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.  437

and

REGISTRATION STATEMENT UNDER THE INVESTMENT COMPANY ACT OF 1940 x

x Amendment No.  437

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) of Rule 485.

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. 437to 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 14th day of September 2018.

	Fidelity Salem Street Trust
	By	/s/Laura M. Del Prato
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||		Laura M. Del Prato, 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/Laura M. Del Prato		President and Treasurer	September 14, 2018
Laura M. Del Prato		(Principal Executive Officer)
/s/John J. Burke III		Chief Financial Officer	September 14, 2018
John J. Burke III		(Principal Financial Officer)
/s/Abigail P. Johnson	†	Trustee	September 14, 2018
Abigail P. Johnson
/s/Elizabeth S. Acton	*	Trustee	September 14, 2018
Elizabeth S. Acton
/s/Jonathan Chiel	*	Trustee	September 14, 2018
Jonathan Chiel
/s/John Engler	*	Trustee	September 14, 2018
John Engler
/s/Albert R. Gamper, Jr.	*	Trustee	September 14, 2018
Albert R. Gamper, Jr.
/s/Robert F. Gartland	*	Trustee	September 14, 2018
Robert F. Gartland
/s/Arthur E. Johnson	*	Trustee	September 14, 2018
Arthur E. Johnson
/s/Michael E. Kenneally	*	Trustee	September 14, 2018
Michael E. Kenneally
/s/Marie L. Knowles	*	Trustee	September 14, 2018
Marie L. Knowles
/s/Mark A. Murray	*	Trustee	September 14, 2018
Mark A. Murray
/s/Jennifer Toolin McAuliffe	*	Trustee	September 14, 2018
Jennifer Toolin McAuliffe

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

POWER OF ATTORNEY

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

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

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

WITNESS our hands on this first day of September 2017.

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

POWER OF ATTORNEY

I, the undersigned Trustee of the following investment companies:

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

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

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

/s/Abigail P. Johnson

Abigail P. Johnson

POWER OF ATTORNEY

I, the undersigned Trustee of the following investment company:

Fidelity Salem Street Trust

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

WITNESS my hand on this 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	Jun. 30, 2018
Registrant Name	dei_EntityRegistrantName	FIDELITY SALEM STREET TRUST
Central Index Key	dei_EntityCentralIndexKey	0000035315
Amendment Flag	dei_AmendmentFlag	false
Document Creation Date	dei_DocumentCreationDate	Aug. 27, 2018
Document Effective Date	dei_DocumentEffectiveDate	Aug. 29, 2018
Prospectus Date	rr_ProspectusDate	Aug. 29, 2018

XML

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2019 Fund

Fund Summary

Fund/Class:
Fidelity® Municipal Income 2019 Fund/Fidelity Advisor® Municipal Income 2019 Fund A, I

Investment Objective

The fund seeks as high a level of current income, exempt from federal income tax, as is consistent with the preservation of capital.

Fee Table

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund. In addition to the fees and expenses described below, your broker may also require you to pay brokerage commissions on purchases and sales of certain share classes of the fund.

You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $50,000 in the fund or certain other Fidelity^® funds. More information about these and other discounts is available from your investment professional and in the "Fund Distribution" section beginning on page 50 of the prospectus. Different intermediaries may provide additional waivers or reductions of the sales charge. Please see “Sales Charge Waiver Policies Applied by Certain Intermediaries” in the “Appendix” section of the prospectus.

Shareholder fees

(fees paid directly from your investment)

Shareholder Fees {- Fidelity® Municipal Income 2019 Fund} - 06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 - Fidelity® Municipal Income 2019 Fund	Fidelity Municipal Income 2019 Fund-Class A		Fidelity Municipal Income 2019 Fund - Class I
Maximum sales charge (load) on purchases (as a % of offering price)	2.75%		none
Maximum contingent deferred sales charge (as a % of the lesser of original purchase price or redemption proceeds)	none	[1]	none

Annual Operating Expenses

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

Annual Operating Expenses {- Fidelity® Municipal Income 2019 Fund} - 06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 - Fidelity® Municipal Income 2019 Fund	Fidelity Municipal Income 2019 Fund-Class A	Fidelity Municipal Income 2019 Fund - Class I
Management fee	0.30%	0.30%
Distribution and/or Service (12b-1) fees	0.25%	none
Other expenses	0.10%	0.10%
Total annual operating expenses	0.65%	0.40%

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® Municipal Income 2019 Fund} - 06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 - Fidelity® Municipal Income 2019 Fund - USD ($)	Fidelity Municipal Income 2019 Fund-Class A	Fidelity Municipal Income 2019 Fund - Class I
1 year	$ 340	$ 41
3 years	477	128
5 years	627	224
10 years	$ 1,063	$ 505

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 7% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in investment-grade municipal securities whose interest is exempt from federal income tax (but may be subject to the federal alternative minimum tax).

• Managing the fund to have similar overall interest rate risk compared to a customized municipal bond index representing Fidelity Management & Research Company (FMR)'s view of the performance of the fund’s competitive universe.

• Allocating assets across different market sectors and maturities.

• Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects.

• Analyzing the credit quality of the issuer, security-specific features, current and potential future valuation, and trading opportunities to select investments.

• Liquidating shortly after the fund's target end-date of June 30, 2019.

Principal Investment Risks

• Municipal Market Volatility. The municipal market is volatile and can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities.

• Interest Rate Changes. Interest rate increases can cause the price of a debt security to decrease.

• 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. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a security to decrease.

• Declining Yield Risk. As the fund approaches its liquidation date, the fund's securities will mature and the fund may reinvest the proceeds in money market securities with lower yields than the securities previously held by the fund.

• Fluctuation of Yield & Liquidation Amount Risk. The amount of the fund's income distributions will vary over time, and the breakdown of returns between fund distributions and liquidation proceeds will not be predictable at the time of your investment, and you may experience a gain or loss for tax purposes.

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. Unlike individual debt securities, which typically pay principal at maturity, the value of an investment in the fund will fluctuate. 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 and an additional index over various periods of time. The indexes have characteristics relevant to the fund's investment strategies. Index descriptions appear in the "Additional Index Information" section of the prospectus. Past performance (before and after taxes) is not an indication of future performance.

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

The returns in the bar chart do not reflect any applicable sales charges; if sales charges were reflected, returns would be lower than those shown.

Year-by-Year Returns

Calendar Years

During the periods shown in the chart for Class A:	Returns	Quarter ended
Highest Quarter Return	2.18%	September 30, 2012
Lowest Quarter Return	-2.61%	June 30, 2013
Year-to-Date Return	0.63%	June 30, 2018

Average Annual Returns

Unlike the returns in the bar chart, the returns in the table reflect the maximum applicable sales charges. 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 Class A 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. Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.

For the periods ended December 31, 2017

Average Annual Total Returns{- Fidelity® Municipal Income 2019 Fund} - 06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 - Fidelity® Municipal Income 2019 Fund	Past 1 year	Past 5 years	Since Inception	[1]	Inception Date
Fidelity Municipal Income 2019 Fund-Class A					May 19, 2011
Fidelity Municipal Income 2019 Fund-Class A | Return Before Taxes	(1.74%)	0.38%	2.02%
Fidelity Municipal Income 2019 Fund-Class A | After Taxes on Distributions	(1.75%)	0.36%	2.00%		May 19, 2011
Fidelity Municipal Income 2019 Fund-Class A | After Taxes on Distributions and Sales	(0.52%)	0.62%	1.92%		May 19, 2011
Fidelity Municipal Income 2019 Fund - Class I					May 19, 2011
Fidelity Municipal Income 2019 Fund - Class I | Return Before Taxes	1.29%	1.19%	2.70%
Bloomberg Barclays Municipal Bond Index(reflects no deduction for fees, expenses, or taxes)	5.45%	3.02%	4.28%
Bloomberg Barclays Municipal Bond 2019 Index Ex Tobacco/Prepaid Gas(reflects no deduction for fees, expenses, or taxes)	1.45%	1.33%	2.85%

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2019 Fund | Fidelity Municipal Income 2019 Fund-Class A

Purchases of $250,000 or more will not be subject to a front-end sales charge, but may be subject to a 0.50% contingent deferred sales charge (CDSC) if the intermediary firm has elected an upfront finder's fee at the time the shares are purchased, or a 0.75% CDSC if the shares purchased are recordkept in a Fidelity Advisor® 401(k) Retirement Plan.

[1] Purchases of $250,000 or more will not be subject to a front-end sales charge, but may be subject to a 0.50% contingent deferred sales charge (CDSC) if the intermediary firm has elected an upfront finder's fee at the time the shares are purchased, or a 0.75% CDSC if the shares purchased are recordkept in a Fidelity Advisor® 401(k) Retirement Plan.

[1] From May 19, 2011

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Aug. 29, 2018

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2019 Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® Municipal Income 2019 Fund/Fidelity Advisor® Municipal Income 2019 Fund A, I

Objective [Heading]

rr_ObjectiveHeading

Investment Objective

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks as high a level of current income, exempt from federal income tax, as is consistent with the preservation of capital.

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. In addition to the fees and expenses described below, your broker may also require you to pay brokerage commissions on purchases and sales of certain share classes of the fund.

You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $50,000 in the fund or certain other Fidelity^® funds. More information about these and other discounts is available from your investment professional and in the "Fund Distribution" section beginning on page 50 of the prospectus. Different intermediaries may provide additional waivers or reductions of the sales charge. Please see “Sales Charge Waiver Policies Applied by Certain Intermediaries” in the “Appendix” section of the prospectus.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

Shareholder fees

(fees paid directly from your investment)

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

Annual Operating Expenses

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

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 7% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

7.00%

Expense Breakpoint Discounts [Text]

rr_ExpenseBreakpointDiscounts

You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $50,000 in the fund or certain other Fidelity® funds. More information about these and other discounts is available from your investment professional and in the "Fund Distribution" section beginning on page 50 of the prospectus.

Expense Breakpoint, Minimum Investment Required [Amount]

rr_ExpenseBreakpointMinimumInvestmentRequiredAmount

$ 50,000

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 investment-grade municipal securities whose interest is exempt from federal income tax (but may be subject to the federal alternative minimum tax).

• Managing the fund to have similar overall interest rate risk compared to a customized municipal bond index representing Fidelity Management & Research Company (FMR)'s view of the performance of the fund’s competitive universe.

• Allocating assets across different market sectors and maturities.

• Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects.

• Analyzing the credit quality of the issuer, security-specific features, current and potential future valuation, and trading opportunities to select investments.

• Liquidating shortly after the fund's target end-date of June 30, 2019.

Risk [Heading]

rr_RiskHeading

Principal Investment Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Municipal Market Volatility. The municipal market is volatile and can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities.

• Interest Rate Changes. Interest rate increases can cause the price of a debt security to decrease.

• 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. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a security to decrease.

• Declining Yield Risk. As the fund approaches its liquidation date, the fund's securities will mature and the fund may reinvest the proceeds in money market securities with lower yields than the securities previously held by the fund.

• Fluctuation of Yield & Liquidation Amount Risk. The amount of the fund's income distributions will vary over time, and the breakdown of returns between fund distributions and liquidation proceeds will not be predictable at the time of your investment, and you may experience a gain or loss for tax purposes.

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. Unlike individual debt securities, which typically pay principal at maturity, the value of an investment in the fund will fluctuate. 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 and an additional index over various periods of time. The indexes have characteristics relevant to the fund's investment strategies. Index descriptions appear in the "Additional Index Information" section of the prospectus. Past performance (before and after taxes) is not an indication of future performance.

Visit institutional.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 and an additional index over various periods of time.

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

institutional.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 Narrative [Text Block]

rr_BarChartNarrativeTextBlock

The returns in the bar chart do not reflect any applicable sales charges; if sales charges were reflected, returns would be lower than those shown.

Bar Chart Does Not Reflect Sales Loads [Text]

rr_BarChartDoesNotReflectSalesLoads

The returns in the bar chart do not reflect any applicable sales charges; if sales charges were reflected, returns would be lower than those shown.

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart for Class A:	Returns	Quarter ended
Highest Quarter Return	2.18%	September 30, 2012
Lowest Quarter Return	-2.61%	June 30, 2013
Year-to-Date Return	0.63%	June 30, 2018

Performance Table Heading

rr_PerformanceTableHeading

Average Annual Returns

Performance Table Does Reflect Sales Loads

rr_PerformanceTableDoesReflectSalesLoads

Unlike the returns in the bar chart, the returns in the table reflect the maximum applicable sales charges.

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 One Class of after Tax Shown [Text]

rr_PerformanceTableOneClassOfAfterTaxShown

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

Performance Table Explanation after Tax Higher

rr_PerformanceTableExplanationAfterTaxHigher

Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

Unlike the returns in the bar chart, the returns in the table reflect the maximum applicable sales charges. 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 Class A 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. Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.

For the periods ended December 31, 2017

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2019 Fund | Fidelity Municipal Income 2019 Fund-Class A

Risk/Return:

rr_RiskReturnAbstract

Maximum sales charge (load) on purchases (as a % of offering price)

rr_MaximumCumulativeSalesChargeOverOfferingPrice

2.75%

Maximum contingent deferred sales charge (as a % of the lesser of original purchase price or redemption proceeds)

rr_MaximumDeferredSalesChargeOverOther

none

^[1]

Management fee

rr_ManagementFeesOverAssets

0.30%

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

rr_DistributionAndService12b1FeesOverAssets

0.25%

Other expenses

rr_OtherExpensesOverAssets

0.10%

Total annual operating expenses

rr_ExpensesOverAssets

0.65%

Expenses Deferred Charges [Text Block]

rr_ExpensesDeferredChargesTextBlock

Purchases of $250,000 or more will not be subject to a front-end sales charge, but may be subject to a 0.50% contingent deferred sales charge (CDSC) if the intermediary firm has elected an upfront finder's fee at the time the shares are purchased, or a 0.75% CDSC if the shares purchased are recordkept in a Fidelity Advisor® 401(k) Retirement Plan.

1 year

rr_ExpenseExampleYear01

$ 340

3 years

rr_ExpenseExampleYear03

477

5 years

rr_ExpenseExampleYear05

627

10 years

rr_ExpenseExampleYear10

$ 1,063

2012

rr_AnnualReturn2012

4.56%

2013

rr_AnnualReturn2013

(1.17%)

2014

rr_AnnualReturn2014

3.39%

2015

rr_AnnualReturn2015

1.34%

2016

rr_AnnualReturn2016

0.15%

2017

rr_AnnualReturn2017

1.04%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Jun. 30, 2018

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.63%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Sep. 30, 2012

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

2.18%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Jun. 30, 2013

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(2.61%)

Inception Date

rr_AverageAnnualReturnInceptionDate

May 19, 2011

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2019 Fund | Fidelity Municipal Income 2019 Fund - Class I

Risk/Return:

rr_RiskReturnAbstract

Maximum sales charge (load) on purchases (as a % of offering price)

rr_MaximumCumulativeSalesChargeOverOfferingPrice

none

Maximum contingent deferred sales charge (as a % of the lesser of original purchase price or redemption proceeds)

rr_MaximumDeferredSalesChargeOverOther

none

Management fee

rr_ManagementFeesOverAssets

0.30%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.10%

Total annual operating expenses

rr_ExpensesOverAssets

0.40%

1 year

rr_ExpenseExampleYear01

$ 41

3 years

rr_ExpenseExampleYear03

128

5 years

rr_ExpenseExampleYear05

224

10 years

rr_ExpenseExampleYear10

$ 505

Inception Date

rr_AverageAnnualReturnInceptionDate

May 19, 2011

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2019 Fund | Return Before Taxes | Fidelity Municipal Income 2019 Fund-Class A

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Municipal Income 2019 Fund

Label

rr_AverageAnnualReturnLabel

Class A - Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

(1.74%)

Past 5 years

rr_AverageAnnualReturnYear05

0.38%

Since Inception

rr_AverageAnnualReturnSinceInception

2.02%

^[2]

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2019 Fund | Return Before Taxes | Fidelity Municipal Income 2019 Fund - Class I

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Municipal Income 2019 Fund

Label

rr_AverageAnnualReturnLabel

Class I - Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

1.29%

Past 5 years

rr_AverageAnnualReturnYear05

1.19%

Since Inception

rr_AverageAnnualReturnSinceInception

2.70%

^[2]

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2019 Fund | After Taxes on Distributions | Fidelity Municipal Income 2019 Fund-Class A

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Municipal Income 2019 Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions

Past 1 year

rr_AverageAnnualReturnYear01

(1.75%)

Past 5 years

rr_AverageAnnualReturnYear05

0.36%

Since Inception

rr_AverageAnnualReturnSinceInception

2.00%

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

May 19, 2011

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2019 Fund | After Taxes on Distributions and Sales | Fidelity Municipal Income 2019 Fund-Class A

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Municipal Income 2019 Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions and Sale of Fund Shares

Past 1 year

rr_AverageAnnualReturnYear01

(0.52%)

Past 5 years

rr_AverageAnnualReturnYear05

0.62%

Since Inception

rr_AverageAnnualReturnSinceInception

1.92%

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

May 19, 2011

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2019 Fund | Bloomberg Barclays Municipal Bond Index(reflects no deduction for fees, expenses, or taxes)

Risk/Return:

rr_RiskReturnAbstract

Past 1 year

rr_AverageAnnualReturnYear01

5.45%

Past 5 years

rr_AverageAnnualReturnYear05

3.02%

Since Inception

rr_AverageAnnualReturnSinceInception

4.28%

^[2]

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2019 Fund | Bloomberg Barclays Municipal Bond 2019 Index Ex Tobacco/Prepaid Gas(reflects no deduction for fees, expenses, or taxes)

Risk/Return:

rr_RiskReturnAbstract

Past 1 year

rr_AverageAnnualReturnYear01

1.45%

Past 5 years

rr_AverageAnnualReturnYear05

1.33%

Since Inception

rr_AverageAnnualReturnSinceInception

2.85%

^[2]

[1] Purchases of $250,000 or more will not be subject to a front-end sales charge, but may be subject to a 0.50% contingent deferred sales charge (CDSC) if the intermediary firm has elected an upfront finder's fee at the time the shares are purchased, or a 0.75% CDSC if the shares purchased are recordkept in a Fidelity Advisor® 401(k) Retirement Plan.

[2] From May 19, 2011

XML

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2021 Fund

Fund Summary

Fund/Class:
Fidelity® Municipal Income 2021 Fund/Fidelity Advisor® Municipal Income 2021 Fund A, I

Investment Objective

The fund seeks as high a level of current income, exempt from federal income tax, as is consistent with the preservation of capital.

Fee Table

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund. In addition to the fees and expenses described below, your broker may also require you to pay brokerage commissions on purchases and sales of certain share classes of the fund.

You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $50,000 in the fund or certain other Fidelity^® funds. More information about these and other discounts is available from your investment professional and in the "Fund Distribution" section beginning on page 50 of the prospectus. Different intermediaries may provide additional waivers or reductions of the sales charge. Please see “Sales Charge Waiver Policies Applied by Certain Intermediaries” in the “Appendix” section of the prospectus.

Shareholder fees

(fees paid directly from your investment)

Shareholder Fees {- Fidelity® Municipal Income 2021 Fund} - 06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 - Fidelity® Municipal Income 2021 Fund	Fidelity Municipal Income 2021 Fund-Class A		Fidelity Municipal Income 2021 Fund - Class I
Maximum sales charge (load) on purchases (as a % of offering price)	2.75%		none
Maximum contingent deferred sales charge (as a % of the lesser of original purchase price or redemption proceeds)	none	[1]	none

Annual Operating Expenses

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

Annual Operating Expenses {- Fidelity® Municipal Income 2021 Fund} - 06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 - Fidelity® Municipal Income 2021 Fund	Fidelity Municipal Income 2021 Fund-Class A	Fidelity Municipal Income 2021 Fund - Class I
Management fee	0.30%	0.30%
Distribution and/or Service (12b-1) fees	0.25%	none
Other expenses	0.10%	0.10%
Total annual operating expenses	0.65%	0.40%

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® Municipal Income 2021 Fund} - 06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 - Fidelity® Municipal Income 2021 Fund - USD ($)	Fidelity Municipal Income 2021 Fund-Class A	Fidelity Municipal Income 2021 Fund - Class I
1 year	$ 340	$ 41
3 years	477	128
5 years	627	224
10 years	$ 1,063	$ 505

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 8% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in investment-grade municipal securities whose interest is exempt from federal income tax (but may be subject to the federal alternative minimum tax).

• Managing the fund to have similar overall interest rate risk compared to a customized municipal bond index representing Fidelity Management & Research Company (FMR)'s view of the performance of the fund’s competitive universe.

• Allocating assets across different market sectors and maturities.

• Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects.

• Analyzing the credit quality of the issuer, security-specific features, current and potential future valuation, and trading opportunities to select investments.

• Liquidating shortly after the fund's target end-date of June 30, 2021.

Principal Investment Risks

• Municipal Market Volatility. The municipal market is volatile and can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities.

• Interest Rate Changes. Interest rate increases can cause the price of a debt security to decrease.

• 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. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a security to decrease.

• Declining Yield Risk. As the fund approaches its liquidation date, the fund's securities will mature and the fund may reinvest the proceeds in money market securities with lower yields than the securities previously held by the fund.

• Fluctuation of Yield & Liquidation Amount Risk. The amount of the fund's income distributions will vary over time, and the breakdown of returns between fund distributions and liquidation proceeds will not be predictable at the time of your investment, and you may experience a gain or loss for tax purposes.

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. Unlike individual debt securities, which typically pay principal at maturity, the value of an investment in the fund will fluctuate. 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 and an additional index over various periods of time. The indexes have characteristics relevant to the fund's investment strategies. Index descriptions appear in the "Additional Index Information" section of the prospectus. Past performance (before and after taxes) is not an indication of future performance.

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

The returns in the bar chart do not reflect any applicable sales charges; if sales charges were reflected, returns would be lower than those shown.

Year-by-Year Returns

Calendar Years

During the periods shown in the chart for Class A:	Returns	Quarter ended
Highest Quarter Return	2.53%	June 30, 2014
Lowest Quarter Return	-3.73%	June 30, 2013
Year-to-Date Return	0.32%	June 30, 2018

Average Annual Returns

Unlike the returns in the bar chart, the returns in the table reflect the maximum applicable sales charges. 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 Class A 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. Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.

For the periods ended December 31, 2017

Average Annual Total Returns{- Fidelity® Municipal Income 2021 Fund} - 06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 - Fidelity® Municipal Income 2021 Fund	Past 1 year	Past 5 years	Since Inception	[1]	Inception Date
Fidelity Municipal Income 2021 Fund-Class A					May 19, 2011
Fidelity Municipal Income 2021 Fund-Class A | Return Before Taxes	(0.63%)	1.10%	2.99%
Fidelity Municipal Income 2021 Fund-Class A | After Taxes on Distributions	(0.63%)	1.07%	2.94%		May 19, 2011
Fidelity Municipal Income 2021 Fund-Class A | After Taxes on Distributions and Sales	0.37%	1.29%	2.78%		May 19, 2011
Fidelity Municipal Income 2021 Fund - Class I					May 19, 2011
Fidelity Municipal Income 2021 Fund - Class I | Return Before Taxes	2.43%	1.92%	3.68%
Bloomberg Barclays Municipal Bond Index(reflects no deduction for fees, expenses, or taxes)	5.45%	3.02%	4.28%
Bloomberg Barclays Municipal Bond 2021 Index Ex Tobacco/Prepaid Gas(reflects no deduction for fees, expenses, or taxes)	2.76%	1.93%	3.77%

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2021 Fund | Fidelity Municipal Income 2021 Fund-Class A

Purchases of $250,000 or more will not be subject to a front-end sales charge, but may be subject to a 0.50% contingent deferred sales charge (CDSC) if the intermediary firm has elected an upfront finder's fee at the time the shares are purchased, or a 0.75% CDSC if the shares purchased are recordkept in a Fidelity Advisor® 401(k) Retirement Plan.

[1] Purchases of $250,000 or more will not be subject to a front-end sales charge, but may be subject to a 0.50% contingent deferred sales charge (CDSC) if the intermediary firm has elected an upfront finder's fee at the time the shares are purchased, or a 0.75% CDSC if the shares purchased are recordkept in a Fidelity Advisor® 401(k) Retirement Plan.

[1] From May 19, 2011

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Aug. 29, 2018

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2021 Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® Municipal Income 2021 Fund/Fidelity Advisor® Municipal Income 2021 Fund A, I

Objective [Heading]

rr_ObjectiveHeading

Investment Objective

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks as high a level of current income, exempt from federal income tax, as is consistent with the preservation of capital.

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. In addition to the fees and expenses described below, your broker may also require you to pay brokerage commissions on purchases and sales of certain share classes of the fund.

You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $50,000 in the fund or certain other Fidelity^® funds. More information about these and other discounts is available from your investment professional and in the "Fund Distribution" section beginning on page 50 of the prospectus. Different intermediaries may provide additional waivers or reductions of the sales charge. Please see “Sales Charge Waiver Policies Applied by Certain Intermediaries” in the “Appendix” section of the prospectus.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

Shareholder fees

(fees paid directly from your investment)

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

Annual Operating Expenses

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

Portfolio Turnover [Heading]

rr_PortfolioTurnoverHeading

Portfolio Turnover

Portfolio Turnover [Text Block]

rr_PortfolioTurnoverTextBlock

The fund pays transaction costs, such as commissions, when it buys and sells securities (or "turns over" its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when fund shares are held in a taxable account. These costs, which are not reflected in annual operating expenses or in the example, affect the fund's performance. During the most recent fiscal year, the fund's portfolio turnover rate was 8% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

8.00%

Expense Breakpoint Discounts [Text]

rr_ExpenseBreakpointDiscounts

You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $50,000 in the fund or certain other Fidelity® funds. More information about these and other discounts is available from your investment professional and in the "Fund Distribution" section beginning on page 50 of the prospectus.

Expense Breakpoint, Minimum Investment Required [Amount]

rr_ExpenseBreakpointMinimumInvestmentRequiredAmount

$ 50,000

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 investment-grade municipal securities whose interest is exempt from federal income tax (but may be subject to the federal alternative minimum tax).

• Managing the fund to have similar overall interest rate risk compared to a customized municipal bond index representing Fidelity Management & Research Company (FMR)'s view of the performance of the fund’s competitive universe.

• Allocating assets across different market sectors and maturities.

• Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects.

• Analyzing the credit quality of the issuer, security-specific features, current and potential future valuation, and trading opportunities to select investments.

• Liquidating shortly after the fund's target end-date of June 30, 2021.

Risk [Heading]

rr_RiskHeading

Principal Investment Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Municipal Market Volatility. The municipal market is volatile and can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities.

• Interest Rate Changes. Interest rate increases can cause the price of a debt security to decrease.

• 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. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a security to decrease.

• Declining Yield Risk. As the fund approaches its liquidation date, the fund's securities will mature and the fund may reinvest the proceeds in money market securities with lower yields than the securities previously held by the fund.

• Fluctuation of Yield & Liquidation Amount Risk. The amount of the fund's income distributions will vary over time, and the breakdown of returns between fund distributions and liquidation proceeds will not be predictable at the time of your investment, and you may experience a gain or loss for tax purposes.

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. Unlike individual debt securities, which typically pay principal at maturity, the value of an investment in the fund will fluctuate. 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 and an additional index over various periods of time. The indexes have characteristics relevant to the fund's investment strategies. Index descriptions appear in the "Additional Index Information" section of the prospectus. Past performance (before and after taxes) is not an indication of future performance.

Visit institutional.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 and an additional index over various periods of time.

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

institutional.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 Narrative [Text Block]

rr_BarChartNarrativeTextBlock

The returns in the bar chart do not reflect any applicable sales charges; if sales charges were reflected, returns would be lower than those shown.

Bar Chart Does Not Reflect Sales Loads [Text]

rr_BarChartDoesNotReflectSalesLoads

The returns in the bar chart do not reflect any applicable sales charges; if sales charges were reflected, returns would be lower than those shown.

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart for Class A:	Returns	Quarter ended
Highest Quarter Return	2.53%	June 30, 2014
Lowest Quarter Return	-3.73%	June 30, 2013
Year-to-Date Return	0.32%	June 30, 2018

Performance Table Heading

rr_PerformanceTableHeading

Average Annual Returns

Performance Table Does Reflect Sales Loads

rr_PerformanceTableDoesReflectSalesLoads

Unlike the returns in the bar chart, the returns in the table reflect the maximum applicable sales charges.

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 One Class of after Tax Shown [Text]

rr_PerformanceTableOneClassOfAfterTaxShown

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

Performance Table Explanation after Tax Higher

rr_PerformanceTableExplanationAfterTaxHigher

Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

Unlike the returns in the bar chart, the returns in the table reflect the maximum applicable sales charges. 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 Class A 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. Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.

For the periods ended December 31, 2017

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2021 Fund | Fidelity Municipal Income 2021 Fund-Class A

Risk/Return:

rr_RiskReturnAbstract

Maximum sales charge (load) on purchases (as a % of offering price)

rr_MaximumCumulativeSalesChargeOverOfferingPrice

2.75%

Maximum contingent deferred sales charge (as a % of the lesser of original purchase price or redemption proceeds)

rr_MaximumDeferredSalesChargeOverOther

none

^[1]

Management fee

rr_ManagementFeesOverAssets

0.30%

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

rr_DistributionAndService12b1FeesOverAssets

0.25%

Other expenses

rr_OtherExpensesOverAssets

0.10%

Total annual operating expenses

rr_ExpensesOverAssets

0.65%

Expenses Deferred Charges [Text Block]

rr_ExpensesDeferredChargesTextBlock

Purchases of $250,000 or more will not be subject to a front-end sales charge, but may be subject to a 0.50% contingent deferred sales charge (CDSC) if the intermediary firm has elected an upfront finder's fee at the time the shares are purchased, or a 0.75% CDSC if the shares purchased are recordkept in a Fidelity Advisor® 401(k) Retirement Plan.

1 year

rr_ExpenseExampleYear01

$ 340

3 years

rr_ExpenseExampleYear03

477

5 years

rr_ExpenseExampleYear05

627

10 years

rr_ExpenseExampleYear10

$ 1,063

2012

rr_AnnualReturn2012

5.60%

2013

rr_AnnualReturn2013

(2.95%)

2014

rr_AnnualReturn2014

7.08%

2015

rr_AnnualReturn2015

2.66%

2016

rr_AnnualReturn2016

(0.35%)

2017

rr_AnnualReturn2017

2.18%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Jun. 30, 2018

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.32%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Jun. 30, 2014

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

2.53%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Jun. 30, 2013

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(3.73%)

Inception Date

rr_AverageAnnualReturnInceptionDate

May 19, 2011

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2021 Fund | Fidelity Municipal Income 2021 Fund - Class I

Risk/Return:

rr_RiskReturnAbstract

Maximum sales charge (load) on purchases (as a % of offering price)

rr_MaximumCumulativeSalesChargeOverOfferingPrice

none

Maximum contingent deferred sales charge (as a % of the lesser of original purchase price or redemption proceeds)

rr_MaximumDeferredSalesChargeOverOther

none

Management fee

rr_ManagementFeesOverAssets

0.30%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.10%

Total annual operating expenses

rr_ExpensesOverAssets

0.40%

1 year

rr_ExpenseExampleYear01

$ 41

3 years

rr_ExpenseExampleYear03

128

5 years

rr_ExpenseExampleYear05

224

10 years

rr_ExpenseExampleYear10

$ 505

Inception Date

rr_AverageAnnualReturnInceptionDate

May 19, 2011

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2021 Fund | Return Before Taxes | Fidelity Municipal Income 2021 Fund-Class A

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Municipal Income 2021 Fund

Label

rr_AverageAnnualReturnLabel

Class A - Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

(0.63%)

Past 5 years

rr_AverageAnnualReturnYear05

1.10%

Since Inception

rr_AverageAnnualReturnSinceInception

2.99%

^[2]

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2021 Fund | Return Before Taxes | Fidelity Municipal Income 2021 Fund - Class I

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Municipal Income 2021 Fund

Label

rr_AverageAnnualReturnLabel

Class I - Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

2.43%

Past 5 years

rr_AverageAnnualReturnYear05

1.92%

Since Inception

rr_AverageAnnualReturnSinceInception

3.68%

^[2]

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2021 Fund | After Taxes on Distributions | Fidelity Municipal Income 2021 Fund-Class A

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Municipal Income 2021 Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions

Past 1 year

rr_AverageAnnualReturnYear01

(0.63%)

Past 5 years

rr_AverageAnnualReturnYear05

1.07%

Since Inception

rr_AverageAnnualReturnSinceInception

2.94%

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

May 19, 2011

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2021 Fund | After Taxes on Distributions and Sales | Fidelity Municipal Income 2021 Fund-Class A

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Municipal Income 2021 Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions and Sale of Fund Shares

Past 1 year

rr_AverageAnnualReturnYear01

0.37%

Past 5 years

rr_AverageAnnualReturnYear05

1.29%

Since Inception

rr_AverageAnnualReturnSinceInception

2.78%

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

May 19, 2011

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2021 Fund | Bloomberg Barclays Municipal Bond Index(reflects no deduction for fees, expenses, or taxes)

Risk/Return:

rr_RiskReturnAbstract

Past 1 year

rr_AverageAnnualReturnYear01

5.45%

Past 5 years

rr_AverageAnnualReturnYear05

3.02%

Since Inception

rr_AverageAnnualReturnSinceInception

4.28%

^[2]

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2021 Fund | Bloomberg Barclays Municipal Bond 2021 Index Ex Tobacco/Prepaid Gas(reflects no deduction for fees, expenses, or taxes)

Risk/Return:

rr_RiskReturnAbstract

Past 1 year

rr_AverageAnnualReturnYear01

2.76%

Past 5 years

rr_AverageAnnualReturnYear05

1.93%

Since Inception

rr_AverageAnnualReturnSinceInception

3.77%

^[2]

[1] Purchases of $250,000 or more will not be subject to a front-end sales charge, but may be subject to a 0.50% contingent deferred sales charge (CDSC) if the intermediary firm has elected an upfront finder's fee at the time the shares are purchased, or a 0.75% CDSC if the shares purchased are recordkept in a Fidelity Advisor® 401(k) Retirement Plan.

[2] From May 19, 2011

XML

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2023 Fund

Fund Summary

Fund/Class:
Fidelity® Municipal Income 2023 Fund/Fidelity Advisor® Municipal Income 2023 Fund A, I

Investment Objective

The fund seeks as high a level of current income, exempt from federal income tax, as is consistent with the preservation of capital.

Fee Table

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund. In addition to the fees and expenses described below, your broker may also require you to pay brokerage commissions on purchases and sales of certain share classes of the fund.

You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $50,000 in the fund or certain other Fidelity^® funds. More information about these and other discounts is available from your investment professional and in the "Fund Distribution" section beginning on page 50 of the prospectus. Different intermediaries may provide additional waivers or reductions of the sales charge. Please see “Sales Charge Waiver Policies Applied by Certain Intermediaries” in the “Appendix” section of the prospectus.

Shareholder fees

(fees paid directly from your investment)

Shareholder Fees {- Fidelity® Municipal Income 2023 Fund} - 06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 - Fidelity® Municipal Income 2023 Fund	Fidelity Municipal Income 2023 Fund-Class A		Fidelity Municipal Income 2023 Fund - Class I
Maximum sales charge (load) on purchases (as a % of offering price)	2.75%		none
Maximum contingent deferred sales charge (as a % of the lesser of original purchase price or redemption proceeds)	none	[1]	none

Annual Operating Expenses

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

Annual Operating Expenses {- Fidelity® Municipal Income 2023 Fund} - 06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 - Fidelity® Municipal Income 2023 Fund	Fidelity Municipal Income 2023 Fund-Class A	Fidelity Municipal Income 2023 Fund - Class I
Management fee	0.30%	0.30%
Distribution and/or Service (12b-1) fees	0.25%	none
Other expenses	0.10%	0.10%
Total annual operating expenses	0.65%	0.40%

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® Municipal Income 2023 Fund} - 06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 - Fidelity® Municipal Income 2023 Fund - USD ($)	Fidelity Municipal Income 2023 Fund-Class A	Fidelity Municipal Income 2023 Fund - Class I
1 year	$ 340	$ 41
3 years	477	128
5 years	627	224
10 years	$ 1,063	$ 505

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 12% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in investment-grade municipal securities whose interest is exempt from federal income tax (but may be subject to the federal alternative minimum tax).

• Managing the fund to have similar overall interest rate risk compared to a customized municipal bond index representing Fidelity Management & Research Company (FMR)'s view of the performance of the fund’s competitive universe.

• Allocating assets across different market sectors and maturities.

• Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects.

• Analyzing the credit quality of the issuer, security-specific features, current and potential future valuation, and trading opportunities to select investments.

• Liquidating shortly after the fund's target end-date of June 30, 2023.

Principal Investment Risks

• Municipal Market Volatility. The municipal market is volatile and can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities.

• Interest Rate Changes. Interest rate increases can cause the price of a debt security to decrease.

• 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. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a security to decrease.

• Declining Yield Risk. As the fund approaches its liquidation date, the fund's securities will mature and the fund may reinvest the proceeds in money market securities with lower yields than the securities previously held by the fund.

• Fluctuation of Yield & Liquidation Amount Risk. The amount of the fund's income distributions will vary over time, and the breakdown of returns between fund distributions and liquidation proceeds will not be predictable at the time of your investment, and you may experience a gain or loss for tax purposes.

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. Unlike individual debt securities, which typically pay principal at maturity, the value of an investment in the fund will fluctuate. 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 and an additional index over various periods of time. The indexes have characteristics relevant to the fund's investment strategies. Index descriptions appear in the "Additional Index Information" section of the prospectus. Past performance (before and after taxes) is not an indication of future performance.

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

The returns in the bar chart do not reflect any applicable sales charges; if sales charges were reflected, returns would be lower than those shown.

Year-by-Year Returns

Calendar Years

During the periods shown in the chart for Class A:	Returns	Quarter ended
Highest Quarter Return	3.65%	March 31, 2014
Lowest Quarter Return	-4.66%	December 31, 2016
Year-to-Date Return	-0.31%	June 30, 2018

Average Annual Returns

Unlike the returns in the bar chart, the returns in the table reflect the maximum applicable sales charges. 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 Class A 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. Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.

For the periods ended December 31, 2017

Average Annual Total Returns{- Fidelity® Municipal Income 2023 Fund} - 06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 - Fidelity® Municipal Income 2023 Fund	Past 1 year	Since Inception	[1]	Inception Date
Fidelity Municipal Income 2023 Fund-Class A				Apr. 23, 2013
Fidelity Municipal Income 2023 Fund-Class A | Return Before Taxes	1.27%	1.71%
Fidelity Municipal Income 2023 Fund-Class A | After Taxes on Distributions	1.27%	1.70%		Apr. 23, 2013
Fidelity Municipal Income 2023 Fund-Class A | After Taxes on Distributions and Sales	1.49%	1.73%		Apr. 23, 2013
Fidelity Municipal Income 2023 Fund - Class I				Apr. 23, 2013
Fidelity Municipal Income 2023 Fund - Class I | Return Before Taxes	4.39%	2.57%
Bloomberg Barclays Municipal Bond Index(reflects no deduction for fees, expenses, or taxes)	5.45%	2.97%
Bloomberg Barclays Municipal Bond 2023 Index Ex Tobacco/Prepaid Gas(reflects no deduction for fees, expenses, or taxes)	4.32%	2.35%

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2023 Fund | Fidelity Municipal Income 2023 Fund-Class A

Purchases of $250,000 or more will not be subject to a front-end sales charge, but may be subject to a 0.50% contingent deferred sales charge (CDSC) if the intermediary firm has elected an upfront finder's fee at the time the shares are purchased, or a 0.75% CDSC if the shares purchased are recordkept in a Fidelity Advisor® 401(k) Retirement Plan.

[1] Purchases of $250,000 or more will not be subject to a front-end sales charge, but may be subject to a 0.50% contingent deferred sales charge (CDSC) if the intermediary firm has elected an upfront finder's fee at the time the shares are purchased, or a 0.75% CDSC if the shares purchased are recordkept in a Fidelity Advisor® 401(k) Retirement Plan.

[1] From April 23, 2013

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Aug. 29, 2018

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2023 Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® Municipal Income 2023 Fund/Fidelity Advisor® Municipal Income 2023 Fund A, I

Objective [Heading]

rr_ObjectiveHeading

Investment Objective

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks as high a level of current income, exempt from federal income tax, as is consistent with the preservation of capital.

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. In addition to the fees and expenses described below, your broker may also require you to pay brokerage commissions on purchases and sales of certain share classes of the fund.

You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $50,000 in the fund or certain other Fidelity^® funds. More information about these and other discounts is available from your investment professional and in the "Fund Distribution" section beginning on page 50 of the prospectus. Different intermediaries may provide additional waivers or reductions of the sales charge. Please see “Sales Charge Waiver Policies Applied by Certain Intermediaries” in the “Appendix” section of the prospectus.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

Shareholder fees

(fees paid directly from your investment)

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

Annual Operating Expenses

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

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 12% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

12.00%

Expense Breakpoint Discounts [Text]

rr_ExpenseBreakpointDiscounts

You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $50,000 in the fund or certain other Fidelity® funds. More information about these and other discounts is available from your investment professional and in the "Fund Distribution" section beginning on page 50 of the prospectus.

Expense Breakpoint, Minimum Investment Required [Amount]

rr_ExpenseBreakpointMinimumInvestmentRequiredAmount

$ 50,000

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 investment-grade municipal securities whose interest is exempt from federal income tax (but may be subject to the federal alternative minimum tax).

• Managing the fund to have similar overall interest rate risk compared to a customized municipal bond index representing Fidelity Management & Research Company (FMR)'s view of the performance of the fund’s competitive universe.

• Allocating assets across different market sectors and maturities.

• Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects.

• Analyzing the credit quality of the issuer, security-specific features, current and potential future valuation, and trading opportunities to select investments.

• Liquidating shortly after the fund's target end-date of June 30, 2023.

Risk [Heading]

rr_RiskHeading

Principal Investment Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Municipal Market Volatility. The municipal market is volatile and can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities.

• Interest Rate Changes. Interest rate increases can cause the price of a debt security to decrease.

• 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. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a security to decrease.

• Declining Yield Risk. As the fund approaches its liquidation date, the fund's securities will mature and the fund may reinvest the proceeds in money market securities with lower yields than the securities previously held by the fund.

• Fluctuation of Yield & Liquidation Amount Risk. The amount of the fund's income distributions will vary over time, and the breakdown of returns between fund distributions and liquidation proceeds will not be predictable at the time of your investment, and you may experience a gain or loss for tax purposes.

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. Unlike individual debt securities, which typically pay principal at maturity, the value of an investment in the fund will fluctuate. 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 and an additional index over various periods of time. The indexes have characteristics relevant to the fund's investment strategies. Index descriptions appear in the "Additional Index Information" section of the prospectus. Past performance (before and after taxes) is not an indication of future performance.

Visit institutional.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 and an additional index over various periods of time.

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

institutional.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 Narrative [Text Block]

rr_BarChartNarrativeTextBlock

The returns in the bar chart do not reflect any applicable sales charges; if sales charges were reflected, returns would be lower than those shown.

Bar Chart Does Not Reflect Sales Loads [Text]

rr_BarChartDoesNotReflectSalesLoads

The returns in the bar chart do not reflect any applicable sales charges; if sales charges were reflected, returns would be lower than those shown.

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart for Class A:	Returns	Quarter ended
Highest Quarter Return	3.65%	March 31, 2014
Lowest Quarter Return	-4.66%	December 31, 2016
Year-to-Date Return	-0.31%	June 30, 2018

Performance Table Heading

rr_PerformanceTableHeading

Average Annual Returns

Performance Table Does Reflect Sales Loads

rr_PerformanceTableDoesReflectSalesLoads

Unlike the returns in the bar chart, the returns in the table reflect the maximum applicable sales charges.

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 One Class of after Tax Shown [Text]

rr_PerformanceTableOneClassOfAfterTaxShown

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

Performance Table Explanation after Tax Higher

rr_PerformanceTableExplanationAfterTaxHigher

Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

Unlike the returns in the bar chart, the returns in the table reflect the maximum applicable sales charges. 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 Class A 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. Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.

For the periods ended December 31, 2017

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2023 Fund | Fidelity Municipal Income 2023 Fund-Class A

Risk/Return:

rr_RiskReturnAbstract

Maximum sales charge (load) on purchases (as a % of offering price)

rr_MaximumCumulativeSalesChargeOverOfferingPrice

2.75%

Maximum contingent deferred sales charge (as a % of the lesser of original purchase price or redemption proceeds)

rr_MaximumDeferredSalesChargeOverOther

none

^[1]

Management fee

rr_ManagementFeesOverAssets

0.30%

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

rr_DistributionAndService12b1FeesOverAssets

0.25%

Other expenses

rr_OtherExpensesOverAssets

0.10%

Total annual operating expenses

rr_ExpensesOverAssets

0.65%

Expenses Deferred Charges [Text Block]

rr_ExpensesDeferredChargesTextBlock

Purchases of $250,000 or more will not be subject to a front-end sales charge, but may be subject to a 0.50% contingent deferred sales charge (CDSC) if the intermediary firm has elected an upfront finder's fee at the time the shares are purchased, or a 0.75% CDSC if the shares purchased are recordkept in a Fidelity Advisor® 401(k) Retirement Plan.

1 year

rr_ExpenseExampleYear01

$ 340

3 years

rr_ExpenseExampleYear03

477

5 years

rr_ExpenseExampleYear05

627

10 years

rr_ExpenseExampleYear10

$ 1,063

2014

rr_AnnualReturn2014

9.96%

2015

rr_AnnualReturn2015

3.06%

2016

rr_AnnualReturn2016

(0.42%)

2017

rr_AnnualReturn2017

4.13%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Jun. 30, 2018

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

(0.31%)

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Mar. 31, 2014

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

3.65%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Dec. 31, 2016

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(4.66%)

Inception Date

rr_AverageAnnualReturnInceptionDate

Apr. 23, 2013

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2023 Fund | Fidelity Municipal Income 2023 Fund - Class I

Risk/Return:

rr_RiskReturnAbstract

Maximum sales charge (load) on purchases (as a % of offering price)

rr_MaximumCumulativeSalesChargeOverOfferingPrice

none

Maximum contingent deferred sales charge (as a % of the lesser of original purchase price or redemption proceeds)

rr_MaximumDeferredSalesChargeOverOther

none

Management fee

rr_ManagementFeesOverAssets

0.30%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.10%

Total annual operating expenses

rr_ExpensesOverAssets

0.40%

1 year

rr_ExpenseExampleYear01

$ 41

3 years

rr_ExpenseExampleYear03

128

5 years

rr_ExpenseExampleYear05

224

10 years

rr_ExpenseExampleYear10

$ 505

Inception Date

rr_AverageAnnualReturnInceptionDate

Apr. 23, 2013

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2023 Fund | Return Before Taxes | Fidelity Municipal Income 2023 Fund-Class A

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Municipal Income 2023 Fund

Label

rr_AverageAnnualReturnLabel

Class A - Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

1.27%

Since Inception

rr_AverageAnnualReturnSinceInception

1.71%

^[2]

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2023 Fund | Return Before Taxes | Fidelity Municipal Income 2023 Fund - Class I

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Municipal Income 2023 Fund

Label

rr_AverageAnnualReturnLabel

Class I - Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

4.39%

Since Inception

rr_AverageAnnualReturnSinceInception

2.57%

^[2]

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2023 Fund | After Taxes on Distributions | Fidelity Municipal Income 2023 Fund-Class A

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Municipal Income 2023 Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions

Past 1 year

rr_AverageAnnualReturnYear01

1.27%

Since Inception

rr_AverageAnnualReturnSinceInception

1.70%

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

Apr. 23, 2013

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2023 Fund | After Taxes on Distributions and Sales | Fidelity Municipal Income 2023 Fund-Class A

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Municipal Income 2023 Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions and Sale of Fund Shares

Past 1 year

rr_AverageAnnualReturnYear01

1.49%

Since Inception

rr_AverageAnnualReturnSinceInception

1.73%

^[2]

Inception Date

rr_AverageAnnualReturnInceptionDate

Apr. 23, 2013

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2023 Fund | Bloomberg Barclays Municipal Bond Index(reflects no deduction for fees, expenses, or taxes)

Risk/Return:

rr_RiskReturnAbstract

Past 1 year

rr_AverageAnnualReturnYear01

5.45%

Since Inception

rr_AverageAnnualReturnSinceInception

2.97%

^[2]

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2023 Fund | Bloomberg Barclays Municipal Bond 2023 Index Ex Tobacco/Prepaid Gas(reflects no deduction for fees, expenses, or taxes)

Risk/Return:

rr_RiskReturnAbstract

Past 1 year

rr_AverageAnnualReturnYear01

4.32%

Since Inception

rr_AverageAnnualReturnSinceInception

2.35%

^[2]

[1] Purchases of $250,000 or more will not be subject to a front-end sales charge, but may be subject to a 0.50% contingent deferred sales charge (CDSC) if the intermediary firm has elected an upfront finder's fee at the time the shares are purchased, or a 0.75% CDSC if the shares purchased are recordkept in a Fidelity Advisor® 401(k) Retirement Plan.

[2] From April 23, 2013

XML

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2025 Fund

Fund Summary

Fund/Class:
Fidelity® Municipal Income 2025 Fund/Fidelity Advisor® Municipal Income 2025 Fund A, I

Investment Objective

The fund seeks as high a level of current income, exempt from federal income tax, as is consistent with the preservation of capital.

Fee Table

The following table describes the fees and expenses that may be incurred when you buy and hold shares of the fund. In addition to the fees and expenses described below, your broker may also require you to pay brokerage commissions on purchases and sales of certain share classes of the fund.

You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $50,000 in the fund or certain other Fidelity^® funds. More information about these and other discounts is available from your investment professional and in the "Fund Distribution" section beginning on page 50 of the prospectus. Different intermediaries may provide additional waivers or reductions of the sales charge. Please see “Sales Charge Waiver Policies Applied by Certain Intermediaries” in the “Appendix” section of the prospectus.

Shareholder fees

(fees paid directly from your investment)

Shareholder Fees {- Fidelity® Municipal Income 2025 Fund} - 06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 - Fidelity® Municipal Income 2025 Fund	Fidelity Municipal Income 2025 Fund-Class A		Fidelity Municipal Income 2025 Fund-Class I Advisor
Maximum sales charge (load) on purchases (as a % of offering price)	2.75%		none
Maximum contingent deferred sales charge (as a % of the lesser of original purchase price or redemption proceeds)	none	[1]	none

Annual Operating Expenses

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

Annual Operating Expenses {- Fidelity® Municipal Income 2025 Fund} - 06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 - Fidelity® Municipal Income 2025 Fund	Fidelity Municipal Income 2025 Fund-Class A	Fidelity Municipal Income 2025 Fund-Class I Advisor
Management fee	0.30%	0.30%
Distribution and/or Service (12b-1) fees	0.25%	none
Other expenses	0.10%	0.10%
Total annual operating expenses	0.65%	0.40%

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® Municipal Income 2025 Fund} - 06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 - Fidelity® Municipal Income 2025 Fund - USD ($)	Fidelity Municipal Income 2025 Fund-Class A	Fidelity Municipal Income 2025 Fund-Class I Advisor
1 year	$ 340	$ 41
3 years	477	128
5 years	627	224
10 years	$ 1,063	$ 505

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 44% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in investment-grade municipal securities whose interest is exempt from federal income tax (but may be subject to the federal alternative minimum tax).

• Managing the fund to have similar overall interest rate risk compared to a customized municipal bond index representing Fidelity Management & Research Company (FMR)'s view of the performance of the fund’s competitive universe.

• Allocating assets across different market sectors and maturities.

• Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects.

• Analyzing the credit quality of the issuer, security-specific features, current and potential future valuation, and trading opportunities to select investments.

• Liquidating shortly after the fund's target end-date of June 30, 2025.

Principal Investment Risks

• Municipal Market Volatility. The municipal market is volatile and can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities.

• Interest Rate Changes. Interest rate increases can cause the price of a debt security to decrease.

• 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. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a security to decrease.

• Declining Yield Risk. As the fund approaches its liquidation date, the fund's securities will mature and the fund may reinvest the proceeds in money market securities with lower yields than the securities previously held by the fund.

• Fluctuation of Yield & Liquidation Amount Risk. The amount of the fund's income distributions will vary over time, and the breakdown of returns between fund distributions and liquidation proceeds will not be predictable at the time of your investment, and you may experience a gain or loss for tax purposes.

In addition, the fund is considered non-diversified and can invest a greater portion of assets in securities of a smaller number of individual issuers than a diversified fund. As a result, changes in the market value of a single investment could cause greater fluctuations in share price than would occur in a more diversified fund.

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. Unlike individual debt securities, which typically pay principal at maturity, the value of an investment in the fund will fluctuate. 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.

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2025 Fund | Fidelity Municipal Income 2025 Fund-Class A

Purchases of $250,000 or more will not be subject to a front-end sales charge, but may be subject to a 0.50% contingent deferred sales charge (CDSC) if the intermediary firm has elected an upfront finder's fee at the time the shares are purchased, or a 0.75% CDSC if the shares purchased are recordkept in a Fidelity Advisor® 401(k) Retirement Plan.

[1] Purchases of $250,000 or more will not be subject to a front-end sales charge, but may be subject to a 0.50% contingent deferred sales charge (CDSC) if the intermediary firm has elected an upfront finder's fee at the time the shares are purchased, or a 0.75% CDSC if the shares purchased are recordkept in a Fidelity Advisor® 401(k) Retirement Plan.

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Aug. 29, 2018

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2025 Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® Municipal Income 2025 Fund/Fidelity Advisor® Municipal Income 2025 Fund A, I

Objective [Heading]

rr_ObjectiveHeading

Investment Objective

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks as high a level of current income, exempt from federal income tax, as is consistent with the preservation of capital.

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. In addition to the fees and expenses described below, your broker may also require you to pay brokerage commissions on purchases and sales of certain share classes of the fund.

You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $50,000 in the fund or certain other Fidelity^® funds. More information about these and other discounts is available from your investment professional and in the "Fund Distribution" section beginning on page 50 of the prospectus. Different intermediaries may provide additional waivers or reductions of the sales charge. Please see “Sales Charge Waiver Policies Applied by Certain Intermediaries” in the “Appendix” section of the prospectus.

Shareholder Fees Caption [Text]

rr_ShareholderFeesCaption

Shareholder fees

(fees paid directly from your investment)

Operating Expenses Caption [Text]

rr_OperatingExpensesCaption

Annual Operating Expenses

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

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 44% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

44.00%

Expense Breakpoint Discounts [Text]

rr_ExpenseBreakpointDiscounts

You may qualify for sales charge discounts if you and your family invest, or agree to invest in the future, at least $50,000 in the fund or certain other Fidelity® funds. More information about these and other discounts is available from your investment professional and in the "Fund Distribution" section beginning on page 50 of the prospectus.

Expense Breakpoint, Minimum Investment Required [Amount]

rr_ExpenseBreakpointMinimumInvestmentRequiredAmount

$ 50,000

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 investment-grade municipal securities whose interest is exempt from federal income tax (but may be subject to the federal alternative minimum tax).

• Managing the fund to have similar overall interest rate risk compared to a customized municipal bond index representing Fidelity Management & Research Company (FMR)'s view of the performance of the fund’s competitive universe.

• Allocating assets across different market sectors and maturities.

• Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects.

• Analyzing the credit quality of the issuer, security-specific features, current and potential future valuation, and trading opportunities to select investments.

• Liquidating shortly after the fund's target end-date of June 30, 2025.

Risk [Heading]

rr_RiskHeading

Principal Investment Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Municipal Market Volatility. The municipal market is volatile and can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities.

• Interest Rate Changes. Interest rate increases can cause the price of a debt security to decrease.

• 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. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a security to decrease.

• Declining Yield Risk. As the fund approaches its liquidation date, the fund's securities will mature and the fund may reinvest the proceeds in money market securities with lower yields than the securities previously held by the fund.

• Fluctuation of Yield & Liquidation Amount Risk. The amount of the fund's income distributions will vary over time, and the breakdown of returns between fund distributions and liquidation proceeds will not be predictable at the time of your investment, and you may experience a gain or loss for tax purposes.

In addition, the fund is considered non-diversified and can invest a greater portion of assets in securities of a smaller number of individual issuers than a diversified fund. As a result, changes in the market value of a single investment could cause greater fluctuations in share price than would occur in a more diversified fund.

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. Unlike individual debt securities, which typically pay principal at maturity, the value of an investment in the fund will fluctuate. 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 Nondiversified Status [Text]

rr_RiskNondiversifiedStatus

In addition, the fund is considered non-diversified and can invest a greater portion of assets in securities of a smaller number of individual issuers than a diversified fund. As a result, changes in the market value of a single investment could cause greater fluctuations in share price than would occur in a more diversified 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.

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2025 Fund | Fidelity Municipal Income 2025 Fund-Class A

Risk/Return:

rr_RiskReturnAbstract

Maximum sales charge (load) on purchases (as a % of offering price)

rr_MaximumCumulativeSalesChargeOverOfferingPrice

2.75%

Maximum contingent deferred sales charge (as a % of the lesser of original purchase price or redemption proceeds)

rr_MaximumDeferredSalesChargeOverOther

none

^[1]

Management fee

rr_ManagementFeesOverAssets

0.30%

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

rr_DistributionAndService12b1FeesOverAssets

0.25%

Other expenses

rr_OtherExpensesOverAssets

0.10%

Total annual operating expenses

rr_ExpensesOverAssets

0.65%

Expenses Deferred Charges [Text Block]

rr_ExpensesDeferredChargesTextBlock

Purchases of $250,000 or more will not be subject to a front-end sales charge, but may be subject to a 0.50% contingent deferred sales charge (CDSC) if the intermediary firm has elected an upfront finder's fee at the time the shares are purchased, or a 0.75% CDSC if the shares purchased are recordkept in a Fidelity Advisor® 401(k) Retirement Plan.

1 year

rr_ExpenseExampleYear01

$ 340

3 years

rr_ExpenseExampleYear03

477

5 years

rr_ExpenseExampleYear05

627

10 years

rr_ExpenseExampleYear10

$ 1,063

06.30 Fidelity Defined Maturity Funds AI Combo PRO-06 | Fidelity® Municipal Income 2025 Fund | Fidelity Municipal Income 2025 Fund-Class I Advisor

Risk/Return:

rr_RiskReturnAbstract

Maximum sales charge (load) on purchases (as a % of offering price)

rr_MaximumCumulativeSalesChargeOverOfferingPrice

none

Maximum contingent deferred sales charge (as a % of the lesser of original purchase price or redemption proceeds)

rr_MaximumDeferredSalesChargeOverOther

none

Management fee

rr_ManagementFeesOverAssets

0.30%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.10%

Total annual operating expenses

rr_ExpensesOverAssets

0.40%

1 year

rr_ExpenseExampleYear01

$ 41

3 years

rr_ExpenseExampleYear03

128

5 years

rr_ExpenseExampleYear05

224

10 years

rr_ExpenseExampleYear10

$ 505

[1] Purchases of $250,000 or more will not be subject to a front-end sales charge, but may be subject to a 0.50% contingent deferred sales charge (CDSC) if the intermediary firm has elected an upfront finder's fee at the time the shares are purchased, or a 0.75% CDSC if the shares purchased are recordkept in a Fidelity Advisor® 401(k) Retirement Plan.

XML

06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 | Fidelity® Municipal Income 2019 Fund

Fund Summary

Fund/Class:
Fidelity® Municipal Income 2019 Fund/Fidelity® Municipal Income 2019 Fund

Investment Objective

The fund seeks as high a level of current income, exempt from federal income tax, as is consistent with the preservation of capital.

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® Municipal Income 2019 Fund}	06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 Fidelity® Municipal Income 2019 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® Municipal Income 2019 Fund}	06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 Fidelity® Municipal Income 2019 Fund Fidelity Municipal Income 2019 Fund-Retail Class
Management fee	0.30%
Distribution and/or Service (12b-1) fees	none
Other expenses	0.10%
Total annual operating expenses	0.40%

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® Municipal Income 2019 Fund}	06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 Fidelity® Municipal Income 2019 Fund Fidelity Municipal Income 2019 Fund-Retail Class USD ($)
1 year	$ 41
3 years	128
5 years	224
10 years	$ 505

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 7% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in investment-grade municipal securities whose interest is exempt from federal income tax (but may be subject to the federal alternative minimum tax).

• Managing the fund to have similar overall interest rate risk compared to a customized municipal bond index representing Fidelity Management & Research Company (FMR)'s view of the performance of the fund’s competitive universe.

• Allocating assets across different market sectors and maturities.

• Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects.

• Analyzing the credit quality of the issuer, security-specific features, current and potential future valuation, and trading opportunities to select investments.

• Liquidating shortly after the fund's target end-date of June 30, 2019.

Principal Investment Risks

• Municipal Market Volatility. The municipal market is volatile and can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities.

• Interest Rate Changes. Interest rate increases can cause the price of a debt security to decrease.

• 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. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a security to decrease.

• Declining Yield Risk. As the fund approaches its liquidation date, the fund's securities will mature and the fund may reinvest the proceeds in money market securities with lower yields than the securities previously held by the fund.

• Fluctuation of Yield & Liquidation Amount Risk. The amount of the fund's income distributions will vary over time, and the breakdown of returns between fund distributions and liquidation proceeds will not be predictable at the time of your investment, and you may experience a gain or loss for tax purposes.

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. Unlike individual debt securities, which typically pay principal at maturity, the value of an investment in the fund will fluctuate. 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 and an additional index over various periods of time. The indexes have characteristics relevant to the fund's investment strategies. Index descriptions appear in the "Additional Index Information" section of the prospectus. Past performance (before and after taxes) is not an indication of future performance.

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

Year-by-Year Returns

Calendar Years

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	2.25%	September 30, 2012
Lowest Quarter Return	-2.55%	June 30, 2013
Year-to-Date Return	0.76%	June 30, 2018

Average Annual Returns

After-tax returns are calculated using the historical highest individual federal marginal income tax rates, but do not reflect the impact of state or local taxes. Actual after-tax returns may differ depending on your individual circumstances. Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.

For the periods ended December 31, 2017

Average Annual Total Returns{- Fidelity® Municipal Income 2019 Fund} - 06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 - Fidelity® Municipal Income 2019 Fund	Past 1 year	Past 5 years	Since Inception	[1]	Inception Date
Fidelity Municipal Income 2019 Fund-Retail Class					May 19, 2011
Fidelity Municipal Income 2019 Fund-Retail Class | Return Before Taxes	1.29%	1.19%	2.70%
Fidelity Municipal Income 2019 Fund-Retail Class | After Taxes on Distributions	1.28%	1.18%	2.69%		May 19, 2011
Fidelity Municipal Income 2019 Fund-Retail Class | After Taxes on Distributions and Sales	1.32%	1.30%	2.52%		May 19, 2011
Bloomberg Barclays Municipal Bond Index(reflects no deduction for fees, expenses, or taxes)	5.45%	3.02%	4.28%
Bloomberg Barclays Municipal Bond 2019 Index Ex Tobacco/Prepaid Gas(reflects no deduction for fees, expenses, or taxes)	1.45%	1.33%	2.85%

[1] From May 19, 2011

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Aug. 29, 2018

06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 | Fidelity® Municipal Income 2019 Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® Municipal Income 2019 Fund/Fidelity® Municipal Income 2019 Fund

Objective [Heading]

rr_ObjectiveHeading

Investment Objective

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks as high a level of current income, exempt from federal income tax, as is consistent with the preservation of capital.

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 7% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

7.00%

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 investment-grade municipal securities whose interest is exempt from federal income tax (but may be subject to the federal alternative minimum tax).

• Managing the fund to have similar overall interest rate risk compared to a customized municipal bond index representing Fidelity Management & Research Company (FMR)'s view of the performance of the fund’s competitive universe.

• Allocating assets across different market sectors and maturities.

• Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects.

• Analyzing the credit quality of the issuer, security-specific features, current and potential future valuation, and trading opportunities to select investments.

• Liquidating shortly after the fund's target end-date of June 30, 2019.

Risk [Heading]

rr_RiskHeading

Principal Investment Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Municipal Market Volatility. The municipal market is volatile and can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities.

• Interest Rate Changes. Interest rate increases can cause the price of a debt security to decrease.

• 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. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a security to decrease.

• Declining Yield Risk. As the fund approaches its liquidation date, the fund's securities will mature and the fund may reinvest the proceeds in money market securities with lower yields than the securities previously held by the fund.

• Fluctuation of Yield & Liquidation Amount Risk. The amount of the fund's income distributions will vary over time, and the breakdown of returns between fund distributions and liquidation proceeds will not be predictable at the time of your investment, and you may experience a gain or loss for tax purposes.

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. Unlike individual debt securities, which typically pay principal at maturity, the value of an investment in the fund will fluctuate. 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 and an additional index over various periods of time. The indexes have characteristics relevant to the fund's investment strategies. Index descriptions appear in the "Additional Index Information" section of the prospectus. Past performance (before and after taxes) is not an indication of future performance.

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

Performance Information Illustrates Variability of Returns [Text]

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 and an additional index over various periods of time.

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

www.fidelity.com

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

Past performance (before and after taxes) is not an indication of future performance.

Bar Chart [Heading]

rr_BarChartHeading

Year-by-Year Returns

Calendar Years

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	2.25%	September 30, 2012
Lowest Quarter Return	-2.55%	June 30, 2013
Year-to-Date Return	0.76%	June 30, 2018

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 Explanation after Tax Higher

rr_PerformanceTableExplanationAfterTaxHigher

Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

After-tax returns are calculated using the historical highest individual federal marginal income tax rates, but do not reflect the impact of state or local taxes. Actual after-tax returns may differ depending on your individual circumstances. Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.

For the periods ended December 31, 2017

06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 | Fidelity® Municipal Income 2019 Fund | Fidelity Municipal Income 2019 Fund-Retail Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.30%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.10%

Total annual operating expenses

rr_ExpensesOverAssets

0.40%

1 year

rr_ExpenseExampleYear01

$ 41

3 years

rr_ExpenseExampleYear03

128

5 years

rr_ExpenseExampleYear05

224

10 years

rr_ExpenseExampleYear10

$ 505

2012

rr_AnnualReturn2012

4.82%

2013

rr_AnnualReturn2013

(0.93%)

2014

rr_AnnualReturn2014

3.65%

2015

rr_AnnualReturn2015

1.59%

2016

rr_AnnualReturn2016

0.40%

2017

rr_AnnualReturn2017

1.29%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Jun. 30, 2018

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.76%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Sep. 30, 2012

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

2.25%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Jun. 30, 2013

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(2.55%)

Inception Date

rr_AverageAnnualReturnInceptionDate

May 19, 2011

06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 | Fidelity® Municipal Income 2019 Fund | Return Before Taxes | Fidelity Municipal Income 2019 Fund-Retail Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Municipal Income 2019 Fund

Label

rr_AverageAnnualReturnLabel

Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

1.29%

Past 5 years

rr_AverageAnnualReturnYear05

1.19%

Since Inception

rr_AverageAnnualReturnSinceInception

2.70%

^[1]

06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 | Fidelity® Municipal Income 2019 Fund | After Taxes on Distributions | Fidelity Municipal Income 2019 Fund-Retail Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Municipal Income 2019 Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions

Past 1 year

rr_AverageAnnualReturnYear01

1.28%

Past 5 years

rr_AverageAnnualReturnYear05

1.18%

Since Inception

rr_AverageAnnualReturnSinceInception

2.69%

^[1]

Inception Date

rr_AverageAnnualReturnInceptionDate

May 19, 2011

06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 | Fidelity® Municipal Income 2019 Fund | After Taxes on Distributions and Sales | Fidelity Municipal Income 2019 Fund-Retail Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Municipal Income 2019 Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions and Sale of Fund Shares

Past 1 year

rr_AverageAnnualReturnYear01

1.32%

Past 5 years

rr_AverageAnnualReturnYear05

1.30%

Since Inception

rr_AverageAnnualReturnSinceInception

2.52%

^[1]

Inception Date

rr_AverageAnnualReturnInceptionDate

May 19, 2011

06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 | Fidelity® Municipal Income 2019 Fund | Bloomberg Barclays Municipal Bond Index(reflects no deduction for fees, expenses, or taxes)

Risk/Return:

rr_RiskReturnAbstract

Past 1 year

rr_AverageAnnualReturnYear01

5.45%

Past 5 years

rr_AverageAnnualReturnYear05

3.02%

Since Inception

rr_AverageAnnualReturnSinceInception

4.28%

^[1]

06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 | Fidelity® Municipal Income 2019 Fund | Bloomberg Barclays Municipal Bond 2019 Index Ex Tobacco/Prepaid Gas(reflects no deduction for fees, expenses, or taxes)

Risk/Return:

rr_RiskReturnAbstract

Past 1 year

rr_AverageAnnualReturnYear01

1.45%

Past 5 years

rr_AverageAnnualReturnYear05

1.33%

Since Inception

rr_AverageAnnualReturnSinceInception

2.85%

^[1]

[1] From May 19, 2011

XML

06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 | Fidelity® Municipal Income 2021 Fund

Fund Summary

Fund/Class:
Fidelity® Municipal Income 2021 Fund/Fidelity® Municipal Income 2021 Fund

Investment Objective

The fund seeks as high a level of current income, exempt from federal income tax, as is consistent with the preservation of capital.

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® Municipal Income 2021 Fund}	06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 Fidelity® Municipal Income 2021 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® Municipal Income 2021 Fund}	06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 Fidelity® Municipal Income 2021 Fund Fidelity Municipal Income 2021 Fund-Retail Class
Management fee	0.30%
Distribution and/or Service (12b-1) fees	none
Other expenses	0.10%
Total annual operating expenses	0.40%

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® Municipal Income 2021 Fund}	06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 Fidelity® Municipal Income 2021 Fund Fidelity Municipal Income 2021 Fund-Retail Class USD ($)
1 year	$ 41
3 years	128
5 years	224
10 years	$ 505

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 8% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in investment-grade municipal securities whose interest is exempt from federal income tax (but may be subject to the federal alternative minimum tax).

• Managing the fund to have similar overall interest rate risk compared to a customized municipal bond index representing Fidelity Management & Research Company (FMR)'s view of the performance of the fund’s competitive universe.

• Allocating assets across different market sectors and maturities.

• Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects.

• Analyzing the credit quality of the issuer, security-specific features, current and potential future valuation, and trading opportunities to select investments.

• Liquidating shortly after the fund's target end-date of June 30, 2021.

Principal Investment Risks

• Municipal Market Volatility. The municipal market is volatile and can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities.

• Interest Rate Changes. Interest rate increases can cause the price of a debt security to decrease.

• 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. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a security to decrease.

• Declining Yield Risk. As the fund approaches its liquidation date, the fund's securities will mature and the fund may reinvest the proceeds in money market securities with lower yields than the securities previously held by the fund.

• Fluctuation of Yield & Liquidation Amount Risk. The amount of the fund's income distributions will vary over time, and the breakdown of returns between fund distributions and liquidation proceeds will not be predictable at the time of your investment, and you may experience a gain or loss for tax purposes.

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. Unlike individual debt securities, which typically pay principal at maturity, the value of an investment in the fund will fluctuate. 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 and an additional index over various periods of time. The indexes have characteristics relevant to the fund's investment strategies. Index descriptions appear in the "Additional Index Information" section of the prospectus. Past performance (before and after taxes) is not an indication of future performance.

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

Year-by-Year Returns

Calendar Years

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	2.59%	June 30, 2014
Lowest Quarter Return	-3.67%	June 30, 2013
Year-to-Date Return	0.44%	June 30, 2018

Average Annual Returns

After-tax returns are calculated using the historical highest individual federal marginal income tax rates, but do not reflect the impact of state or local taxes. Actual after-tax returns may differ depending on your individual circumstances. Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.

For the periods ended December 31, 2017

Average Annual Total Returns{- Fidelity® Municipal Income 2021 Fund} - 06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 - Fidelity® Municipal Income 2021 Fund	Past 1 year	Past 5 years	Since Inception	[1]	Inception Date
Fidelity Municipal Income 2021 Fund-Retail Class					May 19, 2011
Fidelity Municipal Income 2021 Fund-Retail Class | Return Before Taxes	2.43%	1.92%	3.68%
Fidelity Municipal Income 2021 Fund-Retail Class | After Taxes on Distributions	2.43%	1.89%	3.63%		May 19, 2011
Fidelity Municipal Income 2021 Fund-Retail Class | After Taxes on Distributions and Sales	2.24%	1.99%	3.39%		May 19, 2011
Bloomberg Barclays Municipal Bond Index(reflects no deduction for fees, expenses, or taxes)	5.45%	3.02%	4.28%
Bloomberg Barclays Municipal Bond 2021 Index Ex Tobacco/Prepaid Gas(reflects no deduction for fees, expenses, or taxes)	2.76%	1.93%	3.77%

[1] From May 19, 2011

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Aug. 29, 2018

06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 | Fidelity® Municipal Income 2021 Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® Municipal Income 2021 Fund/Fidelity® Municipal Income 2021 Fund

Objective [Heading]

rr_ObjectiveHeading

Investment Objective

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks as high a level of current income, exempt from federal income tax, as is consistent with the preservation of capital.

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 8% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

8.00%

Expense Example Narrative [Text Block]

rr_ExpenseExampleNarrativeTextBlock

This example helps compare the cost of investing in the fund with the cost of investing in other funds.

Let's say, hypothetically, that the annual return for shares of the fund is 5% and that your shareholder fees and the annual operating expenses for shares of the fund are exactly as described in the fee table. This example illustrates the effect of fees and expenses, but is not meant to suggest actual or expected fees and expenses or returns, all of which may vary. For every $10,000 you invested, here's how much you would pay in total expenses if you sell all of your shares at the end of each time period indicated:

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 investment-grade municipal securities whose interest is exempt from federal income tax (but may be subject to the federal alternative minimum tax).

• Managing the fund to have similar overall interest rate risk compared to a customized municipal bond index representing Fidelity Management & Research Company (FMR)'s view of the performance of the fund’s competitive universe.

• Allocating assets across different market sectors and maturities.

• Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects.

• Analyzing the credit quality of the issuer, security-specific features, current and potential future valuation, and trading opportunities to select investments.

• Liquidating shortly after the fund's target end-date of June 30, 2021.

Risk [Heading]

rr_RiskHeading

Principal Investment Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Municipal Market Volatility. The municipal market is volatile and can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities.

• Interest Rate Changes. Interest rate increases can cause the price of a debt security to decrease.

• 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. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a security to decrease.

• Declining Yield Risk. As the fund approaches its liquidation date, the fund's securities will mature and the fund may reinvest the proceeds in money market securities with lower yields than the securities previously held by the fund.

• Fluctuation of Yield & Liquidation Amount Risk. The amount of the fund's income distributions will vary over time, and the breakdown of returns between fund distributions and liquidation proceeds will not be predictable at the time of your investment, and you may experience a gain or loss for tax purposes.

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. Unlike individual debt securities, which typically pay principal at maturity, the value of an investment in the fund will fluctuate. 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 and an additional index over various periods of time. The indexes have characteristics relevant to the fund's investment strategies. Index descriptions appear in the "Additional Index Information" section of the prospectus. Past performance (before and after taxes) is not an indication of future performance.

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

Performance Information Illustrates Variability of Returns [Text]

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 and an additional index over various periods of time.

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

www.fidelity.com

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

Past performance (before and after taxes) is not an indication of future performance.

Bar Chart [Heading]

rr_BarChartHeading

Year-by-Year Returns

Calendar Years

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	2.59%	June 30, 2014
Lowest Quarter Return	-3.67%	June 30, 2013
Year-to-Date Return	0.44%	June 30, 2018

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 Explanation after Tax Higher

rr_PerformanceTableExplanationAfterTaxHigher

Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.

Performance Table Narrative

rr_PerformanceTableNarrativeTextBlock

After-tax returns are calculated using the historical highest individual federal marginal income tax rates, but do not reflect the impact of state or local taxes. Actual after-tax returns may differ depending on your individual circumstances. Return After Taxes on Distributions and Sale of Fund Shares may be higher than other returns for the same period due to a tax benefit of realizing a capital loss upon the sale of fund shares.

For the periods ended December 31, 2017

06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 | Fidelity® Municipal Income 2021 Fund | Fidelity Municipal Income 2021 Fund-Retail Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.30%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.10%

Total annual operating expenses

rr_ExpensesOverAssets

0.40%

1 year

rr_ExpenseExampleYear01

$ 41

3 years

rr_ExpenseExampleYear03

128

5 years

rr_ExpenseExampleYear05

224

10 years

rr_ExpenseExampleYear10

$ 505

2012

rr_AnnualReturn2012

5.87%

2013

rr_AnnualReturn2013

(2.71%)

2014

rr_AnnualReturn2014

7.35%

2015

rr_AnnualReturn2015

2.92%

2016

rr_AnnualReturn2016

(0.11%)

2017

rr_AnnualReturn2017

2.43%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Jun. 30, 2018

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

0.44%

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Jun. 30, 2014

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

2.59%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Jun. 30, 2013

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(3.67%)

Inception Date

rr_AverageAnnualReturnInceptionDate

May 19, 2011

06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 | Fidelity® Municipal Income 2021 Fund | Return Before Taxes | Fidelity Municipal Income 2021 Fund-Retail Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Municipal Income 2021 Fund

Label

rr_AverageAnnualReturnLabel

Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

2.43%

Past 5 years

rr_AverageAnnualReturnYear05

1.92%

Since Inception

rr_AverageAnnualReturnSinceInception

3.68%

^[1]

06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 | Fidelity® Municipal Income 2021 Fund | After Taxes on Distributions | Fidelity Municipal Income 2021 Fund-Retail Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Municipal Income 2021 Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions

Past 1 year

rr_AverageAnnualReturnYear01

2.43%

Past 5 years

rr_AverageAnnualReturnYear05

1.89%

Since Inception

rr_AverageAnnualReturnSinceInception

3.63%

^[1]

Inception Date

rr_AverageAnnualReturnInceptionDate

May 19, 2011

06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 | Fidelity® Municipal Income 2021 Fund | After Taxes on Distributions and Sales | Fidelity Municipal Income 2021 Fund-Retail Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Municipal Income 2021 Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions and Sale of Fund Shares

Past 1 year

rr_AverageAnnualReturnYear01

2.24%

Past 5 years

rr_AverageAnnualReturnYear05

1.99%

Since Inception

rr_AverageAnnualReturnSinceInception

3.39%

^[1]

Inception Date

rr_AverageAnnualReturnInceptionDate

May 19, 2011

06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 | Fidelity® Municipal Income 2021 Fund | Bloomberg Barclays Municipal Bond Index(reflects no deduction for fees, expenses, or taxes)

Risk/Return:

rr_RiskReturnAbstract

Past 1 year

rr_AverageAnnualReturnYear01

5.45%

Past 5 years

rr_AverageAnnualReturnYear05

3.02%

Since Inception

rr_AverageAnnualReturnSinceInception

4.28%

^[1]

06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 | Fidelity® Municipal Income 2021 Fund | Bloomberg Barclays Municipal Bond 2021 Index Ex Tobacco/Prepaid Gas(reflects no deduction for fees, expenses, or taxes)

Risk/Return:

rr_RiskReturnAbstract

Past 1 year

rr_AverageAnnualReturnYear01

2.76%

Past 5 years

rr_AverageAnnualReturnYear05

1.93%

Since Inception

rr_AverageAnnualReturnSinceInception

3.77%

^[1]

[1] From May 19, 2011

XML

06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 | Fidelity® Municipal Income 2023 Fund

Fund Summary

Fund/Class:
Fidelity® Municipal Income 2023 Fund/Fidelity® Municipal Income 2023 Fund

Investment Objective

The fund seeks as high a level of current income, exempt from federal income tax, as is consistent with the preservation of capital.

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® Municipal Income 2023 Fund}	06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 Fidelity® Municipal Income 2023 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® Municipal Income 2023 Fund}	06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 Fidelity® Municipal Income 2023 Fund Fidelity Municipal Income 2023 Fund-Retail Class
Management fee	0.30%
Distribution and/or Service (12b-1) fees	none
Other expenses	0.10%
Total annual operating expenses	0.40%

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® Municipal Income 2023 Fund}	06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 Fidelity® Municipal Income 2023 Fund Fidelity Municipal Income 2023 Fund-Retail Class USD ($)
1 year	$ 41
3 years	128
5 years	224
10 years	$ 505

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 12% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in investment-grade municipal securities whose interest is exempt from federal income tax (but may be subject to the federal alternative minimum tax).

• Managing the fund to have similar overall interest rate risk compared to a customized municipal bond index representing Fidelity Management & Research Company (FMR)'s view of the performance of the fund’s competitive universe.

• Allocating assets across different market sectors and maturities.

• Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects.

• Analyzing the credit quality of the issuer, security-specific features, current and potential future valuation, and trading opportunities to select investments.

• Liquidating shortly after the fund's target end-date of June 30, 2023.

Principal Investment Risks

• Municipal Market Volatility. The municipal market is volatile and can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities.

• Interest Rate Changes. Interest rate increases can cause the price of a debt security to decrease.

• 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. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a security to decrease.

• Declining Yield Risk. As the fund approaches its liquidation date, the fund's securities will mature and the fund may reinvest the proceeds in money market securities with lower yields than the securities previously held by the fund.

• Fluctuation of Yield & Liquidation Amount Risk. The amount of the fund's income distributions will vary over time, and the breakdown of returns between fund distributions and liquidation proceeds will not be predictable at the time of your investment, and you may experience a gain or loss for tax purposes.

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. Unlike individual debt securities, which typically pay principal at maturity, the value of an investment in the fund will fluctuate. 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 and an additional index over various periods of time. The indexes have characteristics relevant to the fund's investment strategies. Index descriptions appear in the "Additional Index Information" section of the prospectus. Past performance (before and after taxes) is not an indication of future performance.

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

Year-by-Year Returns

Calendar Years

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	3.71%	March 31, 2014
Lowest Quarter Return	-4.60%	December 31, 2016
Year-to-Date Return	-0.19%	June 30, 2018

Average Annual Returns

After-tax returns are calculated using the historical highest individual federal marginal income tax rates, but do not reflect the impact of state or local taxes. Actual after-tax returns may differ depending on your individual circumstances.

For the periods ended December 31, 2017

Average Annual Total Returns{- Fidelity® Municipal Income 2023 Fund} - 06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 - Fidelity® Municipal Income 2023 Fund	Past 1 year	Since Inception	[1]	Inception Date
Fidelity Municipal Income 2023 Fund-Retail Class				Apr. 23, 2013
Fidelity Municipal Income 2023 Fund-Retail Class | Return Before Taxes	4.39%	2.57%
Fidelity Municipal Income 2023 Fund-Retail Class | After Taxes on Distributions	4.39%	2.56%		Apr. 23, 2013
Fidelity Municipal Income 2023 Fund-Retail Class | After Taxes on Distributions and Sales	3.39%	2.46%		Apr. 23, 2013
Bloomberg Barclays Municipal Bond Index(reflects no deduction for fees, expenses, or taxes)	5.45%	2.97%
Bloomberg Barclays Municipal Bond 2023 Index Ex Tobacco/Prepaid Gas(reflects no deduction for fees, expenses, or taxes)	4.32%	2.35%

[1] From April 23, 2013

XML

Label

Element

Value

Risk/Return:

rr_RiskReturnAbstract

Registrant Name

dei_EntityRegistrantName

FIDELITY SALEM STREET TRUST

Prospectus Date

rr_ProspectusDate

Aug. 29, 2018

06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 | Fidelity® Municipal Income 2023 Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® Municipal Income 2023 Fund/Fidelity® Municipal Income 2023 Fund

Objective [Heading]

rr_ObjectiveHeading

Investment Objective

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks as high a level of current income, exempt from federal income tax, as is consistent with the preservation of capital.

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 12% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

12.00%

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 investment-grade municipal securities whose interest is exempt from federal income tax (but may be subject to the federal alternative minimum tax).

• Managing the fund to have similar overall interest rate risk compared to a customized municipal bond index representing Fidelity Management & Research Company (FMR)'s view of the performance of the fund’s competitive universe.

• Allocating assets across different market sectors and maturities.

• Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects.

• Analyzing the credit quality of the issuer, security-specific features, current and potential future valuation, and trading opportunities to select investments.

• Liquidating shortly after the fund's target end-date of June 30, 2023.

Risk [Heading]

rr_RiskHeading

Principal Investment Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Municipal Market Volatility. The municipal market is volatile and can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities.

• Interest Rate Changes. Interest rate increases can cause the price of a debt security to decrease.

• 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. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a security to decrease.

• Declining Yield Risk. As the fund approaches its liquidation date, the fund's securities will mature and the fund may reinvest the proceeds in money market securities with lower yields than the securities previously held by the fund.

• Fluctuation of Yield & Liquidation Amount Risk. The amount of the fund's income distributions will vary over time, and the breakdown of returns between fund distributions and liquidation proceeds will not be predictable at the time of your investment, and you may experience a gain or loss for tax purposes.

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. Unlike individual debt securities, which typically pay principal at maturity, the value of an investment in the fund will fluctuate. 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 and an additional index over various periods of time. The indexes have characteristics relevant to the fund's investment strategies. Index descriptions appear in the "Additional Index Information" section of the prospectus. Past performance (before and after taxes) is not an indication of future performance.

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

Performance Information Illustrates Variability of Returns [Text]

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 and an additional index over various periods of time.

Performance Availability Website Address [Text]

rr_PerformanceAvailabilityWebSiteAddress

www.fidelity.com

Performance Past Does Not Indicate Future [Text]

rr_PerformancePastDoesNotIndicateFuture

Past performance (before and after taxes) is not an indication of future performance.

Bar Chart [Heading]

rr_BarChartHeading

Year-by-Year Returns

Calendar Years

Bar Chart Closing [Text Block]

rr_BarChartClosingTextBlock

During the periods shown in the chart:	Returns	Quarter ended
Highest Quarter Return	3.71%	March 31, 2014
Lowest Quarter Return	-4.60%	December 31, 2016
Year-to-Date Return	-0.19%	June 30, 2018

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 Narrative

rr_PerformanceTableNarrativeTextBlock

After-tax returns are calculated using the historical highest individual federal marginal income tax rates, but do not reflect the impact of state or local taxes. Actual after-tax returns may differ depending on your individual circumstances.

For the periods ended December 31, 2017

06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 | Fidelity® Municipal Income 2023 Fund | Fidelity Municipal Income 2023 Fund-Retail Class

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.30%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.10%

Total annual operating expenses

rr_ExpensesOverAssets

0.40%

1 year

rr_ExpenseExampleYear01

$ 41

3 years

rr_ExpenseExampleYear03

128

5 years

rr_ExpenseExampleYear05

224

10 years

rr_ExpenseExampleYear10

$ 505

2014

rr_AnnualReturn2014

10.23%

2015

rr_AnnualReturn2015

3.32%

2016

rr_AnnualReturn2016

(0.17%)

2017

rr_AnnualReturn2017

4.39%

Year to Date Return, Label

rr_YearToDateReturnLabel

Year-to-Date Return

Bar Chart, Year to Date Return, Date

rr_BarChartYearToDateReturnDate

Jun. 30, 2018

Bar Chart, Year to Date Return

rr_BarChartYearToDateReturn

(0.19%)

Highest Quarterly Return, Label

rr_HighestQuarterlyReturnLabel

Highest Quarter Return

Highest Quarterly Return, Date

rr_BarChartHighestQuarterlyReturnDate

Mar. 31, 2014

Highest Quarterly Return

rr_BarChartHighestQuarterlyReturn

3.71%

Lowest Quarterly Return, Label

rr_LowestQuarterlyReturnLabel

Lowest Quarter Return

Lowest Quarterly Return, Date

rr_BarChartLowestQuarterlyReturnDate

Dec. 31, 2016

Lowest Quarterly Return

rr_BarChartLowestQuarterlyReturn

(4.60%)

Inception Date

rr_AverageAnnualReturnInceptionDate

Apr. 23, 2013

06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 | Fidelity® Municipal Income 2023 Fund | Return Before Taxes | Fidelity Municipal Income 2023 Fund-Retail Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Municipal Income 2023 Fund

Label

rr_AverageAnnualReturnLabel

Return Before Taxes

Past 1 year

rr_AverageAnnualReturnYear01

4.39%

Since Inception

rr_AverageAnnualReturnSinceInception

2.57%

^[1]

06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 | Fidelity® Municipal Income 2023 Fund | After Taxes on Distributions | Fidelity Municipal Income 2023 Fund-Retail Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Municipal Income 2023 Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions

Past 1 year

rr_AverageAnnualReturnYear01

4.39%

Since Inception

rr_AverageAnnualReturnSinceInception

2.56%

^[1]

Inception Date

rr_AverageAnnualReturnInceptionDate

Apr. 23, 2013

06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 | Fidelity® Municipal Income 2023 Fund | After Taxes on Distributions and Sales | Fidelity Municipal Income 2023 Fund-Retail Class

Risk/Return:

rr_RiskReturnAbstract

Column

rr_AverageAnnualReturnColumnName

Fidelity® Municipal Income 2023 Fund

Label

rr_AverageAnnualReturnLabel

Return After Taxes on Distributions and Sale of Fund Shares

Past 1 year

rr_AverageAnnualReturnYear01

3.39%

Since Inception

rr_AverageAnnualReturnSinceInception

2.46%

^[1]

Inception Date

rr_AverageAnnualReturnInceptionDate

Apr. 23, 2013

06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 | Fidelity® Municipal Income 2023 Fund | Bloomberg Barclays Municipal Bond Index(reflects no deduction for fees, expenses, or taxes)

Risk/Return:

rr_RiskReturnAbstract

Past 1 year

rr_AverageAnnualReturnYear01

5.45%

Since Inception

rr_AverageAnnualReturnSinceInception

2.97%

^[1]

06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 | Fidelity® Municipal Income 2023 Fund | Bloomberg Barclays Municipal Bond 2023 Index Ex Tobacco/Prepaid Gas(reflects no deduction for fees, expenses, or taxes)

Risk/Return:

rr_RiskReturnAbstract

Past 1 year

rr_AverageAnnualReturnYear01

4.32%

Since Inception

rr_AverageAnnualReturnSinceInception

2.35%

^[1]

[1] From April 23, 2013

XML

06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 | Fidelity® Municipal Income 2025 Fund

Fund Summary

Fund/Class:
Fidelity® Municipal Income 2025 Fund/Fidelity® Municipal Income 2025 Fund

Investment Objective

The fund seeks as high a level of current income, exempt from federal income tax, as is consistent with the preservation of capital.

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® Municipal Income 2025 Fund}	06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 Fidelity® Municipal Income 2025 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® Municipal Income 2025 Fund}	06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 Fidelity® Municipal Income 2025 Fund Fidelity Municipal Income 2025 Fund
Management fee	0.30%
Distribution and/or Service (12b-1) fees	none
Other expenses	0.10%
Total annual operating expenses	0.40%

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® Municipal Income 2025 Fund}	06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 Fidelity® Municipal Income 2025 Fund Fidelity Municipal Income 2025 Fund USD ($)
1 year	$ 41
3 years	128
5 years	224
10 years	$ 505

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 44% of the average value of its portfolio.

Principal Investment Strategies

• Normally investing at least 80% of assets in investment-grade municipal securities whose interest is exempt from federal income tax (but may be subject to the federal alternative minimum tax).

• Managing the fund to have similar overall interest rate risk compared to a customized municipal bond index representing Fidelity Management & Research Company (FMR)'s view of the performance of the fund’s competitive universe.

• Allocating assets across different market sectors and maturities.

• Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects.

• Analyzing the credit quality of the issuer, security-specific features, current and potential future valuation, and trading opportunities to select investments.

• Liquidating shortly after the fund's target end-date of June 30, 2025.

Principal Investment Risks

• Municipal Market Volatility. The municipal market is volatile and can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities.

• Interest Rate Changes. Interest rate increases can cause the price of a debt security to decrease.

• 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. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a security to decrease.

• Declining Yield Risk. As the fund approaches its liquidation date, the fund's securities will mature and the fund may reinvest the proceeds in money market securities with lower yields than the securities previously held by the fund.

• Fluctuation of Yield & Liquidation Amount Risk. The amount of the fund's income distributions will vary over time, and the breakdown of returns between fund distributions and liquidation proceeds will not be predictable at the time of your investment, and you may experience a gain or loss for tax purposes.

In addition, the fund is considered non-diversified and can invest a greater portion of assets in securities of a smaller number of individual issuers than a diversified fund. As a result, changes in the market value of a single investment could cause greater fluctuations in share price than would occur in a more diversified fund.

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. Unlike individual debt securities, which typically pay principal at maturity, the value of an investment in the fund will fluctuate. 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

Aug. 29, 2018

06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 | Fidelity® Municipal Income 2025 Fund

Risk/Return:

rr_RiskReturnAbstract

Risk/Return [Heading]

rr_RiskReturnHeading

Fund Summary

Fund/Class:
Fidelity® Municipal Income 2025 Fund/Fidelity® Municipal Income 2025 Fund

Objective [Heading]

rr_ObjectiveHeading

Investment Objective

Objective, Primary [Text Block]

rr_ObjectivePrimaryTextBlock

The fund seeks as high a level of current income, exempt from federal income tax, as is consistent with the preservation of capital.

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 44% of the average value of its portfolio.

Portfolio Turnover, Rate

rr_PortfolioTurnoverRate

44.00%

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 investment-grade municipal securities whose interest is exempt from federal income tax (but may be subject to the federal alternative minimum tax).

• Managing the fund to have similar overall interest rate risk compared to a customized municipal bond index representing Fidelity Management & Research Company (FMR)'s view of the performance of the fund’s competitive universe.

• Allocating assets across different market sectors and maturities.

• Potentially investing more than 25% of total assets in municipal securities that finance similar types of projects.

• Analyzing the credit quality of the issuer, security-specific features, current and potential future valuation, and trading opportunities to select investments.

• Liquidating shortly after the fund's target end-date of June 30, 2025.

Risk [Heading]

rr_RiskHeading

Principal Investment Risks

Risk Narrative [Text Block]

rr_RiskNarrativeTextBlock

• Municipal Market Volatility. The municipal market is volatile and can be significantly affected by adverse tax, legislative, or political changes and the financial condition of the issuers of municipal securities.

• Interest Rate Changes. Interest rate increases can cause the price of a debt security to decrease.

• 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. A decline in the credit quality of an issuer or a provider of credit support or a maturity-shortening structure for a security can cause the price of a security to decrease.

• Declining Yield Risk. As the fund approaches its liquidation date, the fund's securities will mature and the fund may reinvest the proceeds in money market securities with lower yields than the securities previously held by the fund.

• Fluctuation of Yield & Liquidation Amount Risk. The amount of the fund's income distributions will vary over time, and the breakdown of returns between fund distributions and liquidation proceeds will not be predictable at the time of your investment, and you may experience a gain or loss for tax purposes.

In addition, the fund is considered non-diversified and can invest a greater portion of assets in securities of a smaller number of individual issuers than a diversified fund. As a result, changes in the market value of a single investment could cause greater fluctuations in share price than would occur in a more diversified fund.

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. Unlike individual debt securities, which typically pay principal at maturity, the value of an investment in the fund will fluctuate. 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 Nondiversified Status [Text]

rr_RiskNondiversifiedStatus

In addition, the fund is considered non-diversified and can invest a greater portion of assets in securities of a smaller number of individual issuers than a diversified fund. As a result, changes in the market value of a single investment could cause greater fluctuations in share price than would occur in a more diversified 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.

06.30 Fidelity Defined Maturity Funds Retail Combo PRO-04 | Fidelity® Municipal Income 2025 Fund | Fidelity Municipal Income 2025 Fund

Risk/Return:

rr_RiskReturnAbstract

Management fee

rr_ManagementFeesOverAssets

0.30%

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

rr_DistributionAndService12b1FeesOverAssets

none

Other expenses

rr_OtherExpensesOverAssets

0.10%

Total annual operating expenses

rr_ExpensesOverAssets

0.40%

1 year

rr_ExpenseExampleYear01

$ 41

3 years

rr_ExpenseExampleYear03

128

5 years

rr_ExpenseExampleYear05

224

10 years

rr_ExpenseExampleYear10

$ 505

XML

Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	FIDELITY SALEM STREET TRUST
Prospectus Date	rr_ProspectusDate	Aug. 29, 2018
Document Creation Date	dei_DocumentCreationDate	Aug. 27, 2018