v3.26.1
Segments of Operations - Components of underwriting profit, including significant segment expenses by segment and sub-segment (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Segment Reporting [Line Items]        
Net earned premiums $ 1,694 $ 1,647 $ 3,303 $ 3,227
Losses and loss adjustment expenses 1,000 1,007 1,906 1,972
Commissions and other underwriting expenses 560 534 1,116 1,064
Operating segments | Property and casualty insurance        
Segment Reporting [Line Items]        
Net earned premiums 1,694 1,647 3,303 3,227
Losses and loss adjustment expenses 1,000 1,007 1,906 1,972
Commissions and other underwriting expenses 552 527 1,099 1,048
Property and casualty insurance underwriting profit 142 113 298 207
Operating segments | Property and transportation | Property and casualty insurance        
Segment Reporting [Line Items]        
Net earned premiums 590 576 1,116 1,076
Losses and loss adjustment expenses 369 387 670 698
Commissions and other underwriting expenses 164 162 324 314
Property and casualty insurance underwriting profit 57 27 122 64
Operating segments | Specialty casualty | Property and casualty insurance        
Segment Reporting [Line Items]        
Net earned premiums 814 799 1,613 1,593
Losses and loss adjustment expenses 525 516 1,042 1,052
Commissions and other underwriting expenses 244 234 492 472
Property and casualty insurance underwriting profit 45 49 79 69
Operating segments | Specialty financial | Property and casualty insurance        
Segment Reporting [Line Items]        
Net earned premiums 290 272 574 558
Losses and loss adjustment expenses 104 103 192 221
Commissions and other underwriting expenses 144 131 283 262
Property and casualty insurance underwriting profit 42 38 99 75
Operating segments | Other lines | Property and casualty insurance        
Segment Reporting [Line Items]        
Losses and loss adjustment expenses 2 1 2 1
Property and casualty insurance underwriting profit $ (2) $ (1) $ (2) $ (1)