v3.26.3
Segments of Operations - Components of underwriting profit, including significant segment expenses by segment and sub-segment (Details) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
Segment Reporting [Line Items]        
Net earned premiums $ 2,164 $ 2,013 $ 5,467 $ 5,240
Losses and loss adjustment expenses 1,470 1,355 3,376 3,327
Commissions and other underwriting expenses 571 529 1,687 1,593
Operating segments | Property and casualty insurance        
Segment Reporting [Line Items]        
Net earned premiums 2,164 2,013 5,467 5,240
Losses and loss adjustment expenses 1,470 1,355 3,376 3,327
Commissions and other underwriting expenses 564 520 1,663 1,568
Property and casualty insurance underwriting profit 130 138 428 345
Operating segments | Property and transportation | Property and casualty insurance        
Segment Reporting [Line Items]        
Net earned premiums 1,026 935 2,142 2,011
Losses and loss adjustment expenses 796 728 1,466 1,426
Commissions and other underwriting expenses 162 152 486 466
Property and casualty insurance underwriting profit 68 55 190 119
Operating segments | Specialty casualty | Property and casualty insurance        
Segment Reporting [Line Items]        
Net earned premiums 831 810 2,444 2,403
Losses and loss adjustment expenses 539 541 1,581 1,593
Commissions and other underwriting expenses 250 236 742 708
Property and casualty insurance underwriting profit 42 33 121 102
Operating segments | Specialty financial | Property and casualty insurance        
Segment Reporting [Line Items]        
Net earned premiums 307 268 881 826
Losses and loss adjustment expenses 132 85 324 306
Commissions and other underwriting expenses 152 132 435 394
Property and casualty insurance underwriting profit 23 51 122 126
Operating segments | Other lines | Property and casualty insurance        
Segment Reporting [Line Items]        
Losses and loss adjustment expenses 3 1 5 2
Property and casualty insurance underwriting profit $ (3) $ (1) $ (5) $ (2)