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.  376

and

REGISTRATION STATEMENT UNDER THE INVESTMENT COMPANY ACT OF 1940 x

x Amendment No.  376

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.376to 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 17th day of July 2017.

 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	July 17, 2017
Stephanie J. Dorsey		(Principal Executive Officer)
/s/Howard J. Galligan III		Chief Financial Officer	July 17, 2017
Howard J. Galligan III		(Principal Financial Officer)
/s/Abigail P. Johnson	†	Trustee	July 17, 2017
Abigail P. Johnson
/s/Elizabeth S. Acton	*	Trustee	July 17, 2017
Elizabeth S. Acton
/s/Jonathan Chiel	*	Trustee	July 17, 2017
Jonathan Chiel
/s/John Engler	*	Trustee	July 17, 2017
John Engler
/s/Albert R. Gamper, Jr.	*	Trustee	July 17, 2017
Albert R. Gamper, Jr.
/s/Robert F. Gartland	*	Trustee	July 17, 2017
Robert F. Gartland
/s/Arthur E. Johnson	*	Trustee	July 17, 2017
Arthur E. Johnson
/s/Michael E. Kenneally	*	Trustee	July 17, 2017
Michael E. Kenneally
/s/Marie L. Knowles	*	Trustee	July 17, 2017
Marie L. Knowles
/s/Mark A. Murray	*	Trustee	July 17, 2017
Mark A. Murray
/s/Jennifer Toolin McAuliffe	*	Trustee	July 17, 2017
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 January 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 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 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 January 1, 2017.

WITNESS our hands on this first day of January 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 5th 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	Apr. 30, 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	Jun. 27, 2017
Document Effective Date	dei_DocumentEffectiveDate	Jun. 29, 2017
Prospectus Date	rr_ProspectusDate	Jun. 29, 2017

XML

04.30 Fidelity Large Cap Growth Index Fund Institutional, Institutional Premium PRO-04 | Fidelity® Large Cap Growth Index Fund

Fund Summary

Fund/Class:
Fidelity® Large Cap Growth Index Fund/Institutional, Institutional Premium

Investment Objective

The fund seeks to provide investment results that correspond to the total return of stocks of large capitalization U.S. companies.

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® Large Cap Growth Index Fund}	04.30 Fidelity Large Cap Growth Index Fund Institutional, Institutional Premium PRO-04 Fidelity® Large Cap Growth 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® Large Cap Growth Index Fund} - 04.30 Fidelity Large Cap Growth Index Fund Institutional, Institutional Premium PRO-04 - Fidelity® Large Cap Growth Index Fund	Fidelity Large Cap Growth Index Fund-Institutional Class	Fidelity Large Cap Growth Index Fund-Institutional Premium Class
Management fee	0.05%	0.05%
Distribution and/or Service (12b-1) fees	none	none
Other expenses	0.01%	none
Total annual operating expenses	0.06%	0.05%

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® Large Cap Growth Index Fund} - 04.30 Fidelity Large Cap Growth Index Fund Institutional, Institutional Premium PRO-04 - Fidelity® Large Cap Growth Index Fund - USD ($)	Fidelity Large Cap Growth Index Fund-Institutional Class	Fidelity Large Cap Growth Index Fund-Institutional Premium Class
1 year	$ 6	$ 5
3 years	19	16
5 years	34	28
10 years	$ 77	$ 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 June 7, 2016 to April 30, 2017, the fund's portfolio turnover rate was 17% (annualized) of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in securities included in the Russell 1000^® Growth Index, which is a market capitalization-weighted index designed to measure the performance of the large-cap growth segment of the U.S. equity market.

• 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.

• "Growth" Investing. "Growth" stocks can perform differently from the market as a whole and other types of stocks and can be more volatile than other types of stocks.

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.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Jun. 29, 2017

04.30 Fidelity Large Cap Growth Index Fund Institutional, Institutional Premium PRO-04 | Fidelity® Large Cap Growth Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® Large Cap Growth 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 stocks of large capitalization U.S. companies.

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 June 7, 2016 to April 30, 2017, the fund's portfolio turnover rate was 17% (annualized) of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

17.00%

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 Russell 1000^® Growth Index, which is a market capitalization-weighted index designed to measure the performance of the large-cap growth segment of the 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

• 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.

• "Growth" Investing. "Growth" stocks can perform differently from the market as a whole and other types of stocks and can be more volatile than other types of stocks.

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.

04.30 Fidelity Large Cap Growth Index Fund Institutional, Institutional Premium PRO-04 | Fidelity® Large Cap Growth Index Fund | Fidelity Large Cap Growth Index Fund-Institutional Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.05%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.01%

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

04.30 Fidelity Large Cap Growth Index Fund Institutional, Institutional Premium PRO-04 | Fidelity® Large Cap Growth Index Fund | Fidelity Large Cap Growth Index Fund-Institutional Premium Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.05%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

none

Total annual operating expenses

rr_ExpensesOverAssets

0.05%

1 year

rr_ExpenseExampleYear01

$ 5

3 years

rr_ExpenseExampleYear03

16

5 years

rr_ExpenseExampleYear05

28

10 years

rr_ExpenseExampleYear10

$ 64

XML

04.30 Fidelity Large Cap Growth Index Fund Investor, Premium PRO-04 | Fidelity® Large Cap Growth Index Fund

Fund Summary

Fund/Class:
Fidelity® Large Cap Growth Index Fund/Investor, Premium

Investment Objective

The fund seeks to provide investment results that correspond to the total return of stocks of large capitalization U.S. companies.

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® Large Cap Growth Index Fund}	04.30 Fidelity Large Cap Growth Index Fund Investor, Premium PRO-04 Fidelity® Large Cap Growth 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® Large Cap Growth Index Fund} - 04.30 Fidelity Large Cap Growth Index Fund Investor, Premium PRO-04 - Fidelity® Large Cap Growth Index Fund	Fidelity Large Cap Growth Index Fund-Investor Class	Fidelity Large Cap Growth Index Fund-Premium Class
Management fee	0.05%	0.05%
Distribution and/or Service (12b-1) fees	none	none
Other expenses	0.16%	0.02%
Total annual operating expenses	0.21%	0.07%

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® Large Cap Growth Index Fund} - 04.30 Fidelity Large Cap Growth Index Fund Investor, Premium PRO-04 - Fidelity® Large Cap Growth Index Fund - USD ($)	Fidelity Large Cap Growth Index Fund-Investor Class	Fidelity Large Cap Growth Index Fund-Premium Class
1 year	$ 22	$ 7
3 years	68	23
5 years	118	40
10 years	$ 268	$ 90

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 June 7, 2016 to April 30, 2017, the fund's portfolio turnover rate was 17% (annualized) of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in securities included in the Russell 1000^® Growth Index, which is a market capitalization-weighted index designed to measure the performance of the large-cap growth segment of the U.S. equity market.

• 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.

• "Growth" Investing. "Growth" stocks can perform differently from the market as a whole and other types of stocks and can be more volatile than other types of stocks.

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.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Jun. 29, 2017

04.30 Fidelity Large Cap Growth Index Fund Investor, Premium PRO-04 | Fidelity® Large Cap Growth Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® Large Cap Growth 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 stocks of large capitalization U.S. companies.

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 June 7, 2016 to April 30, 2017, the fund's portfolio turnover rate was 17% (annualized) of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

17.00%

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 Russell 1000^® Growth Index, which is a market capitalization-weighted index designed to measure the performance of the large-cap growth segment of the 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

• 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.

• "Growth" Investing. "Growth" stocks can perform differently from the market as a whole and other types of stocks and can be more volatile than other types of stocks.

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.

04.30 Fidelity Large Cap Growth Index Fund Investor, Premium PRO-04 | Fidelity® Large Cap Growth Index Fund | Fidelity Large Cap Growth Index Fund-Investor Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.05%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.16%

Total annual operating expenses

rr_ExpensesOverAssets

0.21%

1 year

rr_ExpenseExampleYear01

$ 22

3 years

rr_ExpenseExampleYear03

68

5 years

rr_ExpenseExampleYear05

118

10 years

rr_ExpenseExampleYear10

$ 268

04.30 Fidelity Large Cap Growth Index Fund Investor, Premium PRO-04 | Fidelity® Large Cap Growth Index Fund | Fidelity Large Cap Growth Index Fund-Premium Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.05%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.02%

Total annual operating expenses

rr_ExpensesOverAssets

0.07%

1 year

rr_ExpenseExampleYear01

$ 7

3 years

rr_ExpenseExampleYear03

23

5 years

rr_ExpenseExampleYear05

40

10 years

rr_ExpenseExampleYear10

$ 90

XML

04.30 Fidelity Large Cap Value Index Fund Institutional, Institutional Premium PRO-05 | Fidelity® Large Cap Value Index Fund

Fund Summary

Fund/Class:
Fidelity® Large Cap Value Index Fund/Institutional, Institutional Premium

Investment Objective

The fund seeks to provide investment results that correspond to the total return of stocks of large capitalization U.S. companies.

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® Large Cap Value Index Fund}	04.30 Fidelity Large Cap Value Index Fund Institutional, Institutional Premium PRO-05 Fidelity® Large Cap Value 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® Large Cap Value Index Fund} - 04.30 Fidelity Large Cap Value Index Fund Institutional, Institutional Premium PRO-05 - Fidelity® Large Cap Value Index Fund	Fidelity Large Cap Value Index Fund-Institutional Class	Fidelity Large Cap Value Index Fund-Institutional Premium Class
Management fee	0.05%	0.05%
Distribution and/or Service (12b-1) fees	none	none
Other expenses	0.01%	none
Total annual operating expenses	0.06%	0.05%

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® Large Cap Value Index Fund} - 04.30 Fidelity Large Cap Value Index Fund Institutional, Institutional Premium PRO-05 - Fidelity® Large Cap Value Index Fund - USD ($)	Fidelity Large Cap Value Index Fund-Institutional Class	Fidelity Large Cap Value Index Fund-Institutional Premium Class
1 year	$ 6	$ 5
3 years	19	16
5 years	34	28
10 years	$ 77	$ 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 June 7, 2016 to April 30, 2017, the fund's portfolio turnover rate was 23% (annualized) of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in securities included in the Russell 1000^® Value Index, which is a market capitalization-weighted index designed to measure the performance of the large-cap value segment of the U.S. equity market.

• 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.

• "Value" Investing. "Value" stocks can perform differently from the market as a whole and other types of stocks and can continue to be undervalued by the market for long periods of time.

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.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Jun. 29, 2017

04.30 Fidelity Large Cap Value Index Fund Institutional, Institutional Premium PRO-05 | Fidelity® Large Cap Value Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® Large Cap Value 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 stocks of large capitalization U.S. companies.

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 June 7, 2016 to April 30, 2017, the fund's portfolio turnover rate was 23% (annualized) of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

23.00%

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 Russell 1000^® Value Index, which is a market capitalization-weighted index designed to measure the performance of the large-cap value segment of the 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

• 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.

• "Value" Investing. "Value" stocks can perform differently from the market as a whole and other types of stocks and can continue to be undervalued by the market for long periods of time.

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.

04.30 Fidelity Large Cap Value Index Fund Institutional, Institutional Premium PRO-05 | Fidelity® Large Cap Value Index Fund | Fidelity Large Cap Value Index Fund-Institutional Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.05%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.01%

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

04.30 Fidelity Large Cap Value Index Fund Institutional, Institutional Premium PRO-05 | Fidelity® Large Cap Value Index Fund | Fidelity Large Cap Value Index Fund-Institutional Premium Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.05%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

none

Total annual operating expenses

rr_ExpensesOverAssets

0.05%

1 year

rr_ExpenseExampleYear01

$ 5

3 years

rr_ExpenseExampleYear03

16

5 years

rr_ExpenseExampleYear05

28

10 years

rr_ExpenseExampleYear10

$ 64

XML

04.30 Fidelity Large Cap Value Index Fund Investor, Premium PRO-05 | Fidelity® Large Cap Value Index Fund

Fund Summary

Fund/Class:
Fidelity® Large Cap Value Index Fund/Investor, Premium

Investment Objective

The fund seeks to provide investment results that correspond to the total return of stocks of large capitalization U.S. companies.

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® Large Cap Value Index Fund}	04.30 Fidelity Large Cap Value Index Fund Investor, Premium PRO-05 Fidelity® Large Cap Value 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® Large Cap Value Index Fund} - 04.30 Fidelity Large Cap Value Index Fund Investor, Premium PRO-05 - Fidelity® Large Cap Value Index Fund	Fidelity Large Cap Value Index Fund-Investor Class	Fidelity Large Cap Value Index Fund-Premium Class
Management fee	0.05%	0.05%
Distribution and/or Service (12b-1) fees	none	none
Other expenses	0.16%	0.02%
Total annual operating expenses	0.21%	0.07%

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® Large Cap Value Index Fund} - 04.30 Fidelity Large Cap Value Index Fund Investor, Premium PRO-05 - Fidelity® Large Cap Value Index Fund - USD ($)	Fidelity Large Cap Value Index Fund-Investor Class	Fidelity Large Cap Value Index Fund-Premium Class
1 year	$ 22	$ 7
3 years	68	23
5 years	118	40
10 years	$ 268	$ 90

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 June 7, 2016 to April 30, 2017, the fund's portfolio turnover rate was 23% (annualized) of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in securities included in the Russell 1000^® Value Index, which is a market capitalization-weighted index designed to measure the performance of the large-cap value segment of the U.S. equity market.

• 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.

• "Value" Investing. "Value" stocks can perform differently from the market as a whole and other types of stocks and can continue to be undervalued by the market for long periods of time.

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.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Jun. 29, 2017

04.30 Fidelity Large Cap Value Index Fund Investor, Premium PRO-05 | Fidelity® Large Cap Value Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® Large Cap Value 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 stocks of large capitalization U.S. companies.

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 June 7, 2016 to April 30, 2017, the fund's portfolio turnover rate was 23% (annualized) of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

23.00%

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 Russell 1000^® Value Index, which is a market capitalization-weighted index designed to measure the performance of the large-cap value segment of the 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

• 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.

• "Value" Investing. "Value" stocks can perform differently from the market as a whole and other types of stocks and can continue to be undervalued by the market for long periods of time.

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.

04.30 Fidelity Large Cap Value Index Fund Investor, Premium PRO-05 | Fidelity® Large Cap Value Index Fund | Fidelity Large Cap Value Index Fund-Investor Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.05%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.16%

Total annual operating expenses

rr_ExpensesOverAssets

0.21%

1 year

rr_ExpenseExampleYear01

$ 22

3 years

rr_ExpenseExampleYear03

68

5 years

rr_ExpenseExampleYear05

118

10 years

rr_ExpenseExampleYear10

$ 268

04.30 Fidelity Large Cap Value Index Fund Investor, Premium PRO-05 | Fidelity® Large Cap Value Index Fund | Fidelity Large Cap Value Index Fund-Premium Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.05%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.02%

Total annual operating expenses

rr_ExpensesOverAssets

0.07%

1 year

rr_ExpenseExampleYear01

$ 7

3 years

rr_ExpenseExampleYear03

23

5 years

rr_ExpenseExampleYear05

40

10 years

rr_ExpenseExampleYear10

$ 90

XML

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Institutional, Institutional Premium PRO-07 | Fidelity® Mid Cap Index Fund

Fund Summary

Fund/Class:
Fidelity® Mid Cap Index Fund/Institutional, Institutional Premium

Investment Objective

The fund seeks to provide investment results that correspond to the total return of stocks of mid-capitalization United States companies.

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® Mid Cap Index Fund}	04.30 Fidelity Mid Cap Index & Small Cap Index Funds Institutional, Institutional Premium PRO-07 Fidelity® Mid Cap 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® Mid Cap Index Fund} - 04.30 Fidelity Mid Cap Index & Small Cap Index Funds Institutional, Institutional Premium PRO-07 - Fidelity® Mid Cap Index Fund		Fidelity Mid Cap Index Fund-Institutional Class	Fidelity Mid Cap Index Fund-Institutional Premium Class
Management fee	[1]	0.04%	0.04%
Distribution and/or Service (12b-1) fees		none	none
Other expenses		0.02%	none
Total annual operating expenses		0.06%	0.04%

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® Mid Cap Index Fund} - 04.30 Fidelity Mid Cap Index & Small Cap Index Funds Institutional, Institutional Premium PRO-07 - Fidelity® Mid Cap Index Fund - USD ($)	Fidelity Mid Cap Index Fund-Institutional Class	Fidelity Mid Cap Index Fund-Institutional Premium Class
1 year	$ 6	$ 4
3 years	19	13
5 years	34	23
10 years	$ 77	$ 51

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 14% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in securities included in the Russell Midcap^® 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.

• 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

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 Institutional Class:	Returns	Quarter ended
Highest Quarter Return	12.99%	March 31, 2012
Lowest Quarter Return	-8.01%	September 30, 2015
Year-to-Date Return	5.12%	March 31, 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® Mid Cap Index Fund} - 04.30 Fidelity Mid Cap Index & Small Cap Index Funds Institutional, Institutional Premium PRO-07 - Fidelity® Mid Cap Index Fund	Past 1 year	Past 5 years	Since Inception	[1]	Inception Date
Fidelity Mid Cap Index Fund-Institutional Class					Sep. 08, 2011
Fidelity Mid Cap Index Fund-Institutional Class | Return Before Taxes	13.84%	14.67%	14.67%
Fidelity Mid Cap Index Fund-Institutional Class | After Taxes on Distributions	13.21%	13.92%	13.94%		Sep. 08, 2011
Fidelity Mid Cap Index Fund-Institutional Class | After Taxes on Distributions and Sales	8.28%	11.66%	11.70%		Sep. 08, 2011
Fidelity Mid Cap Index Fund-Institutional Premium Class					Sep. 08, 2011
Fidelity Mid Cap Index Fund-Institutional Premium Class | Return Before Taxes	13.86%	14.70%	14.69%
Russell Midcap® Index(reflects no deduction for fees, expenses, or taxes)	13.80%	14.72%	14.73%

[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

Jun. 29, 2017

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Institutional, Institutional Premium PRO-07 | Fidelity® Mid Cap Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® Mid Cap 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 stocks of mid-capitalization United States companies.

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 14% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

14.00%

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 Russell Midcap^® 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.

• 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

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 Institutional Class:	Returns	Quarter ended
Highest Quarter Return	12.99%	March 31, 2012
Lowest Quarter Return	-8.01%	September 30, 2015
Year-to-Date Return	5.12%	March 31, 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

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Institutional, Institutional Premium PRO-07 | Fidelity® Mid Cap Index Fund | Fidelity Mid Cap Index Fund-Institutional Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.04%

^[1]

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.02%

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

2012

rr_AnnualReturn2012

17.19%

2013

rr_AnnualReturn2013

34.75%

2014

rr_AnnualReturn2014

13.09%

2015

rr_AnnualReturn2015

(2.46%)

2016

rr_AnnualReturn2016

13.84%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2017

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

5.12%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Mar. 31, 2012

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

12.99%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Sep. 30, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(8.01%)

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Institutional, Institutional Premium PRO-07 | Fidelity® Mid Cap Index Fund | Fidelity Mid Cap Index Fund-Institutional Premium Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.04%

^[1]

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

none

Total annual operating expenses

rr_ExpensesOverAssets

0.04%

1 year

rr_ExpenseExampleYear01

$ 4

3 years

rr_ExpenseExampleYear03

13

5 years

rr_ExpenseExampleYear05

23

10 years

rr_ExpenseExampleYear10

$ 51

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Institutional, Institutional Premium PRO-07 | Fidelity® Mid Cap Index Fund | Return Before Taxes | Fidelity Mid Cap Index Fund-Institutional Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Mid Cap Index Fund

Label

rr_AverageAnnualReturnLabel

Institutional Class - Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

13.84%

Past 5 years

rr_AverageAnnualReturnYear05

14.67%

Since Inception

rr_AverageAnnualReturnSinceInception

14.67%

^[2]

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Institutional, Institutional Premium PRO-07 | Fidelity® Mid Cap Index Fund | Return Before Taxes | Fidelity Mid Cap Index Fund-Institutional Premium Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Mid Cap Index Fund

Label

rr_AverageAnnualReturnLabel

Institutional Premium Class - Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

13.86%

Past 5 years

rr_AverageAnnualReturnYear05

14.70%

Since Inception

rr_AverageAnnualReturnSinceInception

14.69%

^[2]

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Institutional, Institutional Premium PRO-07 | Fidelity® Mid Cap Index Fund | After Taxes on Distributions | Fidelity Mid Cap Index Fund-Institutional Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Mid Cap Index Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions

Past 1 year

rr_AverageAnnualReturnYear01

13.21%

Past 5 years

rr_AverageAnnualReturnYear05

13.92%

Since Inception

rr_AverageAnnualReturnSinceInception

13.94%

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Institutional, Institutional Premium PRO-07 | Fidelity® Mid Cap Index Fund | After Taxes on Distributions and Sales | Fidelity Mid Cap Index Fund-Institutional Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Mid Cap Index Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions and Sale of Fund Shares

Past 1 year

rr_AverageAnnualReturnYear01

8.28%

Past 5 years

rr_AverageAnnualReturnYear05

11.66%

Since Inception

rr_AverageAnnualReturnSinceInception

11.70%

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Institutional, Institutional Premium PRO-07 | Fidelity® Mid Cap Index Fund | Russell Midcap® Index(reflects no deduction for fees, expenses, or taxes)

Risk/Return:

rr_RiskReturnAbstract

Past 1 year

rr_AverageAnnualReturnYear01

13.80%

Past 5 years

rr_AverageAnnualReturnYear05

14.72%

Since Inception

rr_AverageAnnualReturnSinceInception

14.73%

^[2]

[1] Adjusted to reflect current fees.

[2] From September 8, 2011

XML

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Institutional, Institutional Premium PRO-07 | Fidelity® Small Cap Index Fund

Fund Summary

Fund/Class:
Fidelity® Small Cap Index Fund/Institutional, Institutional Premium

Investment Objective

The fund seeks to provide investment results that correspond to the total return of stocks of small-capitalization United States companies.

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® Small Cap Index Fund}	04.30 Fidelity Mid Cap Index & Small Cap Index Funds Institutional, Institutional Premium PRO-07 Fidelity® Small Cap 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® Small Cap Index Fund} - 04.30 Fidelity Mid Cap Index & Small Cap Index Funds Institutional, Institutional Premium PRO-07 - Fidelity® Small Cap Index Fund		Fidelity Small Cap Index Fund-Institutional Class	Fidelity Small Cap Index Fund-Institutional Premium Class
Management fee	[1]	0.04%	0.04%
Distribution and/or Service (12b-1) fees		none	none
Other expenses		0.02%	none
Total annual operating expenses		0.06%	0.04%

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® Small Cap Index Fund} - 04.30 Fidelity Mid Cap Index & Small Cap Index Funds Institutional, Institutional Premium PRO-07 - Fidelity® Small Cap Index Fund - USD ($)	Fidelity Small Cap Index Fund-Institutional Class	Fidelity Small Cap Index Fund-Institutional Premium Class
1 year	$ 6	$ 4
3 years	19	13
5 years	34	23
10 years	$ 77	$ 51

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 11% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in securities included in the Russell 2000^® 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.

• 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.

• 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

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 Institutional Class:	Returns	Quarter ended
Highest Quarter Return	12.43%	March 31, 2013
Lowest Quarter Return	-11.85%	September 30, 2015
Year-to-Date Return	2.51%	March 31, 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® Small Cap Index Fund} - 04.30 Fidelity Mid Cap Index & Small Cap Index Funds Institutional, Institutional Premium PRO-07 - Fidelity® Small Cap Index Fund	Past 1 year	Past 5 years	Since Inception	[1]	Inception Date
Fidelity Small Cap Index Fund-Institutional Class					Sep. 08, 2011
Fidelity Small Cap Index Fund-Institutional Class | Return Before Taxes	21.60%	14.64%	15.17%
Fidelity Small Cap Index Fund-Institutional Class | After Taxes on Distributions	20.78%	13.72%	14.28%		Sep. 08, 2011
Fidelity Small Cap Index Fund-Institutional Class | After Taxes on Distributions and Sales	12.60%	11.50%	12.00%		Sep. 08, 2011
Fidelity Small Cap Index Fund-Institutional Premium Class					Sep. 08, 2011
Fidelity Small Cap Index Fund-Institutional Premium Class | Return Before Taxes	21.63%	14.66%	15.20%
Russell 2000® Index(reflects no deduction for fees, expenses, or taxes)	21.31%	14.46%	15.04%

[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

Jun. 29, 2017

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Institutional, Institutional Premium PRO-07 | Fidelity® Small Cap Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® Small Cap 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 stocks of small-capitalization United States companies.

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 11% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

11.00%

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 Russell 2000^® 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.

• 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.

• 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

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 Institutional Class:	Returns	Quarter ended
Highest Quarter Return	12.43%	March 31, 2013
Lowest Quarter Return	-11.85%	September 30, 2015
Year-to-Date Return	2.51%	March 31, 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

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Institutional, Institutional Premium PRO-07 | Fidelity® Small Cap Index Fund | Fidelity Small Cap Index Fund-Institutional Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.04%

^[1]

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.02%

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

2012

rr_AnnualReturn2012

16.35%

2013

rr_AnnualReturn2013

38.99%

2014

rr_AnnualReturn2014

5.17%

2015

rr_AnnualReturn2015

(4.26%)

2016

rr_AnnualReturn2016

21.60%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2017

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

2.51%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Mar. 31, 2013

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

12.43%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Sep. 30, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(11.85%)

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Institutional, Institutional Premium PRO-07 | Fidelity® Small Cap Index Fund | Fidelity Small Cap Index Fund-Institutional Premium Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.04%

^[1]

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

none

Total annual operating expenses

rr_ExpensesOverAssets

0.04%

1 year

rr_ExpenseExampleYear01

$ 4

3 years

rr_ExpenseExampleYear03

13

5 years

rr_ExpenseExampleYear05

23

10 years

rr_ExpenseExampleYear10

$ 51

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Institutional, Institutional Premium PRO-07 | Fidelity® Small Cap Index Fund | Return Before Taxes | Fidelity Small Cap Index Fund-Institutional Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Small Cap Index Fund

Label

rr_AverageAnnualReturnLabel

Institutional Class - Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

21.60%

Past 5 years

rr_AverageAnnualReturnYear05

14.64%

Since Inception

rr_AverageAnnualReturnSinceInception

15.17%

^[2]

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Institutional, Institutional Premium PRO-07 | Fidelity® Small Cap Index Fund | Return Before Taxes | Fidelity Small Cap Index Fund-Institutional Premium Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Small Cap Index Fund

Label

rr_AverageAnnualReturnLabel

Institutional Premium Class - Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

21.63%

Past 5 years

rr_AverageAnnualReturnYear05

14.66%

Since Inception

rr_AverageAnnualReturnSinceInception

15.20%

^[2]

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Institutional, Institutional Premium PRO-07 | Fidelity® Small Cap Index Fund | After Taxes on Distributions | Fidelity Small Cap Index Fund-Institutional Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Small Cap Index Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions

Past 1 year

rr_AverageAnnualReturnYear01

20.78%

Past 5 years

rr_AverageAnnualReturnYear05

13.72%

Since Inception

rr_AverageAnnualReturnSinceInception

14.28%

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Institutional, Institutional Premium PRO-07 | Fidelity® Small Cap Index Fund | After Taxes on Distributions and Sales | Fidelity Small Cap Index Fund-Institutional Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Small Cap Index Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions and Sale of Fund Shares

Past 1 year

rr_AverageAnnualReturnYear01

12.60%

Past 5 years

rr_AverageAnnualReturnYear05

11.50%

Since Inception

rr_AverageAnnualReturnSinceInception

12.00%

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Institutional, Institutional Premium PRO-07 | Fidelity® Small Cap Index Fund | Russell 2000® Index(reflects no deduction for fees, expenses, or taxes)

Risk/Return:

rr_RiskReturnAbstract

Past 1 year

rr_AverageAnnualReturnYear01

21.31%

Past 5 years

rr_AverageAnnualReturnYear05

14.46%

Since Inception

rr_AverageAnnualReturnSinceInception

15.04%

^[2]

[1] Adjusted to reflect current fees.

[2] From September 8, 2011

XML

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Investor, Premium Combo PRO-06 | Fidelity® Mid Cap Index Fund

Fund Summary

Fund/Class:
Fidelity® Mid Cap Index Fund/Investor, Premium

Investment Objective

The fund seeks to provide investment results that correspond to the total return of stocks of mid-capitalization United States companies.

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® Mid Cap Index Fund}	04.30 Fidelity Mid Cap Index & Small Cap Index Funds Investor, Premium Combo PRO-06 Fidelity® Mid Cap 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® Mid Cap Index Fund} - 04.30 Fidelity Mid Cap Index & Small Cap Index Funds Investor, Premium Combo PRO-06 - Fidelity® Mid Cap Index Fund		Fidelity Mid Cap Index Fund-Investor Class	Fidelity Mid Cap Index Fund-Premium Class
Management fee	[1]	0.04%	0.04%
Distribution and/or Service (12b-1) fees		none	none
Other expenses	[1]	0.15%	0.03%
Total annual operating expenses		0.19%	0.07%

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® Mid Cap Index Fund} - 04.30 Fidelity Mid Cap Index & Small Cap Index Funds Investor, Premium Combo PRO-06 - Fidelity® Mid Cap Index Fund - USD ($)	Fidelity Mid Cap Index Fund-Investor Class	Fidelity Mid Cap Index Fund-Premium Class
1 year	$ 19	$ 7
3 years	61	23
5 years	107	40
10 years	$ 243	$ 90

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 14% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in securities included in the Russell Midcap^® 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.

• 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

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	12.89%	March 31, 2013
Lowest Quarter Return	-8.07%	September 30, 2015
Year-to-Date Return	5.07%	March 31, 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® Mid Cap Index Fund} - 04.30 Fidelity Mid Cap Index & Small Cap Index Funds Investor, Premium Combo PRO-06 - Fidelity® Mid Cap Index Fund	Past 1 year	Past 5 years	Since Inception	[1]	Inception Date
Fidelity Mid Cap Index Fund-Investor Class					Sep. 08, 2011
Fidelity Mid Cap Index Fund-Investor Class | Return Before Taxes	13.62%	14.48%	14.48%
Fidelity Mid Cap Index Fund-Investor Class | After Taxes on Distributions	13.05%	13.80%	13.82%		Sep. 08, 2011
Fidelity Mid Cap Index Fund-Investor Class | After Taxes on Distributions and Sales	8.15%	11.53%	11.58%		Sep. 08, 2011
Fidelity Mid Cap Index Fund-Premium Class					Sep. 08, 2011
Fidelity Mid Cap Index Fund-Premium Class | Return Before Taxes	13.82%	14.65%	14.65%
Russell Midcap® Index(reflects no deduction for fees, expenses, or taxes)	13.80%	14.72%	14.73%

[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

Jun. 29, 2017

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Investor, Premium Combo PRO-06 | Fidelity® Mid Cap Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® Mid Cap 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 stocks of mid-capitalization United States companies.

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 14% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

14.00%

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 Russell Midcap^® 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.

• 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

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	12.89%	March 31, 2013
Lowest Quarter Return	-8.07%	September 30, 2015
Year-to-Date Return	5.07%	March 31, 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

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Investor, Premium Combo PRO-06 | Fidelity® Mid Cap Index Fund | Fidelity Mid Cap Index Fund-Investor Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.04%

^[1]

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.15%

^[1]

Total annual operating expenses

rr_ExpensesOverAssets

0.19%

1 year

rr_ExpenseExampleYear01

$ 19

3 years

rr_ExpenseExampleYear03

61

5 years

rr_ExpenseExampleYear05

107

10 years

rr_ExpenseExampleYear10

$ 243

2012

rr_AnnualReturn2012

16.96%

2013

rr_AnnualReturn2013

34.48%

2014

rr_AnnualReturn2014

12.99%

2015

rr_AnnualReturn2015

(2.62%)

2016

rr_AnnualReturn2016

13.62%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2017

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

5.07%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Mar. 31, 2013

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

12.89%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Sep. 30, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(8.07%)

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Investor, Premium Combo PRO-06 | Fidelity® Mid Cap Index Fund | Fidelity Mid Cap Index Fund-Premium Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.04%

^[1]

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.03%

^[1]

Total annual operating expenses

rr_ExpensesOverAssets

0.07%

1 year

rr_ExpenseExampleYear01

$ 7

3 years

rr_ExpenseExampleYear03

23

5 years

rr_ExpenseExampleYear05

40

10 years

rr_ExpenseExampleYear10

$ 90

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Investor, Premium Combo PRO-06 | Fidelity® Mid Cap Index Fund | Return Before Taxes | Fidelity Mid Cap Index Fund-Investor Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Mid Cap Index Fund

Label

rr_AverageAnnualReturnLabel

Investor Class - Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

13.62%

Past 5 years

rr_AverageAnnualReturnYear05

14.48%

Since Inception

rr_AverageAnnualReturnSinceInception

14.48%

^[2]

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Investor, Premium Combo PRO-06 | Fidelity® Mid Cap Index Fund | Return Before Taxes | Fidelity Mid Cap Index Fund-Premium Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Mid Cap Index Fund

Label

rr_AverageAnnualReturnLabel

Premium Class - Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

13.82%

Past 5 years

rr_AverageAnnualReturnYear05

14.65%

Since Inception

rr_AverageAnnualReturnSinceInception

14.65%

^[2]

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Investor, Premium Combo PRO-06 | Fidelity® Mid Cap Index Fund | After Taxes on Distributions | Fidelity Mid Cap Index Fund-Investor Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Mid Cap Index Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions

Past 1 year

rr_AverageAnnualReturnYear01

13.05%

Past 5 years

rr_AverageAnnualReturnYear05

13.80%

Since Inception

rr_AverageAnnualReturnSinceInception

13.82%

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Investor, Premium Combo PRO-06 | Fidelity® Mid Cap Index Fund | After Taxes on Distributions and Sales | Fidelity Mid Cap Index Fund-Investor Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Mid Cap Index Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions and Sale of Fund Shares

Past 1 year

rr_AverageAnnualReturnYear01

8.15%

Past 5 years

rr_AverageAnnualReturnYear05

11.53%

Since Inception

rr_AverageAnnualReturnSinceInception

11.58%

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Investor, Premium Combo PRO-06 | Fidelity® Mid Cap Index Fund | Russell Midcap® Index(reflects no deduction for fees, expenses, or taxes)

Risk/Return:

rr_RiskReturnAbstract

Past 1 year

rr_AverageAnnualReturnYear01

13.80%

Past 5 years

rr_AverageAnnualReturnYear05

14.72%

Since Inception

rr_AverageAnnualReturnSinceInception

14.73%

^[2]

[1] Adjusted to reflect current fees.

[2] From September 8, 2011

XML

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Investor, Premium Combo PRO-06 | Fidelity® Small Cap Index Fund

Fund Summary

Fund/Class:
Fidelity® Small Cap Index Fund/Investor, Premium

Investment Objective

The fund seeks to provide investment results that correspond to the total return of stocks of small-capitalization United States companies.

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® Small Cap Index Fund}	04.30 Fidelity Mid Cap Index & Small Cap Index Funds Investor, Premium Combo PRO-06 Fidelity® Small Cap 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® Small Cap Index Fund} - 04.30 Fidelity Mid Cap Index & Small Cap Index Funds Investor, Premium Combo PRO-06 - Fidelity® Small Cap Index Fund		Fidelity Small Cap Index Fund-Investor Class	Fidelity Small Cap Index Fund-Premium Class
Management fee	[1]	0.04%	0.04%
Distribution and/or Service (12b-1) fees		none	none
Other expenses	[1]	0.15%	0.03%
Total annual operating expenses		0.19%	0.07%

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® Small Cap Index Fund} - 04.30 Fidelity Mid Cap Index & Small Cap Index Funds Investor, Premium Combo PRO-06 - Fidelity® Small Cap Index Fund - USD ($)	Fidelity Small Cap Index Fund-Investor Class	Fidelity Small Cap Index Fund-Premium Class
1 year	$ 19	$ 7
3 years	61	23
5 years	107	40
10 years	$ 243	$ 90

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 11% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in securities included in the Russell 2000^® 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.

• 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.

• 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

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	12.36%	March 31, 2013
Lowest Quarter Return	-11.92%	September 30, 2015
Year-to-Date Return	2.51%	March 31, 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® Small Cap Index Fund} - 04.30 Fidelity Mid Cap Index & Small Cap Index Funds Investor, Premium Combo PRO-06 - Fidelity® Small Cap Index Fund	Past 1 year	Past 5 years	Since Inception	[1]	Inception Date
Fidelity Small Cap Index Fund-Investor Class					Sep. 08, 2011
Fidelity Small Cap Index Fund-Investor Class | Return Before Taxes	21.37%	14.45%	14.99%
Fidelity Small Cap Index Fund-Investor Class | After Taxes on Distributions	20.63%	13.60%	14.17%		Sep. 08, 2011
Fidelity Small Cap Index Fund-Investor Class | After Taxes on Distributions and Sales	12.47%	11.38%	11.88%		Sep. 08, 2011
Fidelity Small Cap Index Fund-Premium Class					Sep. 08, 2011
Fidelity Small Cap Index Fund-Premium Class | Return Before Taxes	21.58%	14.62%	15.15%
Russell 2000® Index(reflects no deduction for fees, expenses, or taxes)	21.31%	14.46%	15.04%

[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

Jun. 29, 2017

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Investor, Premium Combo PRO-06 | Fidelity® Small Cap Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® Small Cap 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 stocks of small-capitalization United States companies.

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 11% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

11.00%

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 Russell 2000^® 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.

• 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.

• 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

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	12.36%	March 31, 2013
Lowest Quarter Return	-11.92%	September 30, 2015
Year-to-Date Return	2.51%	March 31, 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

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Investor, Premium Combo PRO-06 | Fidelity® Small Cap Index Fund | Fidelity Small Cap Index Fund-Investor Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.04%

^[1]

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.15%

^[1]

Total annual operating expenses

rr_ExpensesOverAssets

0.19%

1 year

rr_ExpenseExampleYear01

$ 19

3 years

rr_ExpenseExampleYear03

61

5 years

rr_ExpenseExampleYear05

107

10 years

rr_ExpenseExampleYear10

$ 243

2012

rr_AnnualReturn2012

16.12%

2013

rr_AnnualReturn2013

38.71%

2014

rr_AnnualReturn2014

5.01%

2015

rr_AnnualReturn2015

(4.36%)

2016

rr_AnnualReturn2016

21.37%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Mar. 31, 2017

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

2.51%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Mar. 31, 2013

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

12.36%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Sep. 30, 2015

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(11.92%)

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Investor, Premium Combo PRO-06 | Fidelity® Small Cap Index Fund | Fidelity Small Cap Index Fund-Premium Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.04%

^[1]

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.03%

^[1]

Total annual operating expenses

rr_ExpensesOverAssets

0.07%

1 year

rr_ExpenseExampleYear01

$ 7

3 years

rr_ExpenseExampleYear03

23

5 years

rr_ExpenseExampleYear05

40

10 years

rr_ExpenseExampleYear10

$ 90

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Investor, Premium Combo PRO-06 | Fidelity® Small Cap Index Fund | Return Before Taxes | Fidelity Small Cap Index Fund-Investor Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Small Cap Index Fund

Label

rr_AverageAnnualReturnLabel

Investor Class - Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

21.37%

Past 5 years

rr_AverageAnnualReturnYear05

14.45%

Since Inception

rr_AverageAnnualReturnSinceInception

14.99%

^[2]

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Investor, Premium Combo PRO-06 | Fidelity® Small Cap Index Fund | Return Before Taxes | Fidelity Small Cap Index Fund-Premium Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Small Cap Index Fund

Label

rr_AverageAnnualReturnLabel

Premium Class - Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

21.58%

Past 5 years

rr_AverageAnnualReturnYear05

14.62%

Since Inception

rr_AverageAnnualReturnSinceInception

15.15%

^[2]

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Investor, Premium Combo PRO-06 | Fidelity® Small Cap Index Fund | After Taxes on Distributions | Fidelity Small Cap Index Fund-Investor Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Small Cap Index Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions

Past 1 year

rr_AverageAnnualReturnYear01

20.63%

Past 5 years

rr_AverageAnnualReturnYear05

13.60%

Since Inception

rr_AverageAnnualReturnSinceInception

14.17%

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Investor, Premium Combo PRO-06 | Fidelity® Small Cap Index Fund | After Taxes on Distributions and Sales | Fidelity Small Cap Index Fund-Investor Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Small Cap Index Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions and Sale of Fund Shares

Past 1 year

rr_AverageAnnualReturnYear01

12.47%

Past 5 years

rr_AverageAnnualReturnYear05

11.38%

Since Inception

rr_AverageAnnualReturnSinceInception

11.88%

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

Sep. 08, 2011

04.30 Fidelity Mid Cap Index & Small Cap Index Funds Investor, Premium Combo PRO-06 | Fidelity® Small Cap Index Fund | Russell 2000® Index(reflects no deduction for fees, expenses, or taxes)

Risk/Return:

rr_RiskReturnAbstract

Past 1 year

rr_AverageAnnualReturnYear01

21.31%

Past 5 years

rr_AverageAnnualReturnYear05

14.46%

Since Inception

rr_AverageAnnualReturnSinceInception

15.04%

^[2]

[1] Adjusted to reflect current fees.

[2] From September 8, 2011

XML

04.30 Fidelity Flex Mid Cap Index Fund & Fidelity Flex Small Cap Index Fund combo PRO-02 | Fidelity Flex℠ Mid Cap Index Fund

Fund Summary

Fund:
Fidelity Flex℠ Mid Cap Index Fund

Investment Objective

The fund seeks to provide investment results that correspond to the total return of stocks of mid-capitalization United States companies.

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℠ Mid Cap Index Fund}	04.30 Fidelity Flex Mid Cap Index Fund & Fidelity Flex Small Cap Index Fund combo PRO-02 Fidelity Flex℠ Mid Cap 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℠ Mid Cap Index Fund}	04.30 Fidelity Flex Mid Cap Index Fund & Fidelity Flex Small Cap Index Fund combo PRO-02 Fidelity Flex℠ Mid Cap Index Fund Fidelity Flex Mid Cap Index Fund-Default
Management fee	none
Distribution and/or Service (12b-1) fees	none
Other expenses	none	[1]
Total annual operating expenses	none	[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℠ Mid Cap Index Fund}	04.30 Fidelity Flex Mid Cap Index Fund & Fidelity Flex Small Cap Index Fund combo PRO-02 Fidelity Flex℠ Mid Cap Index Fund Fidelity Flex Mid Cap Index Fund-Default USD ($)
1 year	none
3 years	none

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 April 30, 2017, the fund's portfolio turnover rate was 1% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in securities included in the Russell Midcap® Index, which is a market capitalization-weighted index of medium-capitalization U.S. company stocks.

• 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.

[2] The fund is available only to certain fee-based accounts offered by Fidelity. 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

Jun. 29, 2017

04.30 Fidelity Flex Mid Cap Index Fund & Fidelity Flex Small Cap Index Fund combo PRO-02 | Fidelity Flex℠ Mid Cap Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund:
Fidelity Flex℠ Mid Cap 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 stocks of mid-capitalization United States companies.

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 April 30, 2017, the fund's portfolio turnover rate was 1% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

1.00%

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. 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 Russell Midcap® Index, which is a market capitalization-weighted index of medium-capitalization U.S. company stocks.

• 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.

04.30 Fidelity Flex Mid Cap Index Fund & Fidelity Flex Small Cap Index Fund combo PRO-02 | Fidelity Flex℠ Mid Cap Index Fund | Fidelity Flex Mid Cap 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

^[1]

Total annual operating expenses

rr_ExpensesOverAssets

none

^[2]

1 year

rr_ExpenseExampleYear01

none

3 years

rr_ExpenseExampleYear03

none

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

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

XML

04.30 Fidelity Flex Mid Cap Index Fund & Fidelity Flex Small Cap Index Fund combo PRO-02 | Fidelity Flex℠ Small Cap Index Fund

Fund Summary

Fund:
Fidelity Flex℠ Small Cap Index Fund

Investment Objective

The fund seeks to provide investment results that correspond to the total return of stocks of small-capitalization United States companies.

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℠ Small Cap Index Fund}	04.30 Fidelity Flex Mid Cap Index Fund & Fidelity Flex Small Cap Index Fund combo PRO-02 Fidelity Flex℠ Small Cap 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℠ Small Cap Index Fund}	04.30 Fidelity Flex Mid Cap Index Fund & Fidelity Flex Small Cap Index Fund combo PRO-02 Fidelity Flex℠ Small Cap Index Fund FIdelity Flex Small Cap Index Fund-Default
Management fee	none
Distribution and/or Service (12b-1) fees	none
Other expenses	none	[1]
Total annual operating expenses	none	[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℠ Small Cap Index Fund}	04.30 Fidelity Flex Mid Cap Index Fund & Fidelity Flex Small Cap Index Fund combo PRO-02 Fidelity Flex℠ Small Cap Index Fund FIdelity Flex Small Cap Index Fund-Default USD ($)
1 year	none
3 years	none

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 April 30, 2017, 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 Russell 2000® Index, which is a market capitalization-weighted index designed to measure the performance of the small-cap segment of the U.S. equity market.

• 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.

• 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] Based on estimated amounts for the current fiscal year.

[2] The fund is available only to certain fee-based accounts offered by Fidelity. 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

Jun. 29, 2017

04.30 Fidelity Flex Mid Cap Index Fund & Fidelity Flex Small Cap Index Fund combo PRO-02 | Fidelity Flex℠ Small Cap Index Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund:
Fidelity Flex℠ Small Cap 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 stocks of small-capitalization United States companies.

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 April 30, 2017, the fund's portfolio turnover rate was 2% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

2.00%

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. 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 Russell 2000® Index, which is a market capitalization-weighted index designed to measure the performance of the small-cap segment of the 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

• 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.

• 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.

04.30 Fidelity Flex Mid Cap Index Fund & Fidelity Flex Small Cap Index Fund combo PRO-02 | Fidelity Flex℠ Small Cap Index Fund | FIdelity Flex Small Cap 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

^[1]

Total annual operating expenses

rr_ExpensesOverAssets

none

^[2]

1 year

rr_ExpenseExampleYear01

none

3 years

rr_ExpenseExampleYear03

none

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

[2] The fund is available only to certain fee-based accounts offered by Fidelity. 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	Jun. 29, 2017
Document Creation Date	dei_DocumentCreationDate	Jun. 27, 2017