v3.26.3
Shareholder Report, Holdings (Details)
Aug. 31, 2026
Fidelity Short-Term Treasury Bond Index Fund | TH0Member  
Holdings [Line Items]  
Percent of Net Asset Value 99.20%
Fidelity Short-Term Treasury Bond Index Fund | CD0Member  
Holdings [Line Items]  
Percent of Net Asset Value 6.80%
Fidelity Short-Term Treasury Bond Index Fund | CD1Member  
Holdings [Line Items]  
Percent of Net Asset Value 11.60%
Fidelity Short-Term Treasury Bond Index Fund | CD2Member  
Holdings [Line Items]  
Percent of Net Asset Value 4.90%
Fidelity Short-Term Treasury Bond Index Fund | CD3Member  
Holdings [Line Items]  
Percent of Net Asset Value 45.30%
Fidelity Short-Term Treasury Bond Index Fund | CD4Member  
Holdings [Line Items]  
Percent of Net Asset Value 30.60%
Fidelity Short-Term Treasury Bond Index Fund | AAUSTreasuryObligationsMember  
Holdings [Line Items]  
Percent of Net Asset Value 99.20%
Fidelity Short-Term Treasury Bond Index Fund | AAShortTermInvestmentsandNetOtherAssetsLiabilitiesMember  
Holdings [Line Items]  
Percent of Net Asset Value 0.80%
Fidelity Series Long-Term Treasury Bond Index Fund | TH0Member  
Holdings [Line Items]  
Percent of Net Asset Value 98.30%
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 6.10%
Fidelity Series Long-Term Treasury Bond Index Fund | CD1Member  
Holdings [Line Items]  
Percent of Net Asset Value 28.30%
Fidelity Series Long-Term Treasury Bond Index Fund | CD2Member  
Holdings [Line Items]  
Percent of Net Asset Value 20.20%
Fidelity Series Long-Term Treasury Bond Index Fund | CD3Member  
Holdings [Line Items]  
Percent of Net Asset Value 41.30%
Fidelity Series Long-Term Treasury Bond Index Fund | CD4Member  
Holdings [Line Items]  
Percent of Net Asset Value 3.30%
Fidelity Series Long-Term Treasury Bond Index Fund | AAUSTreasuryObligationsMember  
Holdings [Line Items]  
Percent of Net Asset Value 99.20%
Fidelity Series Long-Term Treasury Bond Index Fund | AAShortTermInvestmentsandNetOtherAssetsLiabilitiesMember  
Holdings [Line Items]  
Percent of Net Asset Value 0.80%
Fidelity SAI U.S. Treasury Bond Index Fund | TH0Member  
Holdings [Line Items]  
Percent of Net Asset Value 75.10%
Fidelity SAI U.S. Treasury Bond Index Fund | TH1Member  
Holdings [Line Items]  
Percent of Net Asset Value 23.90%
Fidelity SAI U.S. Treasury Bond Index Fund | CD0Member  
Holdings [Line Items]  
Percent of Net Asset Value 6.20%
Fidelity SAI U.S. Treasury Bond Index Fund | CD1Member  
Holdings [Line Items]  
Percent of Net Asset Value 17.00%
Fidelity SAI U.S. Treasury Bond Index Fund | CD2Member  
Holdings [Line Items]  
Percent of Net Asset Value 15.30%
Fidelity SAI U.S. Treasury Bond Index Fund | CD3Member  
Holdings [Line Items]  
Percent of Net Asset Value 23.40%
Fidelity SAI U.S. Treasury Bond Index Fund | CD4Member  
Holdings [Line Items]  
Percent of Net Asset Value 32.20%
Fidelity SAI U.S. Treasury Bond Index Fund | CD5Member  
Holdings [Line Items]  
Percent of Net Asset Value 1.50%
Fidelity SAI U.S. Treasury Bond Index Fund | CD6Member  
Holdings [Line Items]  
Percent of Net Asset Value 3.40%
Fidelity SAI U.S. Treasury Bond Index Fund | AAUSTreasuryObligationsMember  
Holdings [Line Items]  
Percent of Net Asset Value 99.00%
Fidelity SAI U.S. Treasury Bond Index Fund | AAShortTermInvestmentsandNetOtherAssetsLiabilitiesMember  
Holdings [Line Items]  
Percent of Net Asset Value 1.00%
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 11.50%
Fidelity SAI Long-Term Treasury Bond Index Fund | CD1Member  
Holdings [Line Items]  
Percent of Net Asset Value 17.70%
Fidelity SAI Long-Term Treasury Bond Index Fund | CD2Member  
Holdings [Line Items]  
Percent of Net Asset Value 19.60%
Fidelity SAI Long-Term Treasury Bond Index Fund | CD3Member  
Holdings [Line Items]  
Percent of Net Asset Value 46.10%
Fidelity SAI Long-Term Treasury Bond Index Fund | CD4Member  
Holdings [Line Items]  
Percent of Net Asset Value 4.30%
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 97.80%
Fidelity Long-Term Treasury Bond Index Fund | TH1Member  
Holdings [Line Items]  
Percent of Net Asset Value 1.40%
Fidelity Long-Term Treasury Bond Index Fund | CD0Member  
Holdings [Line Items]  
Percent of Net Asset Value 11.60%
Fidelity Long-Term Treasury Bond Index Fund | CD1Member  
Holdings [Line Items]  
Percent of Net Asset Value 18.30%
Fidelity Long-Term Treasury Bond Index Fund | CD2Member  
Holdings [Line Items]  
Percent of Net Asset Value 24.00%
Fidelity Long-Term Treasury Bond Index Fund | CD3Member  
Holdings [Line Items]  
Percent of Net Asset Value 40.00%
Fidelity Long-Term Treasury Bond Index Fund | CD4Member  
Holdings [Line Items]  
Percent of Net Asset Value 5.30%
Fidelity Long-Term Treasury Bond Index Fund | AAUSTreasuryObligationsMember  
Holdings [Line Items]  
Percent of Net Asset Value 99.20%
Fidelity Long-Term Treasury Bond Index Fund | AAShortTermInvestmentsandNetOtherAssetsLiabilitiesMember  
Holdings [Line Items]  
Percent of Net Asset Value 0.80%
Fidelity Intermediate Treasury Bond Index Fund | TH0Member  
Holdings [Line Items]  
Percent of Net Asset Value 99.30%
Fidelity Intermediate Treasury Bond Index Fund | TH1Member  
Holdings [Line Items]  
Percent of Net Asset Value 0.00%
Fidelity Intermediate Treasury Bond Index Fund | CD0Member  
Holdings [Line Items]  
Percent of Net Asset Value 7.30%
Fidelity Intermediate Treasury Bond Index Fund | CD1Member  
Holdings [Line Items]  
Percent of Net Asset Value 5.60%
Fidelity Intermediate Treasury Bond Index Fund | CD2Member  
Holdings [Line Items]  
Percent of Net Asset Value 20.50%
Fidelity Intermediate Treasury Bond Index Fund | CD3Member  
Holdings [Line Items]  
Percent of Net Asset Value 65.90%
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 98.90%
Fidelity SAI Intermediate Treasury Bond Index Fund | CD0Member  
Holdings [Line Items]  
Percent of Net Asset Value 11.70%
Fidelity SAI Intermediate Treasury Bond Index Fund | CD1Member  
Holdings [Line Items]  
Percent of Net Asset Value 8.70%
Fidelity SAI Intermediate Treasury Bond Index Fund | CD2Member  
Holdings [Line Items]  
Percent of Net Asset Value 21.50%
Fidelity SAI Intermediate Treasury Bond Index Fund | CD3Member  
Holdings [Line Items]  
Percent of Net Asset Value 57.00%
Fidelity SAI Intermediate Treasury Bond Index Fund | AAUSTreasuryObligationsMember  
Holdings [Line Items]  
Percent of Net Asset Value 98.90%
Fidelity SAI Intermediate Treasury Bond Index Fund | AAShortTermInvestmentsandNetOtherAssetsLiabilitiesMember  
Holdings [Line Items]  
Percent of Net Asset Value 1.10%
Fidelity SAI Short-Term Treasury Bond Index Fund | TH0Member  
Holdings [Line Items]  
Percent of Net Asset Value 99.00%
Fidelity SAI Short-Term Treasury Bond Index Fund | TH1Member  
Holdings [Line Items]  
Percent of Net Asset Value 0.10%
Fidelity SAI Short-Term Treasury Bond Index Fund | CD0Member  
Holdings [Line Items]  
Percent of Net Asset Value 7.60%
Fidelity SAI Short-Term Treasury Bond Index Fund | CD1Member  
Holdings [Line Items]  
Percent of Net Asset Value 12.40%
Fidelity SAI Short-Term Treasury Bond Index Fund | CD2Member  
Holdings [Line Items]  
Percent of Net Asset Value 7.70%
Fidelity SAI Short-Term Treasury Bond Index Fund | CD3Member  
Holdings [Line Items]  
Percent of Net Asset Value 39.60%
Fidelity SAI Short-Term Treasury Bond Index Fund | CD4Member  
Holdings [Line Items]  
Percent of Net Asset Value 31.70%
Fidelity SAI Short-Term Treasury Bond Index Fund | CD5Member  
Holdings [Line Items]  
Percent of Net Asset Value 0.00%
Fidelity SAI Short-Term Treasury Bond Index Fund | CD6Member  
Holdings [Line Items]  
Percent of Net Asset Value 0.10%
Fidelity SAI Short-Term Treasury Bond Index Fund | AAUSTreasuryObligationsMember  
Holdings [Line Items]  
Percent of Net Asset Value 99.10%
Fidelity SAI Short-Term Treasury Bond Index Fund | AAShortTermInvestmentsandNetOtherAssetsLiabilitiesMember  
Holdings [Line Items]  
Percent of Net Asset Value 0.90%