExpensesOverAssets

0.11%

1 year

rr_ExpenseExampleYear01

$ 11

3 years

rr_ExpenseExampleYear03

$ 35

[1] Based on estimated amounts for the current fiscal year.

XML

10.31 Fidelity Sustainability Index Funds Combo Investor Premium PRO-02 | Fidelity® International Sustainability Index Fund

Fund Summary

Fund/Class:
Fidelity® International Sustainability Index Fund/Investor, Premium

Investment Objective

The fund seeks to provide investment results that correspond to the total return of the MSCI ACWI (All Country World Index) ex USA ESG Leaders Index.

Fee Table

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder fees

Shareholder Fees {- Fidelity® International Sustainability Index Fund}	10.31 Fidelity Sustainability Index Funds Combo Investor Premium PRO-02 Fidelity® International Sustainability Index Fund USD ($)
(fees paid directly from your investment)	none

Annual Operating Expenses

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

Annual Operating Expenses {- Fidelity® International Sustainability Index Fund} - 10.31 Fidelity Sustainability Index Funds Combo Investor Premium PRO-02 - Fidelity® International Sustainability Index Fund		Fidelity International Sustainability Index Fund-Investor Class	Fidelity International Sustainability Index Fund-Premium Class
Management fee		0.17%	0.17%
Distribution and/or Service (12b-1) fees		none	none
Other expenses	[1]	0.13%	0.08%
Total annual operating expenses		0.30%	0.25%

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:

Sell All Shares

Expense Example {- Fidelity® International Sustainability Index Fund} - 10.31 Fidelity Sustainability Index Funds Combo Investor Premium PRO-02 - Fidelity® International Sustainability Index Fund - USD ($)	Fidelity International Sustainability Index Fund-Investor Class	Fidelity International Sustainability Index Fund-Premium Class
1 year	$ 31	$ 26
3 years	$ 97	$ 80

Portfolio Turnover

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 May 9, 2017 to October 31, 2017, the fund's portfolio turnover rate was 4% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in securities included in the MSCI ACWI (All Country World Index) ex USA ESG Leaders Index and in depository receipts representing securities included in the index. The MSCI ACWI (All Country World Index) ex USA ESG Leaders Index is a capitalization-weighted index that provides exposure to companies with high environmental, social, and governance (ESG) performance relative to their sector peers, as rated by MSCI ESG Research. The MSCI ACWI (All Country World Index) ex USA ESG Leaders Index consists of large- and mid-cap companies across developed and emerging markets, excluding the United States.

• 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 MSCI ACWI (All Country World Index) ex USA ESG Leaders Index.

• Lending securities to earn income for the fund.

Principal Investment Risks

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

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.

Performance

Performance history will be available for the fund after the fund has been in operation for one calendar year.

[1] Based on estimated amounts for the current fiscal year.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Dec. 30, 2017

10.31 Fidelity Sustainability Index Funds Combo Investor Premium PRO-02 | Fidelity® International Sustainability Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® International Sustainability Index Fund/Investor, Premium

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 MSCI ACWI (All Country World Index) ex USA ESG Leaders Index.

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. For the period from May 9, 2017 to October 31, 2017, the fund's portfolio turnover rate was 4% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

4.00%

Other Expenses, New Fund, Based on Estimates [Text]

rr_OtherExpensesNewFundBasedOnEstimates

Based on estimated amounts for the current fiscal year.

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:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

Sell All Shares

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 MSCI ACWI (All Country World Index) ex USA ESG Leaders Index and in depository receipts representing securities included in the index. The MSCI ACWI (All Country World Index) ex USA ESG Leaders Index is a capitalization-weighted index that provides exposure to companies with high environmental, social, and governance (ESG) performance relative to their sector peers, as rated by MSCI ESG Research. The MSCI ACWI (All Country World Index) ex USA ESG Leaders Index consists of large- and mid-cap companies across developed and emerging markets, excluding the United States.

• 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 MSCI ACWI (All Country World Index) ex USA ESG Leaders Index.

• Lending securities to earn income for the fund.

Risk [Heading]

rr_RiskHeading

Principal Investment Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

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]

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

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]

rr_PerformanceOneYearOrLess

Performance history will be available for the fund after the fund has been in operation for one calendar year.

10.31 Fidelity Sustainability Index Funds Combo Investor Premium PRO-02 | Fidelity® International Sustainability Index Fund | Fidelity International Sustainability Index Fund-Investor Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.17%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.13%

^[1]

Total annual operating expenses

rr_ExpensesOverAssets

0.30%

1 year

rr_ExpenseExampleYear01

$ 31

3 years

rr_ExpenseExampleYear03

$ 97

10.31 Fidelity Sustainability Index Funds Combo Investor Premium PRO-02 | Fidelity® International Sustainability Index Fund | Fidelity International Sustainability Index Fund-Premium Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.17%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.08%

^[1]

Total annual operating expenses

rr_ExpensesOverAssets

0.25%

1 year

rr_ExpenseExampleYear01

$ 26

3 years

rr_ExpenseExampleYear03

$ 80

[1] Based on estimated amounts for the current fiscal year.

XML

10.31 Fidelity Sustainability Index Funds Combo Investor Premium PRO-02 | Fidelity® U.S. Sustainability Index Fund

Fund Summary

Fund/Class:
Fidelity® U.S. Sustainability Index Fund/Investor, Premium

Investment Objective

The fund seeks to provide investment results that correspond to the total return of the MSCI USA ESG Leaders Index.

Fee Table

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder fees

Shareholder Fees {- Fidelity® U.S. Sustainability Index Fund}	10.31 Fidelity Sustainability Index Funds Combo Investor Premium PRO-02 Fidelity® U.S. Sustainability Index Fund USD ($)
(fees paid directly from your investment)	none

Annual Operating Expenses

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

Annual Operating Expenses {- Fidelity® U.S. Sustainability Index Fund} - 10.31 Fidelity Sustainability Index Funds Combo Investor Premium PRO-02 - Fidelity® U.S. Sustainability Index Fund		Fidelity U.S. Sustainability Index Fund-Investor Class	Fidelity U.S. Sustainability Index Fund-Premium Class
Management fee		0.08%	0.08%
Distribution and/or Service (12b-1) fees		none	none
Other expenses	[1]	0.13%	0.06%
Total annual operating expenses		0.21%	0.14%

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:

Sell All Shares

Expense Example {- Fidelity® U.S. Sustainability Index Fund} - 10.31 Fidelity Sustainability Index Funds Combo Investor Premium PRO-02 - Fidelity® U.S. Sustainability Index Fund - USD ($)	Fidelity U.S. Sustainability Index Fund-Investor Class	Fidelity U.S. Sustainability Index Fund-Premium Class
1 year	$ 22	$ 14
3 years	$ 68	$ 45

Portfolio Turnover

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 May 9, 2017 to October 31, 2017, the fund's portfolio turnover rate was 3% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in securities included in the MSCI USA ESG Leaders Index, which represents the performance of stocks of large- to mid-capitalization U.S. companies with high environmental, social, and governance (ESG) performance relative to their sector peers, as rated by MSCI ESG Research.

• Using statistical sampling techniques based on such factors as capitalization, industry exposures, dividend yield, price/earnings (P/E) ratio, price/book (P/B) ratio, and earnings growth to attempt to replicate the returns of the MSCI USA ESG Leaders Index using a smaller number of securities.

• Lending securities to earn income for the fund.

Principal Investment Risks

• 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.

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

• Mid Cap Investing. The value of securities of medium size, less well-known issuers can perform differently from the market as a whole and other types of stocks and can be more volatile than that of larger issuers.

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.

Performance

Performance history will be available for the fund after the fund has been in operation for one calendar year.

[1] Based on estimated amounts for the current fiscal year.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Dec. 30, 2017

10.31 Fidelity Sustainability Index Funds Combo Investor Premium PRO-02 | Fidelity® U.S. Sustainability Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® U.S. Sustainability Index Fund/Investor, Premium

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 MSCI USA ESG Leaders Index.

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. For the period from May 9, 2017 to October 31, 2017, the fund's portfolio turnover rate was 3% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

3.00%

Other Expenses, New Fund, Based on Estimates [Text]

rr_OtherExpensesNewFundBasedOnEstimates

Based on estimated amounts for the current fiscal year.

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:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

Sell All Shares

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 MSCI USA ESG Leaders Index, which represents the performance of stocks of large- to mid-capitalization U.S. companies with high environmental, social, and governance (ESG) performance relative to their sector peers, as rated by MSCI ESG Research.

• Using statistical sampling techniques based on such factors as capitalization, industry exposures, dividend yield, price/earnings (P/E) ratio, price/book (P/B) ratio, and earnings growth to attempt to replicate the returns of the MSCI USA ESG Leaders Index using a smaller number of securities.

• Lending securities to earn income for the fund.

Risk [Heading]

rr_RiskHeading

Principal Investment Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• 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.

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

• Mid Cap Investing. The value of securities of medium size, less well-known issuers can perform differently from the market as a whole and other types of stocks and can be more volatile than that of larger issuers.

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]

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

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]

rr_PerformanceOneYearOrLess

Performance history will be available for the fund after the fund has been in operation for one calendar year.

10.31 Fidelity Sustainability Index Funds Combo Investor Premium PRO-02 | Fidelity® U.S. Sustainability Index Fund | Fidelity U.S. Sustainability Index Fund-Investor Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.08%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.13%

^[1]

Total annual operating expenses

rr_ExpensesOverAssets

0.21%

1 year

rr_ExpenseExampleYear01

$ 22

3 years

rr_ExpenseExampleYear03

$ 68

10.31 Fidelity Sustainability Index Funds Combo Investor Premium PRO-02 | Fidelity® U.S. Sustainability Index Fund | Fidelity U.S. Sustainability Index Fund-Premium Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.08%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.06%

^[1]

Total annual operating expenses

rr_ExpensesOverAssets

0.14%

1 year

rr_ExpenseExampleYear01

$ 14

3 years

rr_ExpenseExampleYear03

$ 45

[1] Based on estimated amounts for the current fiscal year.

XML

10.31 Fidelity International Index Funds Institutional, Institutional Premium Combo PRO-09 | Fidelity® Emerging Markets Index Fund

Fund Summary

Fund/Class:
Fidelity® Emerging Markets Index Fund/Institutional, Institutional Premium

Investment Objective

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

Fee Table

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder fees

Shareholder Fees {- Fidelity® Emerging Markets Index Fund}	10.31 Fidelity International Index Funds Institutional, Institutional Premium Combo PRO-09 Fidelity® Emerging Markets Index Fund USD ($)
(fees paid directly from your investment)	none

Annual Operating Expenses

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

Annual Operating Expenses {- Fidelity® Emerging Markets Index Fund} - 10.31 Fidelity International Index Funds Institutional, Institutional Premium Combo PRO-09 - Fidelity® Emerging Markets Index Fund		Fidelity Emerging Markets Index Fund-Institutional Class	Fidelity Emerging Markets Index Fund-Institutional Premium Class
Management fee	[1]	0.08%	0.08%
Distribution and/or Service (12b-1) fees		none	none
Other expenses		0.02%	none
Total annual operating expenses		0.10%	0.08%

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:

Sell All Shares

Expense Example {- Fidelity® Emerging Markets Index Fund} - 10.31 Fidelity International Index Funds Institutional, Institutional Premium Combo PRO-09 - Fidelity® Emerging Markets Index Fund - USD ($)	Fidelity Emerging Markets Index Fund-Institutional Class	Fidelity Emerging Markets Index Fund-Institutional Premium Class
1 year	$ 10	$ 8
3 years	32	26
5 years	56	45
10 years	$ 128	$ 103

Portfolio Turnover

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 4% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in securities included in the MSCI Emerging Markets 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 MSCI Emerging Markets Index.

• Lending securities to earn income for the fund.

Principal Investment Risks

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

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.

Performance

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. Prior to May 4, 2016, the fund compared its performance to a different benchmark. The fund's historical performance may not represent its current investment policies. Past performance (before and after taxes) is not an indication of future performance.

Visit www.401k.com and log in for more recent performance information.

Year-by-Year Returns

Calendar Years

During the periods shown in the chart for Institutional Class:	Returns	Quarter ended
Highest Quarter Return	13.15%	March 31, 2012
Lowest Quarter Return	-18.34%	September 30, 2015
Year-to-Date Return	28.10%	September 30, 2017

Average Annual Returns

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. After-tax returns for Institutional Class are shown in the table below and after-tax returns for other classes will vary. 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.

For the periods ended December 31, 2016

Effective May 4, 2016, the fund began comparing its performance to MSCI Emerging Markets Index rather than FTSE^® Emerging Index because the MSCI Emerging Markets Index conforms more closely to the fund's investment policies.

Average Annual Total Returns{- Fidelity® Emerging Markets Index Fund} - 10.31 Fidelity International Index Funds Institutional, Institutional Premium Combo PRO-09 - Fidelity® Emerging Markets Index Fund	Past 1 year	Past 5 years	Since Inception	[1]	Inception Date
Fidelity Emerging Markets Index Fund-Institutional Class					Sep. 08, 2011
Fidelity Emerging Markets Index Fund-Institutional Class | Return Before Taxes	11.37%	1.09%	(1.10%)
Fidelity Emerging Markets Index Fund-Institutional Class | After Taxes on Distributions	11.00%	0.66%	(1.51%)		Sep. 08, 2011
Fidelity Emerging Markets Index Fund-Institutional Class | After Taxes on Distributions and Sales	6.86%	0.90%	(0.77%)		Sep. 08, 2011
Fidelity Emerging Markets Index Fund-Institutional Premium Class					Sep. 08, 2011
Fidelity Emerging Markets Index Fund-Institutional Premium Class | Return Before Taxes	11.26%	1.09%	(1.10%)
MSCI Emerging Markets Index(reflects no deduction for fees or expenses)	11.23%	1.33%	(0.56%)
Fidelity Emerging Markets Index Fund Linked Index℠(reflects no deduction for fees, expenses, or taxes)	11.78%	1.84%	(0.19%)

[1] Adjusted to reflect current fees.

[1] From September 8, 2011

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Dec. 30, 2017

10.31 Fidelity International Index Funds Institutional, Institutional Premium Combo PRO-09 | Fidelity® Emerging Markets Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® Emerging Markets Index Fund/Institutional, Institutional Premium

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 emerging stock markets.

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 4% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

4.00%

Expenses Restated to Reflect Current [Text]

rr_ExpensesRestatedToReflectCurrent

Adjusted to reflect current fees.

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:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

Sell All Shares

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 MSCI Emerging Markets 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 MSCI Emerging Markets Index.

• Lending securities to earn income for the fund.

Risk [Heading]

rr_RiskHeading

Principal Investment Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

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]

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. Prior to May 4, 2016, the fund compared its performance to a different benchmark. The fund's historical performance may not represent its current investment policies. Past performance (before and after taxes) is not an indication of future performance.

Visit www.401k.com and log in 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.401k.com and log in

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

Calendar Years

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart for Institutional Class:	Returns	Quarter ended
Highest Quarter Return	13.15%	March 31, 2012
Lowest Quarter Return	-18.34%	September 30, 2015
Year-to-Date Return	28.10%	September 30, 2017

Performance Table Heading

rr_PerformanceTableHeading

Average Annual Returns

Performance Table Market Index Changed

rr_PerformanceTableMarketIndexChanged

Effective May 4, 2016, the fund began comparing its performance to MSCI Emerging Markets Index rather than FTSE® Emerging Index because the MSCI Emerging Markets Index conforms more closely to the fund's investment policies.

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 One Class of after Tax Shown [Text]

rr_PerformanceTableOneClassOfAfterTaxShown

After-tax returns for Institutional Class are shown in the table below and after-tax returns for other classes will vary.

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. After-tax returns for Institutional Class are shown in the table below and after-tax returns for other classes will vary. 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.

For the periods ended December 31, 2016

Effective May 4, 2016, the fund began comparing its performance to MSCI Emerging Markets Index rather than FTSE^® Emerging Index because the MSCI Emerging Markets Index conforms more closely to the fund's investment policies.

10.31 Fidelity International Index Funds Institutional, Institutional Premium Combo PRO-09 | Fidelity® Emerging Markets Index Fund | Fidelity Emerging Markets Index Fund-Institutional Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.08%

^[1]

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.02%

Total annual operating expenses

rr_ExpensesOverAssets

0.10%

1 year

rr_ExpenseExampleYear01

$ 10

3 years

rr_ExpenseExampleYear03

32

5 years

rr_ExpenseExampleYear05

56

10 years

rr_ExpenseExampleYear10

$ 128

2012

rr_AnnualReturn2012

17.58%

2013

rr_AnnualReturn2013

(4.62%)

2014

rr_AnnualReturn2014

0.45%

2015

rr_AnnualReturn2015

(15.87%)

2016

rr_AnnualReturn2016

11.37%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Sep. 30, 2017

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

28.10%

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

Sep. 30, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(18.34%)

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

10.31 Fidelity International Index Funds Institutional, Institutional Premium Combo PRO-09 | Fidelity® Emerging Markets Index Fund | Fidelity Emerging Markets Index Fund-Institutional Premium Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.08%

^[1]

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

none

Total annual operating expenses

rr_ExpensesOverAssets

0.08%

1 year

rr_ExpenseExampleYear01

$ 8

3 years

rr_ExpenseExampleYear03

26

5 years

rr_ExpenseExampleYear05

45

10 years

rr_ExpenseExampleYear10

$ 103

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

10.31 Fidelity International Index Funds Institutional, Institutional Premium Combo PRO-09 | Fidelity® Emerging Markets Index Fund | Return Before Taxes | Fidelity Emerging Markets Index Fund-Institutional Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Emerging Markets Index Fund

Label

rr_AverageAnnualReturnLabel

Institutional Class - Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

11.37%

Past 5 years

rr_AverageAnnualReturnYear05

1.09%

Since Inception

rr_AverageAnnualReturnSinceInception

(1.10%)

^[2]

10.31 Fidelity International Index Funds Institutional, Institutional Premium Combo PRO-09 | Fidelity® Emerging Markets Index Fund | Return Before Taxes | Fidelity Emerging Markets Index Fund-Institutional Premium Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Emerging Markets Index Fund

Label

rr_AverageAnnualReturnLabel

Institutional Premium Class - Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

11.26%

Past 5 years

rr_AverageAnnualReturnYear05

1.09%

Since Inception

rr_AverageAnnualReturnSinceInception

(1.10%)

^[2]

10.31 Fidelity International Index Funds Institutional, Institutional Premium Combo PRO-09 | Fidelity® Emerging Markets Index Fund | After Taxes on Distributions | Fidelity Emerging Markets Index Fund-Institutional Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Emerging Markets Index Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions

Past 1 year

rr_AverageAnnualReturnYear01

11.00%

Past 5 years

rr_AverageAnnualReturnYear05

0.66%

Since Inception

rr_AverageAnnualReturnSinceInception

(1.51%)

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

10.31 Fidelity International Index Funds Institutional, Institutional Premium Combo PRO-09 | Fidelity® Emerging Markets Index Fund | After Taxes on Distributions and Sales | Fidelity Emerging Markets Index Fund-Institutional Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Emerging Markets Index Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions and Sale of Fund Shares

Past 1 year

rr_AverageAnnualReturnYear01

6.86%

Past 5 years

rr_AverageAnnualReturnYear05

0.90%

Since Inception

rr_AverageAnnualReturnSinceInception

(0.77%)

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

10.31 Fidelity International Index Funds Institutional, Institutional Premium Combo PRO-09 | Fidelity® Emerging Markets Index Fund | MSCI Emerging Markets Index(reflects no deduction for fees or expenses)

Risk/Return:

rr_RiskReturnAbstract

Past 1 year

rr_AverageAnnualReturnYear01

11.23%

Past 5 years

rr_AverageAnnualReturnYear05

1.33%

Since Inception

rr_AverageAnnualReturnSinceInception

(0.56%)

^[2]

10.31 Fidelity International Index Funds Institutional, Institutional Premium Combo PRO-09 | Fidelity® Emerging Markets Index Fund | Fidelity Emerging Markets Index Fund Linked Index℠(reflects no deduction for fees, expenses, or taxes)

Risk/Return:

rr_RiskReturnAbstract

Past 1 year

rr_AverageAnnualReturnYear01

11.78%

Past 5 years

rr_AverageAnnualReturnYear05

1.84%

Since Inception

rr_AverageAnnualReturnSinceInception

(0.19%)

^[2]

[1] Adjusted to reflect current fees.

[2] From September 8, 2011

XML

10.31 Fidelity International Index Funds Institutional, Institutional Premium Combo PRO-09 | Fidelity® Global ex U.S. Index Fund

Fund Summary

Fund/Class:
Fidelity® Global ex U.S. Index Fund/Institutional, Institutional Premium

Investment Objective

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

Fee Table

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder fees

Shareholder Fees {- Fidelity® Global ex U.S. Index Fund}	10.31 Fidelity International Index Funds Institutional, Institutional Premium Combo PRO-09 Fidelity® Global ex U.S. Index Fund USD ($)
(fees paid directly from your investment)	none

Annual Operating Expenses

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

Annual Operating Expenses {- Fidelity® Global ex U.S. Index Fund} - 10.31 Fidelity International Index Funds Institutional, Institutional Premium Combo PRO-09 - Fidelity® Global ex U.S. Index Fund		Fidelity Global ex U.S. Index Fund-Institutional Class	Fidelity Global ex U.S. Index Fund-Institutional Premium Class
Management fee		0.06%	0.06%
Distribution and/or Service (12b-1) fees		none	none
Other expenses		0.02%	none
Total annual operating expenses	[1]	0.08%	0.06%

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:

Sell All Shares

Expense Example {- Fidelity® Global ex U.S. Index Fund} - 10.31 Fidelity International Index Funds Institutional, Institutional Premium Combo PRO-09 - Fidelity® Global ex U.S. Index Fund - USD ($)	Fidelity Global ex U.S. Index Fund-Institutional Class	Fidelity Global ex U.S. Index Fund-Institutional Premium Class
1 year	$ 8	$ 6
3 years	26	19
5 years	45	34
10 years	$ 103	$ 77

Portfolio Turnover

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 9% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in securities included in the MSCI ACWI (All Country World Index) ex USA 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 MSCI ACWI (All Country World Index) ex USA Index.

• Lending securities to earn income for the fund.

Principal Investment Risks

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

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.

Performance

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.401k.com and log in for more recent performance information.

Year-by-Year Returns

Calendar Years

During the periods shown in the chart for Institutional Class:	Returns	Quarter ended
Highest Quarter Return	11.44%	March 31, 2012
Lowest Quarter Return	-11.94%	September 30, 2015
Year-to-Date Return	21.60%	September 30, 2017

Average Annual Returns

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. After-tax returns for Institutional Class are shown in the table below and after-tax returns for other classes will vary. 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).

For the periods ended December 31, 2016

Average Annual Total Returns{- Fidelity® Global ex U.S. Index Fund} - 10.31 Fidelity International Index Funds Institutional, Institutional Premium Combo PRO-09 - Fidelity® Global ex U.S. Index Fund	Past 1 year	Past 5 years	Since Inception	[1]	Inception Date
Fidelity Global ex U.S. Index Fund-Institutional Class					Sep. 08, 2011
Fidelity Global ex U.S. Index Fund-Institutional Class | Return Before Taxes	4.56%	4.87%	3.70%
Fidelity Global ex U.S. Index Fund-Institutional Class | After Taxes on Distributions	4.16%	4.48%	3.33%		Sep. 08, 2011
Fidelity Global ex U.S. Index Fund-Institutional Class | After Taxes on Distributions and Sales	3.10%	3.92%	2.99%		Sep. 08, 2011
Fidelity Global ex U.S. Index Fund-Institutional Premium Class					Sep. 08, 2011
Fidelity Global ex U.S. Index Fund-Institutional Premium Class | Return Before Taxes	4.59%	4.90%	3.73%
MSCI ACWI (All Country World Index) ex USA Index(reflects no deduction for fees or expenses)	4.68%	5.15%	3.95%

[1] Adjusted to reflect current fees.

[1] From September 8, 2011

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Dec. 30, 2017

10.31 Fidelity International Index Funds Institutional, Institutional Premium Combo PRO-09 | Fidelity® Global ex U.S. Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® Global ex U.S. Index Fund/Institutional, Institutional Premium

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 foreign developed and emerging stock markets.

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 9% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

9.00%

Expenses Restated to Reflect Current [Text]

rr_ExpensesRestatedToReflectCurrent

Adjusted to reflect current fees.

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:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

Sell All Shares

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 MSCI ACWI (All Country World Index) ex USA 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 MSCI ACWI (All Country World Index) ex USA Index.

• Lending securities to earn income for the fund.

Risk [Heading]

rr_RiskHeading

Principal Investment Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

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]

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.401k.com and log in 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.401k.com and log in

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

Calendar Years

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart for Institutional Class:	Returns	Quarter ended
Highest Quarter Return	11.44%	March 31, 2012
Lowest Quarter Return	-11.94%	September 30, 2015
Year-to-Date Return	21.60%	September 30, 2017

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 One Class of after Tax Shown [Text]

rr_PerformanceTableOneClassOfAfterTaxShown

After-tax returns for Institutional Class are shown in the table below and after-tax returns for other classes will vary.

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. After-tax returns for Institutional Class are shown in the table below and after-tax returns for other classes will vary. 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).

For the periods ended December 31, 2016

10.31 Fidelity International Index Funds Institutional, Institutional Premium Combo PRO-09 | Fidelity® Global ex U.S. Index Fund | Fidelity Global ex U.S. Index Fund-Institutional Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.06%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.02%

Total annual operating expenses

rr_ExpensesOverAssets

0.08%

^[1]

1 year

rr_ExpenseExampleYear01

$ 8

3 years

rr_ExpenseExampleYear03

26

5 years

rr_ExpenseExampleYear05

45

10 years

rr_ExpenseExampleYear10

$ 103

2012

rr_AnnualReturn2012

17.64%

2013

rr_AnnualReturn2013

14.24%

2014

rr_AnnualReturn2014

(4.35%)

2015

rr_AnnualReturn2015

(5.61%)

2016

rr_AnnualReturn2016

4.56%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Sep. 30, 2017

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

21.60%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Mar. 31, 2012

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

11.44%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Sep. 30, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(11.94%)

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

10.31 Fidelity International Index Funds Institutional, Institutional Premium Combo PRO-09 | Fidelity® Global ex U.S. Index Fund | Fidelity Global ex U.S. Index Fund-Institutional Premium Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.06%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

none

Total annual operating expenses

rr_ExpensesOverAssets

0.06%

^[1]

1 year

rr_ExpenseExampleYear01

$ 6

3 years

rr_ExpenseExampleYear03

19

5 years

rr_ExpenseExampleYear05

34

10 years

rr_ExpenseExampleYear10

$ 77

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

10.31 Fidelity International Index Funds Institutional, Institutional Premium Combo PRO-09 | Fidelity® Global ex U.S. Index Fund | Return Before Taxes | Fidelity Global ex U.S. Index Fund-Institutional Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Global ex U.S. Index Fund

Label

rr_AverageAnnualReturnLabel

Institutional Class - Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

4.56%

Past 5 years

rr_AverageAnnualReturnYear05

4.87%

Since Inception

rr_AverageAnnualReturnSinceInception

3.70%

^[2]

10.31 Fidelity International Index Funds Institutional, Institutional Premium Combo PRO-09 | Fidelity® Global ex U.S. Index Fund | Return Before Taxes | Fidelity Global ex U.S. Index Fund-Institutional Premium Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Global ex U.S. Index Fund

Label

rr_AverageAnnualReturnLabel

Institutional Premium Class - Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

4.59%

Past 5 years

rr_AverageAnnualReturnYear05

4.90%

Since Inception

rr_AverageAnnualReturnSinceInception

3.73%

^[2]

10.31 Fidelity International Index Funds Institutional, Institutional Premium Combo PRO-09 | Fidelity® Global ex U.S. Index Fund | After Taxes on Distributions | Fidelity Global ex U.S. Index Fund-Institutional Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Global ex U.S. Index Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions

Past 1 year

rr_AverageAnnualReturnYear01

4.16%

Past 5 years

rr_AverageAnnualReturnYear05

4.48%

Since Inception

rr_AverageAnnualReturnSinceInception

3.33%

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

10.31 Fidelity International Index Funds Institutional, Institutional Premium Combo PRO-09 | Fidelity® Global ex U.S. Index Fund | After Taxes on Distributions and Sales | Fidelity Global ex U.S. Index Fund-Institutional Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Global ex U.S. Index Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions and Sale of Fund Shares

Past 1 year

rr_AverageAnnualReturnYear01

3.10%

Past 5 years

rr_AverageAnnualReturnYear05

3.92%

Since Inception

rr_AverageAnnualReturnSinceInception

2.99%

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

10.31 Fidelity International Index Funds Institutional, Institutional Premium Combo PRO-09 | Fidelity® Global ex U.S. Index Fund | MSCI ACWI (All Country World Index) ex USA Index(reflects no deduction for fees or expenses)

Risk/Return:

rr_RiskReturnAbstract

Past 1 year

rr_AverageAnnualReturnYear01

4.68%

Past 5 years

rr_AverageAnnualReturnYear05

5.15%

Since Inception

rr_AverageAnnualReturnSinceInception

3.95%

^[2]

[1] Adjusted to reflect current fees.

[2] From September 8, 2011

XML

10.31 Fidelity International Index Funds Investor, Premium Combo PRO-07 | Fidelity® Emerging Markets Index Fund

Fund Summary

Fund/Class:
Fidelity® Emerging Markets Index Fund/Investor, Premium

Investment Objective

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

Fee Table

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder fees

Shareholder Fees {- Fidelity® Emerging Markets Index Fund}	10.31 Fidelity International Index Funds Investor, Premium Combo PRO-07 Fidelity® Emerging Markets Index Fund USD ($)
(fees paid directly from your investment)	none

Annual Operating Expenses

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

Annual Operating Expenses {- Fidelity® Emerging Markets Index Fund} - 10.31 Fidelity International Index Funds Investor, Premium Combo PRO-07 - Fidelity® Emerging Markets Index Fund		Fidelity Emerging Markets Index Fund-Investor Class	Fidelity Emerging Markets Index Fund-Premium Class
Management fee		0.08%	0.08%
Distribution and/or Service (12b-1) fees		none	none
Other expenses		0.21%	0.05%
Total annual operating expenses	[1]	0.29%	0.13%

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:

Sell All Shares

Expense Example {- Fidelity® Emerging Markets Index Fund} - 10.31 Fidelity International Index Funds Investor, Premium Combo PRO-07 - Fidelity® Emerging Markets Index Fund - USD ($)	Fidelity Emerging Markets Index Fund-Investor Class	Fidelity Emerging Markets Index Fund-Premium Class
1 year	$ 30	$ 13
3 years	93	42
5 years	163	73
10 years	$ 368	$ 166

Portfolio Turnover

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 4% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in securities included in the MSCI Emerging Markets 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 MSCI Emerging Markets Index.

• Lending securities to earn income for the fund.

Principal Investment Risks

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

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.

Performance

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. Prior to May 4, 2016, the fund compared its performance to a different benchmark. The fund's historical performance may not represent its current investment policies. Past performance (before and after taxes) is not an indication of future performance.

Visit www.fidelity.com for more recent performance information.

Year-by-Year Returns

Calendar Years

During the periods shown in the chart for Investor Class:	Returns	Quarter ended
Highest Quarter Return	12.92%	March 31, 2012
Lowest Quarter Return	-18.38%	September 30, 2015
Year-to-Date Return	28.01%	September 30, 2017

Average Annual Returns

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. After-tax returns for Investor Class are shown in the table below and after-tax returns for other classes will vary. 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.

For the periods ended December 31, 2016

Effective May 4, 2016, the fund began comparing its performance to MSCI Emerging Markets Index rather than FTSE^® Emerging Index because the MSCI Emerging Markets Index conforms more closely to the fund's investment policies.

Average Annual Total Returns{- Fidelity® Emerging Markets Index Fund} - 10.31 Fidelity International Index Funds Investor, Premium Combo PRO-07 - Fidelity® Emerging Markets Index Fund	Past 1 year	Past 5 years	Since Inception	[1]	Inception Date
Fidelity Emerging Markets Index Fund-Investor Class					Sep. 08, 2011
Fidelity Emerging Markets Index Fund-Investor Class | Return Before Taxes	11.17%	0.87%	(1.30%)
Fidelity Emerging Markets Index Fund-Investor Class | After Taxes on Distributions	10.90%	0.52%	(1.64%)		Sep. 08, 2011
Fidelity Emerging Markets Index Fund-Investor Class | After Taxes on Distributions and Sales	6.75%	0.77%	(0.89%)		Sep. 08, 2011
Fidelity Emerging Markets Index Fund-Premium Class					Sep. 08, 2011
Fidelity Emerging Markets Index Fund-Premium Class | Return Before Taxes	11.35%	1.00%	(1.18%)
MSCI Emerging Markets Index(reflects no deduction for fees or expenses)	11.23%	1.33%	(0.56%)
Fidelity Emerging Markets Index Fund Linked Index℠(reflects no deduction for fees, expenses, or taxes)	11.78%	1.84%	(0.19%)

[1] Adjusted to reflect current fees.

[1] From September 8, 2011

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Dec. 30, 2017

10.31 Fidelity International Index Funds Investor, Premium Combo PRO-07 | Fidelity® Emerging Markets Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® Emerging Markets Index Fund/Investor, Premium

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 emerging stock markets.

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 4% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

4.00%

Expenses Restated to Reflect Current [Text]

rr_ExpensesRestatedToReflectCurrent

Adjusted to reflect current fees.

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:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

Sell All Shares

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 MSCI Emerging Markets 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 MSCI Emerging Markets Index.

• Lending securities to earn income for the fund.

Risk [Heading]

rr_RiskHeading

Principal Investment Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

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]

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. Prior to May 4, 2016, the fund compared its performance to a different benchmark. The fund's historical performance may not represent its current investment policies. 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

Calendar Years

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart for Investor Class:	Returns	Quarter ended
Highest Quarter Return	12.92%	March 31, 2012
Lowest Quarter Return	-18.38%	September 30, 2015
Year-to-Date Return	28.01%	September 30, 2017

Performance Table Heading

rr_PerformanceTableHeading

Average Annual Returns

Performance Table Market Index Changed

rr_PerformanceTableMarketIndexChanged

Effective May 4, 2016, the fund began comparing its performance to MSCI Emerging Markets Index rather than FTSE® Emerging Index because the MSCI Emerging Markets Index conforms more closely to the fund's investment policies.

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 One Class of after Tax Shown [Text]

rr_PerformanceTableOneClassOfAfterTaxShown

After-tax returns for Investor Class are shown in the table below and after-tax returns for other classes will vary.

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. After-tax returns for Investor Class are shown in the table below and after-tax returns for other classes will vary. 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.

For the periods ended December 31, 2016

Effective May 4, 2016, the fund began comparing its performance to MSCI Emerging Markets Index rather than FTSE^® Emerging Index because the MSCI Emerging Markets Index conforms more closely to the fund's investment policies.

10.31 Fidelity International Index Funds Investor, Premium Combo PRO-07 | Fidelity® Emerging Markets Index Fund | Fidelity Emerging Markets Index Fund-Investor Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.08%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.21%

Total annual operating expenses

rr_ExpensesOverAssets

0.29%

^[1]

1 year

rr_ExpenseExampleYear01

$ 30

3 years

rr_ExpenseExampleYear03

93

5 years

rr_ExpenseExampleYear05

163

10 years

rr_ExpenseExampleYear10

$ 368

2012

rr_AnnualReturn2012

17.20%

2013

rr_AnnualReturn2013

(4.81%)

2014

rr_AnnualReturn2014

0.27%

2015

rr_AnnualReturn2015

(16.04%)

2016

rr_AnnualReturn2016

11.17%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Sep. 30, 2017

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

28.01%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Mar. 31, 2012

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

12.92%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Sep. 30, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(18.38%)

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

10.31 Fidelity International Index Funds Investor, Premium Combo PRO-07 | Fidelity® Emerging Markets Index Fund | Fidelity Emerging Markets Index Fund-Premium Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.08%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.05%

Total annual operating expenses

rr_ExpensesOverAssets

0.13%

^[1]

1 year

rr_ExpenseExampleYear01

$ 13

3 years

rr_ExpenseExampleYear03

42

5 years

rr_ExpenseExampleYear05

73

10 years

rr_ExpenseExampleYear10

$ 166

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

10.31 Fidelity International Index Funds Investor, Premium Combo PRO-07 | Fidelity® Emerging Markets Index Fund | Return Before Taxes | Fidelity Emerging Markets Index Fund-Investor Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Emerging Markets Index Fund

Label

rr_AverageAnnualReturnLabel

Investor Class - Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

11.17%

Past 5 years

rr_AverageAnnualReturnYear05

0.87%

Since Inception

rr_AverageAnnualReturnSinceInception

(1.30%)

^[2]

10.31 Fidelity International Index Funds Investor, Premium Combo PRO-07 | Fidelity® Emerging Markets Index Fund | Return Before Taxes | Fidelity Emerging Markets Index Fund-Premium Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Emerging Markets Index Fund

Label

rr_AverageAnnualReturnLabel

Premium Class - Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

11.35%

Past 5 years

rr_AverageAnnualReturnYear05

1.00%

Since Inception

rr_AverageAnnualReturnSinceInception

(1.18%)

^[2]

10.31 Fidelity International Index Funds Investor, Premium Combo PRO-07 | Fidelity® Emerging Markets Index Fund | After Taxes on Distributions | Fidelity Emerging Markets Index Fund-Investor Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Emerging Markets Index Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions

Past 1 year

rr_AverageAnnualReturnYear01

10.90%

Past 5 years

rr_AverageAnnualReturnYear05

0.52%

Since Inception

rr_AverageAnnualReturnSinceInception

(1.64%)

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

10.31 Fidelity International Index Funds Investor, Premium Combo PRO-07 | Fidelity® Emerging Markets Index Fund | After Taxes on Distributions and Sales | Fidelity Emerging Markets Index Fund-Investor Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Emerging Markets Index Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions and Sale of Fund Shares

Past 1 year

rr_AverageAnnualReturnYear01

6.75%

Past 5 years

rr_AverageAnnualReturnYear05

0.77%

Since Inception

rr_AverageAnnualReturnSinceInception

(0.89%)

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

10.31 Fidelity International Index Funds Investor, Premium Combo PRO-07 | Fidelity® Emerging Markets Index Fund | MSCI Emerging Markets Index(reflects no deduction for fees or expenses)

Risk/Return:

rr_RiskReturnAbstract

Past 1 year

rr_AverageAnnualReturnYear01

11.23%

Past 5 years

rr_AverageAnnualReturnYear05

1.33%

Since Inception

rr_AverageAnnualReturnSinceInception

(0.56%)

^[2]

10.31 Fidelity International Index Funds Investor, Premium Combo PRO-07 | Fidelity® Emerging Markets Index Fund | Fidelity Emerging Markets Index Fund Linked Index℠(reflects no deduction for fees, expenses, or taxes)

Risk/Return:

rr_RiskReturnAbstract

Past 1 year

rr_AverageAnnualReturnYear01

11.78%

Past 5 years

rr_AverageAnnualReturnYear05

1.84%

Since Inception

rr_AverageAnnualReturnSinceInception

(0.19%)

^[2]

[1] Adjusted to reflect current fees.

[2] From September 8, 2011

XML

10.31 Fidelity International Index Funds Investor, Premium Combo PRO-07 | Fidelity® Global ex U.S. Index Fund

Fund Summary

Fund/Class:
Fidelity® Global ex U.S. Index Fund/Investor, Premium

Investment Objective

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

Fee Table

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder fees

Shareholder Fees {- Fidelity® Global ex U.S. Index Fund}	10.31 Fidelity International Index Funds Investor, Premium Combo PRO-07 Fidelity® Global ex U.S. Index Fund USD ($)
(fees paid directly from your investment)	none

Annual Operating Expenses

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

Annual Operating Expenses {- Fidelity® Global ex U.S. Index Fund} - 10.31 Fidelity International Index Funds Investor, Premium Combo PRO-07 - Fidelity® Global ex U.S. Index Fund		Fidelity Global ex U.S. Index Fund-Investor Class	Fidelity Global ex U.S. Index Fund-Premium Class
Management fee		0.06%	0.06%
Distribution and/or Service (12b-1) fees		none	none
Other expenses		0.11%	0.04%
Total annual operating expenses	[1]	0.17%	0.10%

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:

Sell All Shares

Expense Example {- Fidelity® Global ex U.S. Index Fund} - 10.31 Fidelity International Index Funds Investor, Premium Combo PRO-07 - Fidelity® Global ex U.S. Index Fund - USD ($)	Fidelity Global ex U.S. Index Fund-Investor Class	Fidelity Global ex U.S. Index Fund-Premium Class
1 year	$ 17	$ 10
3 years	55	32
5 years	96	56
10 years	$ 217	$ 128

Portfolio Turnover

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 9% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in securities included in the MSCI ACWI (All Country World Index) ex USA 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 MSCI ACWI (All Country World Index) ex USA Index.

• Lending securities to earn income for the fund.

Principal Investment Risks

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

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.

Performance

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.

Year-by-Year Returns

Calendar Years

During the periods shown in the chart for Investor Class:	Returns	Quarter ended
Highest Quarter Return	11.45%	March 31, 2012
Lowest Quarter Return	-11.95%	September 30, 2015
Year-to-Date Return	21.51%	September 30, 2017

Average Annual Returns

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. After-tax returns for Investor Class are shown in the table below and after-tax returns for other classes will vary. 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).

For the periods ended December 31, 2016

Average Annual Total Returns{- Fidelity® Global ex U.S. Index Fund} - 10.31 Fidelity International Index Funds Investor, Premium Combo PRO-07 - Fidelity® Global ex U.S. Index Fund	Past 1 year	Past 5 years	Since Inception	[1]	Inception Date
Fidelity Global ex U.S. Index Fund-Investor Class					Sep. 08, 2011
Fidelity Global ex U.S. Index Fund-Investor Class | Return Before Taxes	4.47%	4.79%	3.60%
Fidelity Global ex U.S. Index Fund-Investor Class | After Taxes on Distributions	4.10%	4.44%	3.27%		Sep. 08, 2011
Fidelity Global ex U.S. Index Fund-Investor Class | After Taxes on Distributions and Sales	3.05%	3.87%	2.92%		Sep. 08, 2011
Fidelity Global ex U.S. Index Fund-Premium Class					Sep. 08, 2011
Fidelity Global ex U.S. Index Fund-Premium Class | Return Before Taxes	4.64%	4.85%	3.66%
MSCI ACWI (All Country World Index) ex USA Index(reflects no deduction for fees or expenses)	4.68%	5.15%	3.95%

[1] Adjusted to reflect current fees.

[1] From September 8, 2011

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Dec. 30, 2017

10.31 Fidelity International Index Funds Investor, Premium Combo PRO-07 | Fidelity® Global ex U.S. Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® Global ex U.S. Index Fund/Investor, Premium

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 foreign developed and emerging stock markets.

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 9% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

9.00%

Expenses Restated to Reflect Current [Text]

rr_ExpensesRestatedToReflectCurrent

Adjusted to reflect current fees.

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:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

Sell All Shares

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 MSCI ACWI (All Country World Index) ex USA 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 MSCI ACWI (All Country World Index) ex USA Index.

• Lending securities to earn income for the fund.

Risk [Heading]

rr_RiskHeading

Principal Investment Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

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]

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

Calendar Years

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart for Investor Class:	Returns	Quarter ended
Highest Quarter Return	11.45%	March 31, 2012
Lowest Quarter Return	-11.95%	September 30, 2015
Year-to-Date Return	21.51%	September 30, 2017

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 One Class of after Tax Shown [Text]

rr_PerformanceTableOneClassOfAfterTaxShown

After-tax returns for Investor Class are shown in the table below and after-tax returns for other classes will vary.

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. After-tax returns for Investor Class are shown in the table below and after-tax returns for other classes will vary. 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).

For the periods ended December 31, 2016

10.31 Fidelity International Index Funds Investor, Premium Combo PRO-07 | Fidelity® Global ex U.S. Index Fund | Fidelity Global ex U.S. Index Fund-Investor Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.06%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.11%

Total annual operating expenses

rr_ExpensesOverAssets

0.17%

^[1]

1 year

rr_ExpenseExampleYear01

$ 17

3 years

rr_ExpenseExampleYear03

55

5 years

rr_ExpenseExampleYear05

96

10 years

rr_ExpenseExampleYear10

$ 217

2012

rr_AnnualReturn2012

17.60%

2013

rr_AnnualReturn2013

14.14%

2014

rr_AnnualReturn2014

(4.44%)

2015

rr_AnnualReturn2015

(5.71%)

2016

rr_AnnualReturn2016

4.47%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Sep. 30, 2017

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

21.51%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Mar. 31, 2012

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

11.45%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Sep. 30, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(11.95%)

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

10.31 Fidelity International Index Funds Investor, Premium Combo PRO-07 | Fidelity® Global ex U.S. Index Fund | Fidelity Global ex U.S. Index Fund-Premium Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.06%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.04%

Total annual operating expenses

rr_ExpensesOverAssets

0.10%

^[1]

1 year

rr_ExpenseExampleYear01

$ 10

3 years

rr_ExpenseExampleYear03

32

5 years

rr_ExpenseExampleYear05

56

10 years

rr_ExpenseExampleYear10

$ 128

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

10.31 Fidelity International Index Funds Investor, Premium Combo PRO-07 | Fidelity® Global ex U.S. Index Fund | Return Before Taxes | Fidelity Global ex U.S. Index Fund-Investor Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Global ex U.S. Index Fund

Label

rr_AverageAnnualReturnLabel

Investor Class - Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

4.47%

Past 5 years

rr_AverageAnnualReturnYear05

4.79%

Since Inception

rr_AverageAnnualReturnSinceInception

3.60%

^[2]

10.31 Fidelity International Index Funds Investor, Premium Combo PRO-07 | Fidelity® Global ex U.S. Index Fund | Return Before Taxes | Fidelity Global ex U.S. Index Fund-Premium Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Global ex U.S. Index Fund

Label

rr_AverageAnnualReturnLabel

Premium Class - Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

4.64%

Past 5 years

rr_AverageAnnualReturnYear05

4.85%

Since Inception

rr_AverageAnnualReturnSinceInception

3.66%

^[2]

10.31 Fidelity International Index Funds Investor, Premium Combo PRO-07 | Fidelity® Global ex U.S. Index Fund | After Taxes on Distributions | Fidelity Global ex U.S. Index Fund-Investor Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Global ex U.S. Index Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions

Past 1 year

rr_AverageAnnualReturnYear01

4.10%

Past 5 years

rr_AverageAnnualReturnYear05

4.44%

Since Inception

rr_AverageAnnualReturnSinceInception

3.27%

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

10.31 Fidelity International Index Funds Investor, Premium Combo PRO-07 | Fidelity® Global ex U.S. Index Fund | After Taxes on Distributions and Sales | Fidelity Global ex U.S. Index Fund-Investor Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Global ex U.S. Index Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions and Sale of Fund Shares

Past 1 year

rr_AverageAnnualReturnYear01

3.05%

Past 5 years

rr_AverageAnnualReturnYear05

3.87%

Since Inception

rr_AverageAnnualReturnSinceInception

2.92%

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

10.31 Fidelity International Index Funds Investor, Premium Combo PRO-07 | Fidelity® Global ex U.S. Index Fund | MSCI ACWI (All Country World Index) ex USA Index(reflects no deduction for fees or expenses)

Risk/Return:

rr_RiskReturnAbstract

Past 1 year

rr_AverageAnnualReturnYear01

4.68%

Past 5 years

rr_AverageAnnualReturnYear05

5.15%

Since Inception

rr_AverageAnnualReturnSinceInception

3.95%

^[2]

[1] Adjusted to reflect current fees.

[2] From September 8, 2011

XML

10.31 Fidelity SAI Minimum Volatility Index Funds Combo PRO-04 | Fidelity® SAI International Minimum Volatility Index Fund

Fund Summary

Fund:
Fidelity® SAI International Minimum Volatility Index Fund

Investment Objective

The fund seeks to provide investment results that correspond to the total return of low volatility foreign stocks.

Fee Table

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder fees

Shareholder Fees {- Fidelity® SAI International Minimum Volatility Index Fund}	10.31 Fidelity SAI Minimum Volatility Index Funds Combo PRO-04 Fidelity® SAI International Minimum Volatility Index Fund USD ($)
(fees paid directly from your investment)	none

Annual Operating Expenses

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

Annual Operating Expenses {- Fidelity® SAI International Minimum Volatility Index Fund}	10.31 Fidelity SAI Minimum Volatility Index Funds Combo PRO-04 Fidelity® SAI International Minimum Volatility Index Fund Fidelity SAI International Minimum Volatility Index Fund-Default
Management fee	0.15%
Distribution and/or Service (12b-1) fees	none
Other expenses	0.16%
Total annual operating expenses	0.31%
Fee waiver and/or expense reimbursement	0.11%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.20%

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:

Sell All Shares

Expense Example {- Fidelity® SAI International Minimum Volatility Index Fund}	10.31 Fidelity SAI Minimum Volatility Index Funds Combo PRO-04 Fidelity® SAI International Minimum Volatility Index Fund Fidelity SAI International Minimum Volatility Index Fund-Default USD ($)
1 year	$ 20
3 years	87
5 years	161
10 years	$ 381

Portfolio Turnover

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 25% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in common stocks included in the MSCI ACWI (All Country World Index) ex USA Minimum Volatility (USD) Index, which reflects the performance of a broad range of stocks across developed (excluding U.S.) and emerging markets that in the aggregate have lower volatility relative to the broader foreign equity markets, and in depository receipts representing securities included in the index.

• Lending securities to earn income for the fund.

Principal Investment Risks

• Minimum Volatility Strategy. Although the fund’s underlying index was created to seek lower volatility relative to a broad range of foreign stocks, there is no guarantee that the fund’s minimum volatility strategy will be successful. There is a risk that the fund may experience more than minimum volatility.

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

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.

Performance

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the performance of the fund's shares over the past 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.

Year-by-Year Returns

Calendar Years

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	4.76%	March 31, 2016
Lowest Quarter Return	-6.75%	December 31, 2016
Year-to-Date Return	16.20%	September 30, 2017

Average Annual Returns

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.

For the periods ended December 31, 2016

Average Annual Total Returns{- Fidelity® SAI International Minimum Volatility Index Fund} - 10.31 Fidelity SAI Minimum Volatility Index Funds Combo PRO-04 - Fidelity® SAI International Minimum Volatility Index Fund	Past 1 year	Since Inception	[1]	Inception Date
Fidelity SAI International Minimum Volatility Index Fund-Default				May 29, 2015
Fidelity SAI International Minimum Volatility Index Fund-Default | Return Before Taxes	2.43%	(2.70%)
Fidelity SAI International Minimum Volatility Index Fund-Default | After Taxes on Distributions	1.71%	(3.32%)		May 29, 2015
Fidelity SAI International Minimum Volatility Index Fund-Default | After Taxes on Distributions and Sales	1.89%	(2.10%)		May 29, 2015
MSCI ACWI (All Country World Index) ex USA Minimum Volatility (USD) Index(reflects no deduction for fees or expenses)	2.08%	(2.28%)

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse the fund to the extent that total operating expenses (excluding interest, certain taxes, certain securities lending costs, brokerage commissions, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any), as a percentage of its average net assets, exceed 0.20%. This arrangement will remain in effect through December 31, 2018. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees.

[1] From May 29, 2015

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Dec. 30, 2017

10.31 Fidelity SAI Minimum Volatility Index Funds Combo PRO-04 | Fidelity® SAI International Minimum Volatility Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund:
Fidelity® SAI International Minimum Volatility 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 low volatility foreign stocks.

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)

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

December 31, 2018

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 25% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

25.00%

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:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

Sell All Shares

Strategy [Heading]

rr_StrategyHeading

Principal Investment Strategies

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Normally investing at least 80% of assets in common stocks included in the MSCI ACWI (All Country World Index) ex USA Minimum Volatility (USD) Index, which reflects the performance of a broad range of stocks across developed (excluding U.S.) and emerging markets that in the aggregate have lower volatility relative to the broader foreign equity markets, and in depository receipts representing securities included in the index.

• Lending securities to earn income for the fund.

Risk [Heading]

rr_RiskHeading

Principal Investment Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Minimum Volatility Strategy. Although the fund’s underlying index was created to seek lower volatility relative to a broad range of foreign stocks, there is no guarantee that the fund’s minimum volatility strategy will be successful. There is a risk that the fund may experience more than minimum volatility.

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

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]

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 performance of the fund's shares over the past 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 performance of the fund's shares over the past 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

Calendar Years

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	4.76%	March 31, 2016
Lowest Quarter Return	-6.75%	December 31, 2016
Year-to-Date Return	16.20%	September 30, 2017

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.

For the periods ended December 31, 2016

10.31 Fidelity SAI Minimum Volatility Index Funds Combo PRO-04 | Fidelity® SAI International Minimum Volatility Index Fund | Fidelity SAI International Minimum Volatility Index Fund-Default

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.15%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.16%

Total annual operating expenses

rr_ExpensesOverAssets

0.31%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.11%

^[1]

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

rr_NetExpensesOverAssets

0.20%

1 year

rr_ExpenseExampleYear01

$ 20

3 years

rr_ExpenseExampleYear03

87

5 years

rr_ExpenseExampleYear05

161

10 years

rr_ExpenseExampleYear10

$ 381

2016

rr_AnnualReturn2016

2.43%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Sep. 30, 2017

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

16.20%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Mar. 31, 2016

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

4.76%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Dec. 31, 2016

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(6.75%)

Inception Date

rr_AverageAnnualReturnInceptionDate

May 29, 2015

10.31 Fidelity SAI Minimum Volatility Index Funds Combo PRO-04 | Fidelity® SAI International Minimum Volatility Index Fund | Return Before Taxes | Fidelity SAI International Minimum Volatility Index Fund-Default

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® SAI International Minimum Volatility Index Fund

Label

rr_AverageAnnualReturnLabel

Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

2.43%

Since Inception

rr_AverageAnnualReturnSinceInception

(2.70%)

^[2]

10.31 Fidelity SAI Minimum Volatility Index Funds Combo PRO-04 | Fidelity® SAI International Minimum Volatility Index Fund | After Taxes on Distributions | Fidelity SAI International Minimum Volatility Index Fund-Default

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® SAI International Minimum Volatility Index Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions

Past 1 year

rr_AverageAnnualReturnYear01

1.71%

Since Inception

rr_AverageAnnualReturnSinceInception

(3.32%)

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

May 29, 2015

10.31 Fidelity SAI Minimum Volatility Index Funds Combo PRO-04 | Fidelity® SAI International Minimum Volatility Index Fund | After Taxes on Distributions and Sales | Fidelity SAI International Minimum Volatility Index Fund-Default

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® SAI International Minimum Volatility Index Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions and Sale of Fund Shares

Past 1 year

rr_AverageAnnualReturnYear01

1.89%

Since Inception

rr_AverageAnnualReturnSinceInception

(2.10%)

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

May 29, 2015

10.31 Fidelity SAI Minimum Volatility Index Funds Combo PRO-04 | Fidelity® SAI International Minimum Volatility Index Fund | MSCI ACWI (All Country World Index) ex USA Minimum Volatility (USD) Index(reflects no deduction for fees or expenses)

Risk/Return:

rr_RiskReturnAbstract

Past 1 year

rr_AverageAnnualReturnYear01

2.08%

Since Inception

rr_AverageAnnualReturnSinceInception

(2.28%)

^[2]

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse the fund to the extent that total operating expenses (excluding interest, certain taxes, certain securities lending costs, brokerage commissions, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any), as a percentage of its average net assets, exceed 0.20%. This arrangement will remain in effect through December 31, 2018. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees.

[2] From May 29, 2015

XML

10.31 Fidelity SAI Minimum Volatility Index Funds Combo PRO-04 | Fidelity® SAI U.S. Minimum Volatility Index Fund

Fund Summary

Fund:
Fidelity® SAI U.S. Minimum Volatility Index Fund

Investment Objective

The fund seeks to provide investment results that correspond to the total return of low volatility U.S. stocks.

Fee Table

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder fees

Shareholder Fees {- Fidelity® SAI U.S. Minimum Volatility Index Fund}	10.31 Fidelity SAI Minimum Volatility Index Funds Combo PRO-04 Fidelity® SAI U.S. Minimum Volatility Index Fund USD ($)
(fees paid directly from your investment)	none

Annual Operating Expenses

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

Annual Operating Expenses {- Fidelity® SAI U.S. Minimum Volatility Index Fund}	10.31 Fidelity SAI Minimum Volatility Index Funds Combo PRO-04 Fidelity® SAI U.S. Minimum Volatility Index Fund Fidelity SAI U.S. Minimum Volatility Index Fund-Default
Management fee	0.10%
Distribution and/or Service (12b-1) fees	none
Other expenses	0.16%
Total annual operating expenses	0.26%
Fee waiver and/or expense reimbursement	0.11%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.15%

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:

Sell All Shares

Expense Example {- Fidelity® SAI U.S. Minimum Volatility Index Fund}	10.31 Fidelity SAI Minimum Volatility Index Funds Combo PRO-04 Fidelity® SAI U.S. Minimum Volatility Index Fund Fidelity SAI U.S. Minimum Volatility Index Fund-Default USD ($)
1 year	$ 15
3 years	71
5 years	133
10 years	$ 318

Portfolio Turnover

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 108% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in common stocks included in the MSCI USA Minimum Volatility (USD) Index, which represents the performance of a broad range of U.S. stocks that in the aggregate have lower volatility relative to the broader U.S. equity market.

• Lending securities to earn income for the fund.

Principal Investment Risks

• Minimum Volatility Strategy. Although the fund’s underlying index was created to seek lower volatility relative to the broader U.S. equity market, there is no guarantee that the fund’s minimum volatility strategy will be successful. There is a risk that the fund may experience more than minimum volatility.

• 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.

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

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.

Performance

The following information is intended to help you understand the risks of investing in the fund. The information illustrates the performance of the fund's shares over the past 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.

Year-by-Year Returns

Calendar Years

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	5.71%	June 30, 2016
Lowest Quarter Return	-1.06%	September 30, 2016
Year-to-Date Return	12.97%	September 30, 2017

Average Annual Returns

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).

For the periods ended December 31, 2016

Average Annual Total Returns{- Fidelity® SAI U.S. Minimum Volatility Index Fund} - 10.31 Fidelity SAI Minimum Volatility Index Funds Combo PRO-04 - Fidelity® SAI U.S. Minimum Volatility Index Fund	Past 1 year	Since Inception	[1]	Inception Date
Fidelity SAI U.S. Minimum Volatility Index Fund-Default				May 29, 2015
Fidelity SAI U.S. Minimum Volatility Index Fund-Default | Return Before Taxes	10.61%	8.21%
Fidelity SAI U.S. Minimum Volatility Index Fund-Default | After Taxes on Distributions	10.04%	7.64%		May 29, 2015
Fidelity SAI U.S. Minimum Volatility Index Fund-Default | After Taxes on Distributions and Sales	6.48%	6.25%		May 29, 2015
MSCI USA Minimum Volatility (USD) Index(reflects no deduction for fees, expenses, or taxes)	10.67%	8.37%

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse the fund to the extent that total operating expenses (excluding interest, certain taxes, certain securities lending costs, brokerage commissions, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any), as a percentage of its average net assets, exceed 0.15%. This arrangement will remain in effect through December 31, 2018. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees.

[1] From May 29, 2015

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Dec. 30, 2017

10.31 Fidelity SAI Minimum Volatility Index Funds Combo PRO-04 | Fidelity® SAI U.S. Minimum Volatility Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund:
Fidelity® SAI U.S. Minimum Volatility 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 low volatility U.S. stocks.

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)

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

December 31, 2018

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 108% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

108.00%

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:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

Sell All Shares

Strategy [Heading]

rr_StrategyHeading

Principal Investment Strategies

Strategy Narrative [Text Block]

rr_StrategyNarrativeTextBlock

• Normally investing at least 80% of assets in common stocks included in the MSCI USA Minimum Volatility (USD) Index, which represents the performance of a broad range of U.S. stocks that in the aggregate have lower volatility relative to the broader U.S. equity market.

• Lending securities to earn income for the fund.

Risk [Heading]

rr_RiskHeading

Principal Investment Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Minimum Volatility Strategy. Although the fund’s underlying index was created to seek lower volatility relative to the broader U.S. equity market, there is no guarantee that the fund’s minimum volatility strategy will be successful. There is a risk that the fund may experience more than minimum volatility.

• 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.

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

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]

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 performance of the fund's shares over the past 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 performance of the fund's shares over the past 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

Calendar Years

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	5.71%	June 30, 2016
Lowest Quarter Return	-1.06%	September 30, 2016
Year-to-Date Return	12.97%	September 30, 2017

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 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).

For the periods ended December 31, 2016

10.31 Fidelity SAI Minimum Volatility Index Funds Combo PRO-04 | Fidelity® SAI U.S. Minimum Volatility Index Fund | Fidelity SAI U.S. Minimum Volatility Index Fund-Default

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.10%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.16%

Total annual operating expenses

rr_ExpensesOverAssets

0.26%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.11%

^[1]

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

rr_NetExpensesOverAssets

0.15%

1 year

rr_ExpenseExampleYear01

$ 15

3 years

rr_ExpenseExampleYear03

71

5 years

rr_ExpenseExampleYear05

133

10 years

rr_ExpenseExampleYear10

$ 318

2016

rr_AnnualReturn2016

10.61%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Sep. 30, 2017

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

12.97%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Jun. 30, 2016

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

5.71%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Sep. 30, 2016

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(1.06%)

Inception Date

rr_AverageAnnualReturnInceptionDate

May 29, 2015

10.31 Fidelity SAI Minimum Volatility Index Funds Combo PRO-04 | Fidelity® SAI U.S. Minimum Volatility Index Fund | Return Before Taxes | Fidelity SAI U.S. Minimum Volatility Index Fund-Default

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® SAI U.S. Minimum Volatility Index Fund

Label

rr_AverageAnnualReturnLabel

Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

10.61%

Since Inception

rr_AverageAnnualReturnSinceInception

8.21%

^[2]

10.31 Fidelity SAI Minimum Volatility Index Funds Combo PRO-04 | Fidelity® SAI U.S. Minimum Volatility Index Fund | After Taxes on Distributions | Fidelity SAI U.S. Minimum Volatility Index Fund-Default

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® SAI U.S. Minimum Volatility Index Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions

Past 1 year

rr_AverageAnnualReturnYear01

10.04%

Since Inception

rr_AverageAnnualReturnSinceInception

7.64%

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

May 29, 2015

10.31 Fidelity SAI Minimum Volatility Index Funds Combo PRO-04 | Fidelity® SAI U.S. Minimum Volatility Index Fund | After Taxes on Distributions and Sales | Fidelity SAI U.S. Minimum Volatility Index Fund-Default

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® SAI U.S. Minimum Volatility Index Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions and Sale of Fund Shares

Past 1 year

rr_AverageAnnualReturnYear01

6.48%

Since Inception

rr_AverageAnnualReturnSinceInception

6.25%

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

May 29, 2015

10.31 Fidelity SAI Minimum Volatility Index Funds Combo PRO-04 | Fidelity® SAI U.S. Minimum Volatility Index Fund | MSCI USA Minimum Volatility (USD) Index(reflects no deduction for fees, expenses, or taxes)

Risk/Return:

rr_RiskReturnAbstract

Past 1 year

rr_AverageAnnualReturnYear01

10.67%

Since Inception

rr_AverageAnnualReturnSinceInception

8.37%

^[2]

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse the fund to the extent that total operating expenses (excluding interest, certain taxes, certain securities lending costs, brokerage commissions, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any), as a percentage of its average net assets, exceed 0.15%. This arrangement will remain in effect through December 31, 2018. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees.

[2] From May 29, 2015

XML

10.31 Fidelity Total International Index Fund Institutional, Institutional Premium PRO-06 | Fidelity® Total International Index Fund

Fund Summary

Fund/Class:
Fidelity® Total International Index Fund/Institutional, Institutional Premium

Investment Objective

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

Fee Table

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder fees

Shareholder Fees {- Fidelity® Total International Index Fund}	10.31 Fidelity Total International Index Fund Institutional, Institutional Premium PRO-06 Fidelity® Total International Index Fund USD ($)
(fees paid directly from your investment)	none

Annual Operating Expenses

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

Annual Operating Expenses {- Fidelity® Total International Index Fund} - 10.31 Fidelity Total International Index Fund Institutional, Institutional Premium PRO-06 - Fidelity® Total International Index Fund	Fidelity Total International Index Fund-Institutional Class		Fidelity Total International Index Fund-Institutional Premium Class
Management fee	0.06%		0.06%
Distribution and/or Service (12b-1) fees	none		none
Other expenses	0.02%	[1]	none
Total annual operating expenses	0.08%	[1]	0.06%

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:

Sell All Shares

Expense Example {- Fidelity® Total International Index Fund} - 10.31 Fidelity Total International Index Fund Institutional, Institutional Premium PRO-06 - Fidelity® Total International Index Fund - USD ($)	Fidelity Total International Index Fund-Institutional Class	Fidelity Total International Index Fund-Institutional Premium Class
1 year	$ 8	$ 6
3 years	26	19
5 years	45	34
10 years	$ 103	$ 77

Portfolio Turnover

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 2% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in securities included in the MSCI ACWI (All Country World Index) ex USA Investable Market Index and in depository receipts representing securities included in the index. The MSCI ACWI (All Country World Index) ex USA Investable Market Index is a market capitalization-weighted index designed to measure the investable equity market performance for global investors of large, mid, and small-cap stocks in developed and emerging markets, excluding the U.S.

• 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 MSCI ACWI (All Country World Index) ex USA Investable Market Index.

• Lending securities to earn income for the fund.

Principal Investment Risks

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

• Mid Cap Investing. The value of securities of medium size, less well-known issuers can perform differently from the market as a whole and other types of stocks and can be more volatile than that of larger issuers.

• Small Cap Investing. The value of securities of smaller, less well-known issuers can perform differently from the market as a whole and other types of stocks and can be more volatile than that of larger issuers.

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.

Performance

Performance history will be available for the fund after the fund has been in operation for one calendar year.

[1] Adjusted to reflect current fees.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Dec. 30, 2017

10.31 Fidelity Total International Index Fund Institutional, Institutional Premium PRO-06 | Fidelity® Total International Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® Total International Index Fund/Institutional, Institutional Premium

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 foreign developed and emerging stock markets.

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 2% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

2.00%

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:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

Sell All Shares

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 MSCI ACWI (All Country World Index) ex USA Investable Market Index and in depository receipts representing securities included in the index. The MSCI ACWI (All Country World Index) ex USA Investable Market Index is a market capitalization-weighted index designed to measure the investable equity market performance for global investors of large, mid, and small-cap stocks in developed and emerging markets, excluding the U.S.

• 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 MSCI ACWI (All Country World Index) ex USA Investable Market Index.

• Lending securities to earn income for the fund.

Risk [Heading]

rr_RiskHeading

Principal Investment Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

• Mid Cap Investing. The value of securities of medium size, less well-known issuers can perform differently from the market as a whole and other types of stocks and can be more volatile than that of larger issuers.

• Small Cap Investing. The value of securities of smaller, less well-known issuers can perform differently from the market as a whole and other types of stocks and can be more volatile than that of larger issuers.

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]

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

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]

rr_PerformanceOneYearOrLess

Performance history will be available for the fund after the fund has been in operation for one calendar year.

10.31 Fidelity Total International Index Fund Institutional, Institutional Premium PRO-06 | Fidelity® Total International Index Fund | Fidelity Total International Index Fund-Institutional Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.06%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.02%

^[1]

Total annual operating expenses

rr_ExpensesOverAssets

0.08%

^[1]

Expenses Restated to Reflect Current [Text]

rr_ExpensesRestatedToReflectCurrent

Adjusted to reflect current fees.

1 year

rr_ExpenseExampleYear01

$ 8

3 years

rr_ExpenseExampleYear03

26

5 years

rr_ExpenseExampleYear05

45

10 years

rr_ExpenseExampleYear10

$ 103

10.31 Fidelity Total International Index Fund Institutional, Institutional Premium PRO-06 | Fidelity® Total International Index Fund | Fidelity Total International Index Fund-Institutional Premium Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.06%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

none

Total annual operating expenses

rr_ExpensesOverAssets

0.06%

1 year

rr_ExpenseExampleYear01

$ 6

3 years

rr_ExpenseExampleYear03

19

5 years

rr_ExpenseExampleYear05

34

10 years

rr_ExpenseExampleYear10

$ 77

[1] Adjusted to reflect current fees.

XML

10.31 Fidelity Total International Index Fund Investor, Premium PRO-06 | Fidelity® Total International Index Fund

Fund Summary

Fund/Class:
Fidelity® Total International Index Fund/Investor, Premium

Investment Objective

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

Fee Table

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder fees

Shareholder Fees {- Fidelity® Total International Index Fund}	10.31 Fidelity Total International Index Fund Investor, Premium PRO-06 Fidelity® Total International Index Fund USD ($)
(fees paid directly from your investment)	none

Annual Operating Expenses

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

Annual Operating Expenses {- Fidelity® Total International Index Fund} - 10.31 Fidelity Total International Index Fund Investor, Premium PRO-06 - Fidelity® Total International Index Fund		Fidelity Total International Index Fund-Investor Class	Fidelity Total International Index Fund-Premium Class
Management fee		0.06%	0.06%
Distribution and/or Service (12b-1) fees		none	none
Other expenses	[1]	0.11%	0.04%
Total annual operating expenses	[1]	0.17%	0.10%

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:

Sell All Shares

Expense Example {- Fidelity® Total International Index Fund} - 10.31 Fidelity Total International Index Fund Investor, Premium PRO-06 - Fidelity® Total International Index Fund - USD ($)	Fidelity Total International Index Fund-Investor Class	Fidelity Total International Index Fund-Premium Class
1 year	$ 17	$ 10
3 years	55	32
5 years	96	56
10 years	$ 217	$ 128

Portfolio Turnover

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 2% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in securities included in the MSCI ACWI (All Country World Index) ex USA Investable Market Index and in depository receipts representing securities included in the index. The MSCI ACWI (All Country World Index) ex USA Investable Market Index is a market capitalization-weighted index designed to measure the investable equity market performance for global investors of large, mid, and small-cap stocks in developed and emerging markets, excluding the U.S.

• 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 MSCI ACWI (All Country World Index) ex USA Investable Market Index.

• Lending securities to earn income for the fund.

Principal Investment Risks

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

• Mid Cap Investing. The value of securities of medium size, less well-known issuers can perform differently from the market as a whole and other types of stocks and can be more volatile than that of larger issuers.

• Small Cap Investing. The value of securities of smaller, less well-known issuers can perform differently from the market as a whole and other types of stocks and can be more volatile than that of larger issuers.

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.

Performance

Performance history will be available for the fund after the fund has been in operation for one calendar year.

[1] Adjusted to reflect current fees.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Dec. 30, 2017

10.31 Fidelity Total International Index Fund Investor, Premium PRO-06 | Fidelity® Total International Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® Total International Index Fund/Investor, Premium

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 foreign developed and emerging stock markets.

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 2% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

2.00%

Expenses Restated to Reflect Current [Text]

rr_ExpensesRestatedToReflectCurrent

Adjusted to reflect current fees.

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:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

Sell All Shares

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 MSCI ACWI (All Country World Index) ex USA Investable Market Index and in depository receipts representing securities included in the index. The MSCI ACWI (All Country World Index) ex USA Investable Market Index is a market capitalization-weighted index designed to measure the investable equity market performance for global investors of large, mid, and small-cap stocks in developed and emerging markets, excluding the U.S.

• 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 MSCI ACWI (All Country World Index) ex USA Investable Market Index.

• Lending securities to earn income for the fund.

Risk [Heading]

rr_RiskHeading

Principal Investment Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

• Mid Cap Investing. The value of securities of medium size, less well-known issuers can perform differently from the market as a whole and other types of stocks and can be more volatile than that of larger issuers.

• Small Cap Investing. The value of securities of smaller, less well-known issuers can perform differently from the market as a whole and other types of stocks and can be more volatile than that of larger issuers.

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]

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

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]

rr_PerformanceOneYearOrLess

Performance history will be available for the fund after the fund has been in operation for one calendar year.

10.31 Fidelity Total International Index Fund Investor, Premium PRO-06 | Fidelity® Total International Index Fund | Fidelity Total International Index Fund-Investor Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.06%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.11%

^[1]

Total annual operating expenses

rr_ExpensesOverAssets

0.17%

^[1]

1 year

rr_ExpenseExampleYear01

$ 17

3 years

rr_ExpenseExampleYear03

55

5 years

rr_ExpenseExampleYear05

96

10 years

rr_ExpenseExampleYear10

$ 217

10.31 Fidelity Total International Index Fund Investor, Premium PRO-06 | Fidelity® Total International Index Fund | Fidelity Total International Index Fund-Premium Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.06%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.04%

^[1]

Total annual operating expenses

rr_ExpensesOverAssets

0.10%

^[1]

1 year

rr_ExpenseExampleYear01

$ 10

3 years

rr_ExpenseExampleYear03

32

5 years

rr_ExpenseExampleYear05

56

10 years

rr_ExpenseExampleYear10

$ 128

[1] Adjusted to reflect current fees.

XML

10.31 Fidelity Series Global ex U.S. Index Fund PRO-04 | Fidelity® Series Global ex U.S. Index Fund

Fund Summary

Fund:
Fidelity® Series Global ex U.S. Index Fund

Investment Objective

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

Fee Table

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder fees

Shareholder Fees {- Fidelity® Series Global ex U.S. Index Fund}	10.31 Fidelity Series Global ex U.S. Index Fund PRO-04 Fidelity® Series Global ex U.S. Index Fund USD ($)
(fees paid directly from your investment)	none

Annual Operating Expenses

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

Annual Operating Expenses {- Fidelity® Series Global ex U.S. Index Fund}	10.31 Fidelity Series Global ex U.S. Index Fund PRO-04 Fidelity® Series Global ex U.S. Index Fund Fidelity Series Global ex U.S. Index Fund-Default
Management fee	0.06%
Distribution and/or Service (12b-1) fees	none
Other expenses	none
Total annual operating expenses	0.06%

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:

Sell All Shares

Expense Example {- Fidelity® Series Global ex U.S. Index Fund}	10.31 Fidelity Series Global ex U.S. Index Fund PRO-04 Fidelity® Series Global ex U.S. Index Fund Fidelity Series Global ex U.S. Index Fund-Default USD ($)
1 year	$ 6
3 years	19
5 years	34
10 years	$ 77

Portfolio Turnover

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 4% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in securities included in the MSCI ACWI (All Country World Index) ex USA 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 MSCI ACWI (All Country World Index) ex USA Index.

• Lending securities to earn income for the fund.

Principal Investment Risks

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

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.

Performance

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.401k.com and log in for more recent performance information.

Year-by-Year Returns

Calendar Years

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	17.49%	September 30, 2010
Lowest Quarter Return	-20.73%	September 30, 2011
Year-to-Date Return	21.69%	September 30, 2017

Average Annual Returns

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).

For the periods ended December 31, 2016

Average Annual Total Returns{- Fidelity® Series Global ex U.S. Index Fund} - 10.31 Fidelity Series Global ex U.S. Index Fund PRO-04 - Fidelity® Series Global ex U.S. Index Fund	Past 1 year	Past 5 years	Since Inception	[1]	Inception Date
Fidelity Series Global ex U.S. Index Fund-Default					Sep. 29, 2009
Fidelity Series Global ex U.S. Index Fund-Default | Return Before Taxes	4.64%	4.91%	3.15%
Fidelity Series Global ex U.S. Index Fund-Default | After Taxes on Distributions	3.97%	4.30%	2.66%		Sep. 29, 2009
Fidelity Series Global ex U.S. Index Fund-Default | After Taxes on Distributions and Sales	3.10%	3.83%	2.49%		Sep. 29, 2009
MSCI ACWI (All Country World Index) ex USA Index(reflects no deduction for fees or expenses)	4.68%	5.15%	3.54%

[1] From September 29, 2009

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Dec. 30, 2017

10.31 Fidelity Series Global ex U.S. Index Fund PRO-04 | Fidelity® Series Global ex U.S. Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund:
Fidelity® Series Global ex U.S. 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 foreign developed and emerging stock markets.

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 4% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

4.00%

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:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

Sell All Shares

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 MSCI ACWI (All Country World Index) ex USA 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 MSCI ACWI (All Country World Index) ex USA Index.

• Lending securities to earn income for the fund.

Risk [Heading]

rr_RiskHeading

Principal Investment Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

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]

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.401k.com and log in 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.401k.com and log in

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

Calendar Years

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	17.49%	September 30, 2010
Lowest Quarter Return	-20.73%	September 30, 2011
Year-to-Date Return	21.69%	September 30, 2017

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 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).

For the periods ended December 31, 2016

10.31 Fidelity Series Global ex U.S. Index Fund PRO-04 | Fidelity® Series Global ex U.S. Index Fund | Fidelity Series Global ex U.S. Index Fund-Default

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.06%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

none

Total annual operating expenses

rr_ExpensesOverAssets

0.06%

1 year

rr_ExpenseExampleYear01

$ 6

3 years

rr_ExpenseExampleYear03

19

5 years

rr_ExpenseExampleYear05

34

10 years

rr_ExpenseExampleYear10

$ 77

2010

rr_AnnualReturn2010

10.87%

2011

rr_AnnualReturn2011

(14.10%)

2012

rr_AnnualReturn2012

18.02%

2013

rr_AnnualReturn2013

14.22%

2014

rr_AnnualReturn2014

(4.38%)

2015

rr_AnnualReturn2015

(5.75%)

2016

rr_AnnualReturn2016

4.64%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Sep. 30, 2017

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

21.69%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Sep. 30, 2010

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

17.49%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Sep. 30, 2011

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(20.73%)

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 29, 2009

10.31 Fidelity Series Global ex U.S. Index Fund PRO-04 | Fidelity® Series Global ex U.S. Index Fund | Return Before Taxes | Fidelity Series Global ex U.S. Index Fund-Default

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Series Global ex U.S. Index Fund

Label

rr_AverageAnnualReturnLabel

Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

4.64%

Past 5 years

rr_AverageAnnualReturnYear05

4.91%

Since Inception

rr_AverageAnnualReturnSinceInception

3.15%

^[1]

10.31 Fidelity Series Global ex U.S. Index Fund PRO-04 | Fidelity® Series Global ex U.S. Index Fund | After Taxes on Distributions | Fidelity Series Global ex U.S. Index Fund-Default

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Series Global ex U.S. Index Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions

Past 1 year

rr_AverageAnnualReturnYear01

3.97%

Past 5 years

rr_AverageAnnualReturnYear05

4.30%

Since Inception

rr_AverageAnnualReturnSinceInception

2.66%

^[1]

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 29, 2009

10.31 Fidelity Series Global ex U.S. Index Fund PRO-04 | Fidelity® Series Global ex U.S. Index Fund | After Taxes on Distributions and Sales | Fidelity Series Global ex U.S. Index Fund-Default

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Series Global ex U.S. Index Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions and Sale of Fund Shares

Past 1 year

rr_AverageAnnualReturnYear01

3.10%

Past 5 years

rr_AverageAnnualReturnYear05

3.83%

Since Inception

rr_AverageAnnualReturnSinceInception

2.49%

^[1]

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 29, 2009

10.31 Fidelity Series Global ex U.S. Index Fund PRO-04 | Fidelity® Series Global ex U.S. Index Fund | MSCI ACWI (All Country World Index) ex USA Index(reflects no deduction for fees or expenses)

Risk/Return:

rr_RiskReturnAbstract

Past 1 year

rr_AverageAnnualReturnYear01

4.68%

Past 5 years

rr_AverageAnnualReturnYear05

5.15%

Since Inception

rr_AverageAnnualReturnSinceInception

3.54%

^[1]

[1] From September 29, 2009

XML

10.31 Fidelity SAI Emerging Markets Index Fund PRO-04 | Fidelity® SAI Emerging Markets Index Fund

Fund Summary

Fund:
Fidelity® SAI Emerging Markets Index Fund

Investment Objective

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

Fee Table

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund.

Shareholder fees

Shareholder Fees {- Fidelity® SAI Emerging Markets Index Fund}	10.31 Fidelity SAI Emerging Markets Index Fund PRO-04 Fidelity® SAI Emerging Markets Index Fund USD ($)
(fees paid directly from your investment)	none

Annual Operating Expenses

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

Annual Operating Expenses {- Fidelity® SAI Emerging Markets Index Fund}	10.31 Fidelity SAI Emerging Markets Index Fund PRO-04 Fidelity® SAI Emerging Markets Index Fund Fidelity SAI Emerging Markets Index Fund-Default
Management fee	0.09%
Distribution and/or Service (12b-1) fees	none
Other expenses	0.075%
Total annual operating expenses	0.165%
Fee waiver and/or expense reimbursement	0.085%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	0.08%

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:

Sell All Shares

Expense Example {- Fidelity® SAI Emerging Markets Index Fund}	10.31 Fidelity SAI Emerging Markets Index Fund PRO-04 Fidelity® SAI Emerging Markets Index Fund Fidelity SAI Emerging Markets Index Fund-Default USD ($)
1 year	$ 8
3 years	43
5 years	83
10 years	$ 201

Portfolio Turnover

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 2% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in securities included in the MSCI Emerging Markets 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, and country weightings to attempt to replicate the returns of the MSCI Emerging Markets Index.

• Lending securities to earn income for the fund.

Principal Investment Risks

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

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.

Performance

Performance history will be available for the fund after the fund has been in operation for one calendar year.

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse the fund to the extent that total operating expenses (excluding interest, certain taxes, certain securities lending costs, brokerage commissions, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any), as a percentage of its average net assets, exceed 0.08%. This arrangement will remain in effect through December 31, 2018. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Dec. 30, 2017

10.31 Fidelity SAI Emerging Markets Index Fund PRO-04 | Fidelity® SAI Emerging Markets Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund:
Fidelity® SAI Emerging Markets 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 emerging stock markets.

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)

Fee Waiver or Reimbursement over Assets, Date of Termination

rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination

December 31, 2018

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 2% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

2.00%

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:

Expense Example by, Year, Caption [Text]

rr_ExpenseExampleByYearCaption

Sell All Shares

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 MSCI Emerging Markets 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, 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]

rr_RiskHeading

Principal Investment Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• 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. Changes in the financial condition of an issuer or counterparty (e.g., broker-dealer or other borrower in a securities lending transaction) can increase the risk of default by an issuer or counterparty, which can affect a security's or instrument's value or result in delays in recovering securities and/or capital from a counterparty.

• Correlation to Index. The performance of the fund and its 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 its index.

• 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.

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]

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

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]

rr_PerformanceOneYearOrLess

Performance history will be available for the fund after the fund has been in operation for one calendar year.

10.31 Fidelity SAI Emerging Markets Index Fund PRO-04 | Fidelity® SAI Emerging Markets Index Fund | Fidelity SAI Emerging Markets Index Fund-Default

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.09%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.075%

Total annual operating expenses

rr_ExpensesOverAssets

0.165%

Fee waiver and/or expense reimbursement

rr_FeeWaiverOrReimbursementOverAssets

0.085%

^[1]

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

rr_NetExpensesOverAssets

0.08%

1 year

rr_ExpenseExampleYear01

$ 8

3 years

rr_ExpenseExampleYear03

43

5 years

rr_ExpenseExampleYear05

83

10 years

rr_ExpenseExampleYear10

$ 201

[1] Fidelity Management & Research Company (FMR) has contractually agreed to reimburse the fund to the extent that total operating expenses (excluding interest, certain taxes, certain securities lending costs, brokerage commissions, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses, if any), as a percentage of its average net assets, exceed 0.08%. This arrangement will remain in effect through December 31, 2018. FMR may not terminate this arrangement before the expiration date without the approval of the Board of Trustees.

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	FIDELITY SALEM STREET TRUST
Prospectus Date	rr_ProspectusDate	Dec. 30, 2017
Document Creation Date	dei_DocumentCreationDate	Dec. 28, 2017