v3.26.3
Shareholder Report
 6 Months Ended
Aug. 31, 2026
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, 2026
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, 2026 to August 31, 2026.
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 $ 3,618,655,497
Holdings Count | shares 112
Investment Company Portfolio Turnover 42.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of August 31, 2026)
 
KEY FACTS 
  
  
Fund Size
$3,618,655,497
  
Number of Holdings
112
  
Portfolio Turnover
42%
  
Holdings [Text Block]   COUPON DISTRIBUTION (% of Fund's net assets) 0.01 - 0.99% 6.8   1 - 1.99% 11.6   2 - 2.99% 4.9   3 - 3.99% 45.3   4 - 4.99% 30.6         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 99.2     99.2  
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, 2026 to August 31, 2026.
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
  
AAmount represents less than $.50
BAmount represents less than 0.005%
 
Expenses Paid, Amount $ 0
Expense Ratio, Percent 0.00%
Net Assets $ 20,577,410,467
Holdings Count | shares 95
Investment Company Portfolio Turnover 15.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of August 31, 2026)
 
KEY FACTS 
  
  
Fund Size
$20,577,410,467
  
Number of Holdings
95
  
Portfolio Turnover
15%
  
Holdings [Text Block]   COUPON DISTRIBUTION (% of Fund's net assets) 1 - 1.99% 6.1   2 - 2.99% 28.3   3 - 3.99% 20.2   4 - 4.99% 41.3   5 - 5.99% 3.3         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 98.3   US Treasury Notes 0.9     99.2  
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, 2026 to August 31, 2026.
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 $ 8,427,896,061
Holdings Count | shares 228
Investment Company Portfolio Turnover 32.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of August 31, 2026)
 
KEY FACTS 
  
  
Fund Size
$8,427,896,061
  
Number of Holdings
228
  
Portfolio Turnover
32%
  
Holdings [Text Block]   COUPON DISTRIBUTION (% of Fund's net assets) 0.01 - 0.99% 6.2   1 - 1.99% 17.0   2 - 2.99% 15.3   3 - 3.99% 23.4   4 - 4.99% 32.2   5 - 5.99% 1.5   6 - 6.99% 3.4         U.S. Treasury Obligations 99.0 Short-Term Investments and Net Other Assets (Liabilities) 1.0 ASSET ALLOCATION (% of Fund's net assets)     U.S. Treasury Obligations - 99.0           Short-Term Investments and Net Other Assets (Liabilities) - 1.0  
Largest Holdings [Text Block] TOP HOLDINGS(% of Fund's net assets)     US Treasury Notes 75.1   US Treasury Bonds 23.9     99.0  
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, 2026 to August 31, 2026.
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 $ 12,850,439,242
Holdings Count | shares 45
Investment Company Portfolio Turnover 34.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of August 31, 2026)
 
KEY FACTS 
  
  
Fund Size
$12,850,439,242
  
Number of Holdings
45
  
Portfolio Turnover
34%
  
Holdings [Text Block]   COUPON DISTRIBUTION (% of Fund's net assets) 1 - 1.99% 11.5   2 - 2.99% 17.7   3 - 3.99% 19.6   4 - 4.99% 46.1   5 - 5.99% 4.3         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, 2026 to August 31, 2026.
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 $ 4,942,865,700
Holdings Count | shares 98
Investment Company Portfolio Turnover 20.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of August 31, 2026)
 
KEY FACTS 
  
  
Fund Size
$4,942,865,700
  
Number of Holdings
98
  
Portfolio Turnover
20%
  
Holdings [Text Block]   COUPON DISTRIBUTION (% of Fund's net assets) 1 - 1.99% 11.6   2 - 2.99% 18.3   3 - 3.99% 24.0   4 - 4.99% 40.0   5 - 5.99% 5.3         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 97.8   US Treasury Notes 1.4     99.2  
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, 2026 to August 31, 2026.
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 
$ 1 
0.03%
  
Expenses Paid, Amount $ 1
Expense Ratio, Percent 0.03%
Net Assets $ 8,447,950,210
Holdings Count | shares 44
Investment Company Portfolio Turnover 38.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of August 31, 2026)
 
KEY FACTS 
  
  
Fund Size
$8,447,950,210
  
Number of Holdings
44
  
Portfolio Turnover
38%
  
Holdings [Text Block]   COUPON DISTRIBUTION (% of Fund's net assets) 1 - 1.99% 7.3   2 - 2.99% 5.6   3 - 3.99% 20.5   4 - 4.99% 65.9         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 99.3   US Treasury Bonds 0.0     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, 2026 to August 31, 2026.
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 $ 1,485,617,207
Holdings Count | shares 27
Investment Company Portfolio Turnover 64.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of August 31, 2026)
 
KEY FACTS 
  
  
Fund Size
$1,485,617,207
  
Number of Holdings
27
  
Portfolio Turnover
64%
  
Holdings [Text Block]   COUPON DISTRIBUTION (% of Fund's net assets) 1 - 1.99% 11.7   2 - 2.99% 8.7   3 - 3.99% 21.5   4 - 4.99% 57.0         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 98.9     98.9  
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, 2026 to August 31, 2026.
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 $ 673,741,788
Holdings Count | shares 133
Investment Company Portfolio Turnover 174.00%
Additional Fund Statistics [Text Block] Key Fund Statistics
(as of August 31, 2026)
 
KEY FACTS 
  
  
Fund Size
$673,741,788
  
Number of Holdings
133
  
Portfolio Turnover
174%
  
Holdings [Text Block]   COUPON DISTRIBUTION (% of Fund's net assets) 0.01 - 0.99% 7.6   1 - 1.99% 12.4   2 - 2.99% 7.7   3 - 3.99% 39.6   4 - 4.99% 31.7   5 - 5.99% 0.0   6 - 6.99% 0.1         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 Notes 99.0   US Treasury Bonds 0.1     99.1