v3.25.3
Shareholder Report
 6 Months Ended
Aug. 31, 2025
USD ($)
shares
Shareholder Report [Line Items]  
Document Type N-CSRS
Amendment Flag false
Registrant Name Fidelity Salem Street Trust
Entity Central Index Key 0000035315
Entity Investment Company Type N-1A
Document Period End Date Aug. 31, 2025
Fidelity Short-Term Treasury Bond Index Fund  
Shareholder Report [Line Items]  
Fund Name Fidelity® Short-Term Treasury Bond Index Fund
Class Name Fidelity® Short-Term Treasury Bond Index Fund
Trading Symbol FUMBX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity® Short-Term Treasury Bond Index Fund for the period March 1, 2025 to August 31, 2025.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-800-544-8544 or by sending an e-mail to fidfunddocuments@fidelity.com.
Additional Information Phone Number 1-800-544-8544
Additional Information Email fidfunddocuments@fidelity.com
Additional Information Website fundresearch.fidelity.com/prospectus/sec
Expenses [Text Block] What were your Fund costs for the last six months?
(based on hypothetical $10,000 investment)
 
 
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Fidelity® Short-Term Treasury Bond Index Fund 
$ 2 
0.03%
  
Expenses Paid, Amount $ 2
Expense Ratio, Percent 0.03%
Net Assets $ 3,266,390,207
Holdings Count | shares 114
Investment Company Portfolio Turnover 32.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of August 31, 2025)
 
KEY FACTS 
  
  
Fund Size
$3,266,390,207
  
Number of Holdings
114
  
Portfolio Turnover
32%
  
Holdings [Text Block]   COUPON DISTRIBUTION (% of Fund's net assets) 0.01 - 0.99% 12.9   1 - 1.99% 17.1   2 - 2.99% 8.4   3 - 3.99% 25.3   4 - 4.99% 34.7   5 - 5.99% 0.4   6 - 6.99% 0.5         U.S. Treasury Obligations 99.3 Short-Term Investments and Net Other Assets (Liabilities) 0.7 ASSET ALLOCATION (% of Fund's net assets)     U.S. Treasury Obligations - 99.3           Short-Term Investments and Net Other Assets (Liabilities) - 0.7  
Largest Holdings [Text Block] TOP HOLDINGS(% of Fund's net assets)     US Treasury Notes 98.4   US Treasury Bonds 0.9     99.3  
Fidelity Series Long-Term Treasury Bond Index Fund  
Shareholder Report [Line Items]  
Fund Name Fidelity® Series Long-Term Treasury Bond Index Fund
Class Name Fidelity® Series Long-Term Treasury Bond Index Fund
Trading Symbol FTLTX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity® Series Long-Term Treasury Bond Index Fund for the period March 1, 2025 to August 31, 2025.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-800-544-8544.
Additional Information Phone Number 1-800-544-8544
Additional Information Website fundresearch.fidelity.com/prospectus/sec
Expenses [Text Block] What were your Fund costs for the last six months?
(based on hypothetical $10,000 investment)
 
 
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Fidelity® Series Long-Term Treasury Bond Index Fund 
$ 0 A
0.00%B
  
A Amount represents less than $.50
B Amount represents less than 0.005%
 
Expenses Paid, Amount $ 0
Expense Ratio, Percent 0.00%
Net Assets $ 22,092,716,503
Holdings Count | shares 94
Investment Company Portfolio Turnover 10.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of August 31, 2025)
 
KEY FACTS 
  
  
Fund Size
$22,092,716,503
  
Number of Holdings
94
  
Portfolio Turnover
10%
  
Holdings [Text Block]   COUPON DISTRIBUTION (% of Fund's net assets) 1 - 1.99% 8.2   2 - 2.99% 30.3   3 - 3.99% 26.2   4 - 4.99% 33.3   5 - 5.99% 1.1         U.S. Treasury Obligations 99.1 Short-Term Investments and Net Other Assets (Liabilities) 0.9 ASSET ALLOCATION (% of Fund's net assets)     U.S. Treasury Obligations - 99.1           Short-Term Investments and Net Other Assets (Liabilities) - 0.9  
Largest Holdings [Text Block] TOP HOLDINGS(% of Fund's net assets)     US Treasury Bonds 98.2   US Treasury Notes 0.9     99.1  
Fidelity SAI U.S. Treasury Bond Index Fund  
Shareholder Report [Line Items]  
Fund Name Fidelity® SAI U.S. Treasury Bond Index Fund
Class Name Fidelity® SAI U.S. Treasury Bond Index Fund
Trading Symbol FUTBX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity® SAI U.S. Treasury Bond Index Fund for the period March 1, 2025 to August 31, 2025.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-800-544-3455 or by sending an e-mail to fidfunddocuments@fidelity.com.
Additional Information Phone Number 1-800-544-3455
Additional Information Email fidfunddocuments@fidelity.com
Additional Information Website fundresearch.fidelity.com/prospectus/sec
Expenses [Text Block] What were your Fund costs for the last six months?
(based on hypothetical $10,000 investment)
 
 
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Fidelity® SAI U.S. Treasury Bond Index Fund 
$ 2 
0.03%
  
Expenses Paid, Amount $ 2
Expense Ratio, Percent 0.03%
Net Assets $ 8,971,463,746
Holdings Count | shares 221
Investment Company Portfolio Turnover 28.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of August 31, 2025)
 
KEY FACTS 
  
  
Fund Size
$8,971,463,746
  
Number of Holdings
221
  
Portfolio Turnover
28%
  
Holdings [Text Block]   COUPON DISTRIBUTION (% of Fund's net assets) 0.01 - 0.99% 11.5   1 - 1.99% 24.9   2 - 2.99% 16.9   3 - 3.99% 17.1   4 - 4.99% 24.1   5 - 5.99% 0.8   6 - 6.99% 3.6         U.S. Treasury Obligations 98.9 Short-Term Investments and Net Other Assets (Liabilities) 1.1 ASSET ALLOCATION (% of Fund's net assets)     U.S. Treasury Obligations - 98.9           Short-Term Investments and Net Other Assets (Liabilities) - 1.1  
Largest Holdings [Text Block] TOP HOLDINGS(% of Fund's net assets)     US Treasury Notes 75.2   US Treasury Bonds 23.7     98.9  
Fidelity SAI Long-Term Treasury Bond Index Fund  
Shareholder Report [Line Items]  
Fund Name Fidelity® SAI Long-Term Treasury Bond Index Fund
Class Name Fidelity® SAI Long-Term Treasury Bond Index Fund
Trading Symbol FBLTX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity® SAI Long-Term Treasury Bond Index Fund for the period March 1, 2025 to August 31, 2025.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-800-544-3455 or by sending an e-mail to fidfunddocuments@fidelity.com.
Additional Information Phone Number 1-800-544-3455
Additional Information Email fidfunddocuments@fidelity.com
Additional Information Website fundresearch.fidelity.com/prospectus/sec
Expenses [Text Block] What were your Fund costs for the last six months?
(based on hypothetical $10,000 investment)
 
 
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Fidelity® SAI Long-Term Treasury Bond Index Fund 
$ 1 
0.03%
  
Expenses Paid, Amount $ 1
Expense Ratio, Percent 0.03%
Net Assets $ 4,209,432,992
Holdings Count | shares 45
Investment Company Portfolio Turnover 35.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of August 31, 2025)
 
KEY FACTS 
  
  
Fund Size
$4,209,432,992
  
Number of Holdings
45
  
Portfolio Turnover
35%
  
Holdings [Text Block]   COUPON DISTRIBUTION (% of Fund's net assets) 1 - 1.99% 9.6   2 - 2.99% 30.1   3 - 3.99% 24.2   4 - 4.99% 35.3         U.S. Treasury Obligations 99.2 Short-Term Investments and Net Other Assets (Liabilities) 0.8 ASSET ALLOCATION (% of Fund's net assets)     U.S. Treasury Obligations - 99.2           Short-Term Investments and Net Other Assets (Liabilities) - 0.8  
Largest Holdings [Text Block] TOP HOLDINGS(% of Fund's net assets)     US Treasury Bonds 99.2     99.2  
Fidelity Long-Term Treasury Bond Index Fund  
Shareholder Report [Line Items]  
Fund Name Fidelity® Long-Term Treasury Bond Index Fund
Class Name Fidelity® Long-Term Treasury Bond Index Fund
Trading Symbol FNBGX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity® Long-Term Treasury Bond Index Fund for the period March 1, 2025 to August 31, 2025.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-800-544-8544 or by sending an e-mail to fidfunddocuments@fidelity.com.
Additional Information Phone Number 1-800-544-8544
Additional Information Email fidfunddocuments@fidelity.com
Additional Information Website fundresearch.fidelity.com/prospectus/sec
Expenses [Text Block] What were your Fund costs for the last six months?
(based on hypothetical $10,000 investment)
 
 
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Fidelity® Long-Term Treasury Bond Index Fund 
$ 1 
0.03%
  
Expenses Paid, Amount $ 1
Expense Ratio, Percent 0.03%
Net Assets $ 5,015,929,002
Holdings Count | shares 91
Investment Company Portfolio Turnover 21.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of August 31, 2025)
 
KEY FACTS 
  
  
Fund Size
$5,015,929,002
  
Number of Holdings
91
  
Portfolio Turnover
21%
  
Holdings [Text Block]   COUPON DISTRIBUTION (% of Fund's net assets) 1 - 1.99% 13.5   2 - 2.99% 19.5   3 - 3.99% 30.4   4 - 4.99% 34.5   5 - 5.99% 1.4         U.S. Treasury Obligations 99.3 Short-Term Investments and Net Other Assets (Liabilities) 0.7 ASSET ALLOCATION (% of Fund's net assets)     U.S. Treasury Obligations - 99.3           Short-Term Investments and Net Other Assets (Liabilities) - 0.7  
Largest Holdings [Text Block] TOP HOLDINGS(% of Fund's net assets)     US Treasury Bonds 98.6   US Treasury Notes 0.7     99.3  
Fidelity Intermediate Treasury Bond Index Fund  
Shareholder Report [Line Items]  
Fund Name Fidelity® Intermediate Treasury Bond Index Fund
Class Name Fidelity® Intermediate Treasury Bond Index Fund
Trading Symbol FUAMX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity® Intermediate Treasury Bond Index Fund for the period March 1, 2025 to August 31, 2025.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-800-544-8544 or by sending an e-mail to fidfunddocuments@fidelity.com.
Additional Information Phone Number 1-800-544-8544
Additional Information Email fidfunddocuments@fidelity.com
Additional Information Website fundresearch.fidelity.com/prospectus/sec
Expenses [Text Block] What were your Fund costs for the last six months?
(based on hypothetical $10,000 investment)
 
 
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Fidelity® Intermediate Treasury Bond Index Fund 
$ 2 
0.03%
  
Expenses Paid, Amount $ 2
Expense Ratio, Percent 0.03%
Net Assets $ 6,824,870,782
Holdings Count | shares 45
Investment Company Portfolio Turnover 45.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of August 31, 2025)
 
KEY FACTS 
  
  
Fund Size
$6,824,870,782
  
Number of Holdings
45
  
Portfolio Turnover
45%
  
Holdings [Text Block]   COUPON DISTRIBUTION (% of Fund's net assets) 0.01 - 0.99% 4.5   1 - 1.99% 13.5   2 - 2.99% 6.8   3 - 3.99% 14.4   4 - 4.99% 59.9   6 - 6.99% 0.2         U.S. Treasury Obligations 99.3 Short-Term Investments and Net Other Assets (Liabilities) 0.7 ASSET ALLOCATION (% of Fund's net assets)     U.S. Treasury Obligations - 99.3           Short-Term Investments and Net Other Assets (Liabilities) - 0.7  
Largest Holdings [Text Block] TOP HOLDINGS(% of Fund's net assets)     US Treasury Notes 99.1   US Treasury Bonds 0.2     99.3  
Fidelity SAI Intermediate Treasury Bond Index Fund  
Shareholder Report [Line Items]  
Fund Name Fidelity® SAI Intermediate Treasury Bond Index Fund
Class Name Fidelity® SAI Intermediate Treasury Bond Index Fund
Trading Symbol FSIWX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity® SAI Intermediate Treasury Bond Index Fund for the period March 1, 2025 to August 31, 2025.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-800-544-3455 or by sending an e-mail to fidfunddocuments@fidelity.com.
Additional Information Phone Number 1-800-544-3455
Additional Information Email fidfunddocuments@fidelity.com
Additional Information Website fundresearch.fidelity.com/prospectus/sec
Expenses [Text Block] What were your Fund costs for the last six months?
(based on hypothetical $10,000 investment)
 
 
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Fidelity® SAI Intermediate Treasury Bond Index Fund 
$ 2 
0.03%
  
Expenses Paid, Amount $ 2
Expense Ratio, Percent 0.03%
Net Assets $ 3,793,942,607
Holdings Count | shares 48
Investment Company Portfolio Turnover 53.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of August 31, 2025)
 
KEY FACTS 
  
  
Fund Size
$3,793,942,607
  
Number of Holdings
48
  
Portfolio Turnover
53%
  
Holdings [Text Block]   COUPON DISTRIBUTION (% of Fund's net assets) 0.01 - 0.99% 3.0   1 - 1.99% 14.5   2 - 2.99% 5.8   3 - 3.99% 15.8   4 - 4.99% 60.0   5 - 5.99% 0.0   6 - 6.99% 0.1         U.S. Treasury Obligations 99.2 Short-Term Investments and Net Other Assets (Liabilities) 0.8 ASSET ALLOCATION (% of Fund's net assets)     U.S. Treasury Obligations - 99.2           Short-Term Investments and Net Other Assets (Liabilities) - 0.8  
Largest Holdings [Text Block] TOP HOLDINGS(% of Fund's net assets)     US Treasury Notes 99.1   US Treasury Bonds 0.1     99.2  
Fidelity SAI Short-Term Treasury Bond Index Fund  
Shareholder Report [Line Items]  
Fund Name Fidelity® SAI Short-Term Treasury Bond Index Fund
Class Name Fidelity® SAI Short-Term Treasury Bond Index Fund
Trading Symbol FSSWX
Annual or Semi-Annual Statement [Text Block] This semi-annual shareholder report contains information about Fidelity® SAI Short-Term Treasury Bond Index Fund for the period March 1, 2025 to August 31, 2025.
Shareholder Report Annual or Semi-Annual semi-annual shareholder report
Additional Information [Text Block] You can find additional information about the Fund at fundresearch.fidelity.com/prospectus/sec. You can also request this information by contacting us at 1-800-544-3455 or by sending an e-mail to fidfunddocuments@fidelity.com.
Additional Information Phone Number 1-800-544-3455
Additional Information Email fidfunddocuments@fidelity.com
Additional Information Website fundresearch.fidelity.com/prospectus/sec
Expenses [Text Block] What were your Fund costs for the last six months?
(based on hypothetical $10,000 investment)
 
 
 
Costs of a $10,000 investment
Costs paid as a percentage of a $10,000 investment
 
Fidelity® SAI Short-Term Treasury Bond Index Fund 
$ 2 
0.03%
  
Expenses Paid, Amount $ 2
Expense Ratio, Percent 0.03%
Net Assets $ 362,276,603
Holdings Count | shares 111
Investment Company Portfolio Turnover 82.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of August 31, 2025)
 
KEY FACTS 
  
  
Fund Size
$362,276,603
  
Number of Holdings
111
  
Portfolio Turnover
82%
  
Holdings [Text Block]   COUPON DISTRIBUTION (% of Fund's net assets) 0.01 - 0.99% 4.6   1 - 1.99% 5.7   2 - 2.99% 3.1   3 - 3.99% 51.0   4 - 4.99% 34.0   5 - 5.99% 0.1   6 - 6.99% 0.3         U.S. Treasury Obligations 98.8 Short-Term Investments and Net Other Assets (Liabilities) 1.2 ASSET ALLOCATION (% of Fund's net assets)     U.S. Treasury Obligations - 98.8           Short-Term Investments and Net Other Assets (Liabilities) - 1.2  
Largest Holdings [Text Block] TOP HOLDINGS(% of Fund's net assets)     US Treasury Notes 98.4   US Treasury Bonds 0.4     98.8