Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	$ 3,817	$ 3,618	$ 11,161	$ 10,581
Total operating revenues	3,824	3,628	11,223	10,623
Net investment gains (losses)	(7)	(10)	(62)	(42)
Operating Segments | Specialty
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	1,437	1,407	4,245	4,167
Operating Segments | Commercial
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	1,653	1,513	4,836	4,331
Operating Segments | International
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	386	343	1,092	1,032
Operating Segments | Life & Group
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	332	350	1,005	1,039
Operating Segments | Corporate & Other
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	18	17	53	62
Eliminations
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	(2)	(2)	(8)	(8)
Management & Professional Liability | Operating Segments | Specialty
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	773	753	2,297	2,215
Surety | Operating Segments | Specialty
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	225	202	618	580
Warranty & Alternative Risks | Operating Segments | Specialty
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	439	452	1,330	1,372
Middle Market | Operating Segments | Commercial
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	483	448	1,430	1,307
Construction | Operating Segments | Commercial
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	535	521	1,562	1,459
Small Business | Operating Segments | Commercial
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	160	161	477	472
Other Commercial | Operating Segments | Commercial
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	475	383	1,367	1,093
Canada | Operating Segments | International
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	114	99	312	296
Europe | Operating Segments | International
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	165	150	465	440
Hardy | Operating Segments | International
Segment Reporting, Revenue Reconciling Item [Line Items]
Total revenues	$ 107	$ 94	$ 315	$ 296