v3.26.1
Segment Information - Operating Results (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Segment Reporting Information [Line Items]    
Net premiums earned $ 20,968 $ 19,409
Fees and other revenues 297 287
Total underwriting revenue 21,265 19,696
Losses (excluding catastrophe losses) 11,677 10,668
Catastrophe losses 268 459
Loss adjustment expenses 1,882 1,677
Losses and loss adjustment expenses 13,827 12,804
Distribution expenses 2,966 2,634
Other underwriting expenses 1,620 1,541
Total underwriting expenses 4,586 4,175
Pretax underwriting profit (loss) 2,852 2,717
Investment profit (loss) 789 595
Service businesses profit (loss) (5) (6)
Interest expense (70) (70)
Income before income taxes 3,566 3,236
Operating Segments | Personal Lines | Underwriting operations    
Segment Reporting Information [Line Items]    
Net premiums earned 18,384 16,710
Fees and other revenues 276 249
Total underwriting revenue 18,660 16,959
Losses (excluding catastrophe losses) 10,219 9,109
Catastrophe losses 263 454
Loss adjustment expenses 1,593 1,390
Losses and loss adjustment expenses 12,075 10,953
Distribution expenses 2,664 2,348
Other underwriting expenses 1,346 1,275
Total underwriting expenses 4,010 3,623
Pretax underwriting profit (loss) 2,575 2,383
Operating Segments | Commercial Lines | Underwriting operations    
Segment Reporting Information [Line Items]    
Net premiums earned 2,583 2,699
Fees and other revenues 20 38
Total underwriting revenue 2,603 2,737
Losses (excluding catastrophe losses) 1,457 1,559
Catastrophe losses 5 5
Loss adjustment expenses 289 287
Losses and loss adjustment expenses 1,751 1,851
Distribution expenses 300 286
Other underwriting expenses 268 262
Total underwriting expenses 568 548
Pretax underwriting profit (loss) 284 338
Operating Segments | Other Indemnity | Underwriting operations    
Segment Reporting Information [Line Items]    
Net premiums earned 1 0
Fees and other revenues 1 0
Total underwriting revenue 2 0
Losses (excluding catastrophe losses) 1 0
Catastrophe losses 0 0
Loss adjustment expenses 0 0
Losses and loss adjustment expenses 1 0
Distribution expenses 2 0
Other underwriting expenses 6 4
Total underwriting expenses 8 4
Pretax underwriting profit (loss) $ (7) $ (4)