v3.26.1
Segment Information - Operating Results (Detail) - 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 premiums earned $ 21,573 $ 20,310 $ 42,541 $ 39,719
Fees and other revenues 305 303 602 590
Total underwriting revenue 21,878 20,613 43,143 40,309
Losses (excluding catastrophe losses) 11,897 11,134 23,574 21,802
Catastrophe losses 773 707 1,041 1,166
Loss adjustment expenses 1,902 1,764 3,784 3,441
Losses and loss adjustment expenses 14,572 13,605 28,399 26,409
Distribution expenses 2,855 2,637 5,821 5,271
Other underwriting expenses 1,721 1,563 3,341 3,104
Total underwriting expenses 4,576 4,200 9,162 8,375
Pretax underwriting profit (loss) 2,730 2,808 5,582 5,525
Investment profit (loss) 1,573 1,249 2,362 1,844
Service businesses profit (loss) (6) (6) (11) (12)
Interest expense (88) (69) (158) (139)
Income before income taxes 4,209 3,982 7,775 7,218
Operating Segments | Personal Lines | Underwriting operations        
Segment Reporting [Line Items]        
Net premiums earned 18,880 17,544 37,264 34,254
Fees and other revenues 288 263 564 512
Total underwriting revenue 19,168 17,807 37,828 34,766
Losses (excluding catastrophe losses) 10,501 9,574 20,720 18,683
Catastrophe losses 759 688 1,022 1,142
Loss adjustment expenses 1,600 1,471 3,193 2,861
Losses and loss adjustment expenses 12,860 11,733 24,935 22,686
Distribution expenses 2,536 2,328 5,200 4,676
Other underwriting expenses 1,426 1,298 2,772 2,573
Total underwriting expenses 3,962 3,626 7,972 7,249
Pretax underwriting profit (loss) 2,346 2,448 4,921 4,831
Operating Segments | Commercial Lines | Underwriting operations        
Segment Reporting [Line Items]        
Net premiums earned 2,691 2,765 5,274 5,464
Fees and other revenues 18 40 38 78
Total underwriting revenue 2,709 2,805 5,312 5,542
Losses (excluding catastrophe losses) 1,390 1,561 2,847 3,120
Catastrophe losses 14 19 19 24
Loss adjustment expenses 302 292 591 579
Losses and loss adjustment expenses 1,706 1,872 3,457 3,723
Distribution expenses 316 308 616 594
Other underwriting expenses 290 261 558 523
Total underwriting expenses 606 569 1,174 1,117
Pretax underwriting profit (loss) 397 364 681 702
Operating Segments | Other Indemnity | Underwriting operations        
Segment Reporting [Line Items]        
Net premiums earned 2 1 3 1
Fees and other revenues (1) 0 0 0
Total underwriting revenue 1 1 3 1
Losses (excluding catastrophe losses) 6 (1) 7 (1)
Catastrophe losses 0 0 0 0
Loss adjustment expenses 0 1 0 1
Losses and loss adjustment expenses 6 0 7 0
Distribution expenses 3 1 5 1
Other underwriting expenses 5 4 11 8
Total underwriting expenses 8 5 16 9
Pretax underwriting profit (loss) $ (13) $ (4) $ (20) $ (8)