Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Total revenues	$ 3,618	$ 3,336	$ 10,581	$ 9,792
Total operating revenues	3,628	3,374	10,623	9,897
Net investment gains (losses)	(10)	(38)	(42)	(105)
Operating Segments | Specialty
Total revenues	1,407	1,372	4,167	4,066
Operating Segments | Commercial
Total revenues	1,513	1,335	4,331	3,828
Operating Segments | International
Total revenues	343	321	1,032	962
Operating Segments | Life & Group
Total revenues	350	328	1,039	999
Operating Segments | Corporate & Other
Total revenues	17	21	62	50
Eliminations
Total revenues	(2)	(3)	(8)	(8)
Management & Professional Liability | Operating Segments | Specialty
Total revenues	753	723	2,215	2,146
Surety | Operating Segments | Specialty
Total revenues	202	189	580	540
Warranty & Alternative Risks | Operating Segments | Specialty
Total revenues	452	460	1,372	1,380
Middle Market | Operating Segments | Commercial
Total revenues	448	437	1,307	1,254
Construction | Operating Segments | Commercial
Total revenues	521	444	1,459	1,249
Small Business | Operating Segments | Commercial
Total revenues	161	160	472	470
Other Commercial | Operating Segments | Commercial
Total revenues	383	294	1,093	855
Canada | Operating Segments | International
Total revenues	99	98	296	287
Europe | Operating Segments | International
Total revenues	150	139	440	401
Hardy | Operating Segments | International
Total revenues	$ 94	$ 84	$ 296	$ 274