v3.25.3
Shareholder Report, Holdings (Details)
Aug. 31, 2025
Fidelity Short-Term Treasury Bond Index Fund | TH0Member  
Holdings [Line Items]  
Percent of Net Asset Value 98.40%
Fidelity Short-Term Treasury Bond Index Fund | TH1Member  
Holdings [Line Items]  
Percent of Net Asset Value 0.90%
Fidelity Short-Term Treasury Bond Index Fund | CD0Member  
Holdings [Line Items]  
Percent of Net Asset Value 12.90%
Fidelity Short-Term Treasury Bond Index Fund | CD1Member  
Holdings [Line Items]  
Percent of Net Asset Value 17.10%
Fidelity Short-Term Treasury Bond Index Fund | CD2Member  
Holdings [Line Items]  
Percent of Net Asset Value 8.40%
Fidelity Short-Term Treasury Bond Index Fund | CD3Member  
Holdings [Line Items]  
Percent of Net Asset Value 25.30%
Fidelity Short-Term Treasury Bond Index Fund | CD4Member  
Holdings [Line Items]  
Percent of Net Asset Value 34.70%
Fidelity Short-Term Treasury Bond Index Fund | CD5Member  
Holdings [Line Items]  
Percent of Net Asset Value 0.40%
Fidelity Short-Term Treasury Bond Index Fund | CD6Member  
Holdings [Line Items]  
Percent of Net Asset Value 0.50%
Fidelity Short-Term Treasury Bond Index Fund | AAUSTreasuryObligationsMember  
Holdings [Line Items]  
Percent of Net Asset Value 99.30%
Fidelity Short-Term Treasury Bond Index Fund | AAShortTermInvestmentsandNetOtherAssetsLiabilitiesMember  
Holdings [Line Items]  
Percent of Net Asset Value 0.70%
Fidelity Series Long-Term Treasury Bond Index Fund | TH0Member  
Holdings [Line Items]  
Percent of Net Asset Value 98.20%
Fidelity Series Long-Term Treasury Bond Index Fund | TH1Member  
Holdings [Line Items]  
Percent of Net Asset Value 0.90%
Fidelity Series Long-Term Treasury Bond Index Fund | CD0Member  
Holdings [Line Items]  
Percent of Net Asset Value 8.20%
Fidelity Series Long-Term Treasury Bond Index Fund | CD1Member  
Holdings [Line Items]  
Percent of Net Asset Value 30.30%
Fidelity Series Long-Term Treasury Bond Index Fund | CD2Member  
Holdings [Line Items]  
Percent of Net Asset Value 26.20%
Fidelity Series Long-Term Treasury Bond Index Fund | CD3Member  
Holdings [Line Items]  
Percent of Net Asset Value 33.30%
Fidelity Series Long-Term Treasury Bond Index Fund | CD4Member  
Holdings [Line Items]  
Percent of Net Asset Value 1.10%
Fidelity Series Long-Term Treasury Bond Index Fund | AAUSTreasuryObligationsMember  
Holdings [Line Items]  
Percent of Net Asset Value 99.10%
Fidelity Series Long-Term Treasury Bond Index Fund | AAShortTermInvestmentsandNetOtherAssetsLiabilitiesMember  
Holdings [Line Items]  
Percent of Net Asset Value 0.90%
Fidelity SAI U.S. Treasury Bond Index Fund | TH0Member  
Holdings [Line Items]  
Percent of Net Asset Value 75.20%
Fidelity SAI U.S. Treasury Bond Index Fund | TH1Member  
Holdings [Line Items]  
Percent of Net Asset Value 23.70%
Fidelity SAI U.S. Treasury Bond Index Fund | CD0Member  
Holdings [Line Items]  
Percent of Net Asset Value 11.50%
Fidelity SAI U.S. Treasury Bond Index Fund | CD1Member  
Holdings [Line Items]  
Percent of Net Asset Value 24.90%
Fidelity SAI U.S. Treasury Bond Index Fund | CD2Member  
Holdings [Line Items]  
Percent of Net Asset Value 16.90%
Fidelity SAI U.S. Treasury Bond Index Fund | CD3Member  
Holdings [Line Items]  
Percent of Net Asset Value 17.10%
Fidelity SAI U.S. Treasury Bond Index Fund | CD4Member  
Holdings [Line Items]  
Percent of Net Asset Value 24.10%
Fidelity SAI U.S. Treasury Bond Index Fund | CD5Member  
Holdings [Line Items]  
Percent of Net Asset Value 0.80%
Fidelity SAI U.S. Treasury Bond Index Fund | CD6Member  
Holdings [Line Items]  
Percent of Net Asset Value 3.60%
Fidelity SAI U.S. Treasury Bond Index Fund | AAUSTreasuryObligationsMember  
Holdings [Line Items]  
Percent of Net Asset Value 98.90%
Fidelity SAI U.S. Treasury Bond Index Fund | AAShortTermInvestmentsandNetOtherAssetsLiabilitiesMember  
Holdings [Line Items]  
Percent of Net Asset Value 1.10%
Fidelity SAI Long-Term Treasury Bond Index Fund | TH0Member  
Holdings [Line Items]  
Percent of Net Asset Value 99.20%
Fidelity SAI Long-Term Treasury Bond Index Fund | CD0Member  
Holdings [Line Items]  
Percent of Net Asset Value 9.60%
Fidelity SAI Long-Term Treasury Bond Index Fund | CD1Member  
Holdings [Line Items]  
Percent of Net Asset Value 30.10%
Fidelity SAI Long-Term Treasury Bond Index Fund | CD2Member  
Holdings [Line Items]  
Percent of Net Asset Value 24.20%
Fidelity SAI Long-Term Treasury Bond Index Fund | CD3Member  
Holdings [Line Items]  
Percent of Net Asset Value 35.30%
Fidelity SAI Long-Term Treasury Bond Index Fund | AAUSTreasuryObligationsMember  
Holdings [Line Items]  
Percent of Net Asset Value 99.20%
Fidelity SAI Long-Term Treasury Bond Index Fund | AAShortTermInvestmentsandNetOtherAssetsLiabilitiesMember  
Holdings [Line Items]  
Percent of Net Asset Value 0.80%
Fidelity Long-Term Treasury Bond Index Fund | TH0Member  
Holdings [Line Items]  
Percent of Net Asset Value 98.60%
Fidelity Long-Term Treasury Bond Index Fund | TH1Member  
Holdings [Line Items]  
Percent of Net Asset Value 0.70%
Fidelity Long-Term Treasury Bond Index Fund | CD0Member  
Holdings [Line Items]  
Percent of Net Asset Value 13.50%
Fidelity Long-Term Treasury Bond Index Fund | CD1Member  
Holdings [Line Items]  
Percent of Net Asset Value 19.50%
Fidelity Long-Term Treasury Bond Index Fund | CD2Member  
Holdings [Line Items]  
Percent of Net Asset Value 30.40%
Fidelity Long-Term Treasury Bond Index Fund | CD3Member  
Holdings [Line Items]  
Percent of Net Asset Value 34.50%
Fidelity Long-Term Treasury Bond Index Fund | CD4Member  
Holdings [Line Items]  
Percent of Net Asset Value 1.40%
Fidelity Long-Term Treasury Bond Index Fund | AAUSTreasuryObligationsMember  
Holdings [Line Items]  
Percent of Net Asset Value 99.30%
Fidelity Long-Term Treasury Bond Index Fund | AAShortTermInvestmentsandNetOtherAssetsLiabilitiesMember  
Holdings [Line Items]  
Percent of Net Asset Value 0.70%
Fidelity Intermediate Treasury Bond Index Fund | TH0Member  
Holdings [Line Items]  
Percent of Net Asset Value 99.10%
Fidelity Intermediate Treasury Bond Index Fund | TH1Member  
Holdings [Line Items]  
Percent of Net Asset Value 0.20%
Fidelity Intermediate Treasury Bond Index Fund | CD0Member  
Holdings [Line Items]  
Percent of Net Asset Value 4.50%
Fidelity Intermediate Treasury Bond Index Fund | CD1Member  
Holdings [Line Items]  
Percent of Net Asset Value 13.50%
Fidelity Intermediate Treasury Bond Index Fund | CD2Member  
Holdings [Line Items]  
Percent of Net Asset Value 6.80%
Fidelity Intermediate Treasury Bond Index Fund | CD3Member  
Holdings [Line Items]  
Percent of Net Asset Value 14.40%
Fidelity Intermediate Treasury Bond Index Fund | CD4Member  
Holdings [Line Items]  
Percent of Net Asset Value 59.90%
Fidelity Intermediate Treasury Bond Index Fund | CD5Member  
Holdings [Line Items]  
Percent of Net Asset Value 0.20%
Fidelity Intermediate Treasury Bond Index Fund | AAUSTreasuryObligationsMember  
Holdings [Line Items]  
Percent of Net Asset Value 99.30%
Fidelity Intermediate Treasury Bond Index Fund | AAShortTermInvestmentsandNetOtherAssetsLiabilitiesMember  
Holdings [Line Items]  
Percent of Net Asset Value 0.70%
Fidelity SAI Intermediate Treasury Bond Index Fund | TH0Member  
Holdings [Line Items]  
Percent of Net Asset Value 99.10%
Fidelity SAI Intermediate Treasury Bond Index Fund | TH1Member  
Holdings [Line Items]  
Percent of Net Asset Value 0.10%
Fidelity SAI Intermediate Treasury Bond Index Fund | CD0Member  
Holdings [Line Items]  
Percent of Net Asset Value 3.00%
Fidelity SAI Intermediate Treasury Bond Index Fund | CD1Member  
Holdings [Line Items]  
Percent of Net Asset Value 14.50%
Fidelity SAI Intermediate Treasury Bond Index Fund | CD2Member  
Holdings [Line Items]  
Percent of Net Asset Value 5.80%
Fidelity SAI Intermediate Treasury Bond Index Fund | CD3Member  
Holdings [Line Items]  
Percent of Net Asset Value 15.80%
Fidelity SAI Intermediate Treasury Bond Index Fund | CD4Member  
Holdings [Line Items]  
Percent of Net Asset Value 60.00%
Fidelity SAI Intermediate Treasury Bond Index Fund | CD5Member  
Holdings [Line Items]  
Percent of Net Asset Value 0.00%
Fidelity SAI Intermediate Treasury Bond Index Fund | CD6Member  
Holdings [Line Items]  
Percent of Net Asset Value 0.10%
Fidelity SAI Intermediate Treasury Bond Index Fund | AAUSTreasuryObligationsMember  
Holdings [Line Items]  
Percent of Net Asset Value 99.20%
Fidelity SAI Intermediate Treasury Bond Index Fund | AAShortTermInvestmentsandNetOtherAssetsLiabilitiesMember  
Holdings [Line Items]  
Percent of Net Asset Value 0.80%
Fidelity SAI Short-Term Treasury Bond Index Fund | TH0Member  
Holdings [Line Items]  
Percent of Net Asset Value 98.40%
Fidelity SAI Short-Term Treasury Bond Index Fund | TH1Member  
Holdings [Line Items]  
Percent of Net Asset Value 0.40%
Fidelity SAI Short-Term Treasury Bond Index Fund | CD0Member  
Holdings [Line Items]  
Percent of Net Asset Value 4.60%
Fidelity SAI Short-Term Treasury Bond Index Fund | CD1Member  
Holdings [Line Items]  
Percent of Net Asset Value 5.70%
Fidelity SAI Short-Term Treasury Bond Index Fund | CD2Member  
Holdings [Line Items]  
Percent of Net Asset Value 3.10%
Fidelity SAI Short-Term Treasury Bond Index Fund | CD3Member  
Holdings [Line Items]  
Percent of Net Asset Value 51.00%
Fidelity SAI Short-Term Treasury Bond Index Fund | CD4Member  
Holdings [Line Items]  
Percent of Net Asset Value 34.00%
Fidelity SAI Short-Term Treasury Bond Index Fund | CD5Member  
Holdings [Line Items]  
Percent of Net Asset Value 0.10%
Fidelity SAI Short-Term Treasury Bond Index Fund | CD6Member  
Holdings [Line Items]  
Percent of Net Asset Value 0.30%
Fidelity SAI Short-Term Treasury Bond Index Fund | AAUSTreasuryObligationsMember  
Holdings [Line Items]  
Percent of Net Asset Value 98.80%
Fidelity SAI Short-Term Treasury Bond Index Fund | AAShortTermInvestmentsandNetOtherAssetsLiabilitiesMember  
Holdings [Line Items]  
Percent of Net Asset Value 1.20%