v3.26.1
Business Segments (Revenues by Line of Business) (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Segment Reporting, Revenue Reconciling Item [Line Items]    
Total revenues $ 3,677 $ 3,627
Total operating revenues 3,695 3,636
Net investment gains (losses) (18) (9)
Operating Segments | Specialty    
Segment Reporting, Revenue Reconciling Item [Line Items]    
Total revenues 1,369 1,379
Operating Segments | Commercial    
Segment Reporting, Revenue Reconciling Item [Line Items]    
Total revenues 1,611 1,565
Operating Segments | International    
Segment Reporting, Revenue Reconciling Item [Line Items]    
Total revenues 377 344
Operating Segments | Life & Group    
Segment Reporting, Revenue Reconciling Item [Line Items]    
Total revenues 327 332
Operating Segments | Corporate & Other    
Segment Reporting, Revenue Reconciling Item [Line Items]    
Total revenues 14 19
Eliminations    
Segment Reporting, Revenue Reconciling Item [Line Items]    
Total revenues (3) (3)
Management & Professional Liability | Operating Segments | Specialty    
Segment Reporting, Revenue Reconciling Item [Line Items]    
Total revenues 761 746
Surety | Operating Segments | Specialty    
Segment Reporting, Revenue Reconciling Item [Line Items]    
Total revenues 191 188
Warranty & Alternative Risks | Operating Segments | Specialty    
Segment Reporting, Revenue Reconciling Item [Line Items]    
Total revenues 417 445
Middle Markets | Operating Segments | Commercial    
Segment Reporting, Revenue Reconciling Item [Line Items]    
Total revenues 479 453
Construction | Operating Segments | Commercial    
Segment Reporting, Revenue Reconciling Item [Line Items]    
Total revenues 501 506
Small Business | Operating Segments | Commercial    
Segment Reporting, Revenue Reconciling Item [Line Items]    
Total revenues 151 156
Other Commercial | Operating Segments | Commercial    
Segment Reporting, Revenue Reconciling Item [Line Items]    
Total revenues 480 450
Canada | Operating Segments | International    
Segment Reporting, Revenue Reconciling Item [Line Items]    
Total revenues 107 94
Europe | Operating Segments | International    
Segment Reporting, Revenue Reconciling Item [Line Items]    
Total revenues 169 144
Hardy | Operating Segments | International    
Segment Reporting, Revenue Reconciling Item [Line Items]    
Total revenues $ 101 $ 106