Label	Element	Value
Risk/Return:	rr_RiskReturnAbstract
Registrant Name	dei_EntityRegistrantName	FIDELITY SALEM STREET TRUST
09.30 Fidelity Strategic Real Return Fund AMCIZ PRO-21
Risk/Return:	rr_RiskReturnAbstract
Supplement to Prospectus [Text Block]	rr_SupplementToProspectusTextBlock	Supplement to theFidelity® Strategic Real Return FundClass A, Class M, Class C, Class I and Class ZNovember 29, 2021Prospectus
09.30 Fidelity Strategic Real Return Fund AMCIZ PRO-21 | Fidelity Strategic Real Return Fund
Risk/Return:	rr_RiskReturnAbstract
Operating Expenses Caption [Text]	rr_OperatingExpensesCaption	Annual Operating Expenses (expenses that you pay each year as a % of the value of your investment)
Fee Waiver or Reimbursement over Assets, Date of Termination	rr_FeeWaiverOrReimbursementOverAssetsDateOfTermination	January 31, 2024
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 and if you hold your shares:
Strategy Narrative [Text Block]	rr_StrategyNarrativeTextBlock	Using a neutral mix of approximately 25% inflation-protected debt securities (which attempts to track the Bloomberg U.S. Treasury Inflation-Protected Securities (TIPS) Index, an index composed of inflation-protected debt securities issued by the U.S. Treasury), 25% floating rate loans, 30% commodity-linked derivative instruments and related investments, and 20% REITs and other real estate related investments.
09.30 Fidelity Strategic Real Return Fund AMCIZ PRO-21 | Fidelity Strategic Real Return Fund | Fidelity Advisor Strategic Real Return Fund: Class A
Risk/Return:	rr_RiskReturnAbstract
Management fee	rr_ManagementFeesOverAssets	0.55%
Distribution and/or Service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Other expenses	rr_OtherExpensesOverAssets	0.31%
Total annual operating expenses	rr_ExpensesOverAssets	1.11%
Fee waiver and/or expense reimbursement	rr_FeeWaiverOrReimbursementOverAssets	0.16%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	rr_NetExpensesOverAssets	0.95%
1 year	rr_ExpenseExampleYear01	$ 493
3 years	rr_ExpenseExampleYear03	702
5 years	rr_ExpenseExampleYear05	951
10 years	rr_ExpenseExampleYear10	1,664
1 Year	rr_ExpenseExampleNoRedemptionYear01	493
3 Years	rr_ExpenseExampleNoRedemptionYear03	702
5 Years	rr_ExpenseExampleNoRedemptionYear05	951
10 Years	rr_ExpenseExampleNoRedemptionYear10	$ 1,664
09.30 Fidelity Strategic Real Return Fund AMCIZ PRO-21 | Fidelity Strategic Real Return Fund | Fidelity Advisor Strategic Real Return Fund: Class M
Risk/Return:	rr_RiskReturnAbstract
Management fee	rr_ManagementFeesOverAssets	0.55%
Distribution and/or Service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	0.25%
Other expenses	rr_OtherExpensesOverAssets	0.32%
Total annual operating expenses	rr_ExpensesOverAssets	1.12%
Fee waiver and/or expense reimbursement	rr_FeeWaiverOrReimbursementOverAssets	0.17%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	rr_NetExpensesOverAssets	0.95%
1 year	rr_ExpenseExampleYear01	$ 493
3 years	rr_ExpenseExampleYear03	703
5 years	rr_ExpenseExampleYear05	954
10 years	rr_ExpenseExampleYear10	1,673
1 Year	rr_ExpenseExampleNoRedemptionYear01	493
3 Years	rr_ExpenseExampleNoRedemptionYear03	703
5 Years	rr_ExpenseExampleNoRedemptionYear05	954
10 Years	rr_ExpenseExampleNoRedemptionYear10	$ 1,673
09.30 Fidelity Strategic Real Return Fund AMCIZ PRO-21 | Fidelity Strategic Real Return Fund | Fidelity Advisor Strategic Real Return Fund: Class C
Risk/Return:	rr_RiskReturnAbstract
Management fee	rr_ManagementFeesOverAssets	0.55%
Distribution and/or Service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	1.00%
Other expenses	rr_OtherExpensesOverAssets	0.33%
Total annual operating expenses	rr_ExpensesOverAssets	1.88%
Fee waiver and/or expense reimbursement	rr_FeeWaiverOrReimbursementOverAssets	0.18%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	rr_NetExpensesOverAssets	1.70%
1 year	rr_ExpenseExampleYear01	$ 273
3 years	rr_ExpenseExampleYear03	549
5 years	rr_ExpenseExampleYear05	976
10 years	rr_ExpenseExampleYear10	1,964
1 Year	rr_ExpenseExampleNoRedemptionYear01	173
3 Years	rr_ExpenseExampleNoRedemptionYear03	549
5 Years	rr_ExpenseExampleNoRedemptionYear05	976
10 Years	rr_ExpenseExampleNoRedemptionYear10	$ 1,964
09.30 Fidelity Strategic Real Return Fund AMCIZ PRO-21 | Fidelity Strategic Real Return Fund | Fidelity Advisor Strategic Real Return Fund: Class I
Risk/Return:	rr_RiskReturnAbstract
Management fee	rr_ManagementFeesOverAssets	0.55%
Distribution and/or Service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.28%
Total annual operating expenses	rr_ExpensesOverAssets	0.83%
Fee waiver and/or expense reimbursement	rr_FeeWaiverOrReimbursementOverAssets	0.13%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	rr_NetExpensesOverAssets	0.70%
1 year	rr_ExpenseExampleYear01	$ 72
3 years	rr_ExpenseExampleYear03	233
5 years	rr_ExpenseExampleYear05	430
10 years	rr_ExpenseExampleYear10	996
1 Year	rr_ExpenseExampleNoRedemptionYear01	72
3 Years	rr_ExpenseExampleNoRedemptionYear03	233
5 Years	rr_ExpenseExampleNoRedemptionYear05	430
10 Years	rr_ExpenseExampleNoRedemptionYear10	$ 996
09.30 Fidelity Strategic Real Return Fund AMCIZ PRO-21 | Fidelity Strategic Real Return Fund | Fidelity Advisor Strategic Real Return Fund: Class Z
Risk/Return:	rr_RiskReturnAbstract
Management fee	rr_ManagementFeesOverAssets	0.55%
Distribution and/or Service (12b-1) fees	rr_DistributionAndService12b1FeesOverAssets	none
Other expenses	rr_OtherExpensesOverAssets	0.18%
Total annual operating expenses	rr_ExpensesOverAssets	0.73%
Fee waiver and/or expense reimbursement	rr_FeeWaiverOrReimbursementOverAssets	0.12%	[1]
Total annual operating expenses after fee waiver and/or expense reimbursement	rr_NetExpensesOverAssets	0.61%
1 year	rr_ExpenseExampleYear01	$ 62
3 years	rr_ExpenseExampleYear03	204
5 years	rr_ExpenseExampleYear05	377
10 years	rr_ExpenseExampleYear10	879
1 Year	rr_ExpenseExampleNoRedemptionYear01	62
3 Years	rr_ExpenseExampleNoRedemptionYear03	204
5 Years	rr_ExpenseExampleNoRedemptionYear05	377
10 Years	rr_ExpenseExampleNoRedemptionYear10	$ 879

[1] ^(a)Fidelity Management & Research Company LLC (FMR) has contractually agreed to reimburse Class A, Class M, Class C, Class I, and Class Z of the fund to the extent that total operating expenses (excluding interest, certain taxes, fees and expenses of the Independent Trustees, proxy and shareholder meeting expenses, extraordinary expenses, and acquired fund fees and expenses (including fees and expenses associated with a wholly owned subsidiary), if any, as well as non-operating expenses such as brokerage commissions and fees and expenses associated with the fund’s securities lending program, if applicable), as a percentage of their respective average net assets, exceed 0.95%, 0.95%, 1.70%, 0.70%, and 0.61% (the Expense Caps). If at any time during the current fiscal year expenses for Class A, Class M, Class C, Class I, or Class Z of the fund fall below the Expense Caps, FMR reserves the right to recoup through the end of the fiscal year any expenses that were reimbursed during the current fiscal year up to, but not in excess of, the Expense Caps. These arrangements will remain in effect through January 31, 2024. FMR may not terminate these arrangements before the expiration date without the approval of the Board of Trustees and may extend them in its discretion after that date.