Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Total revenues	$ 3,717	$ 3,519	$ 7,344	$ 6,963
Total operating revenues	3,763	3,529	7,399	6,995
Net investment gains (losses)	(46)	(10)	(55)	(32)
Operating Segments \| Specialty
Total revenues	1,429	1,388	2,808	2,760
Operating Segments \| Commercial
Total revenues	1,618	1,432	3,183	2,818
Operating Segments \| International
Total revenues	362	343	706	689
Operating Segments \| Life & Group
Total revenues	341	348	673	689
Operating Segments \| Corporate & Other
Total revenues	16	21	35	45
Eliminations
Total revenues	(3)	(3)	(6)	(6)
Management & Professional Liability \| Operating Segments \| Specialty
Total revenues	778	733	1,524	1,462
Surety \| Operating Segments \| Specialty
Total revenues	205	196	393	378
Warranty & Alternative Risks \| Operating Segments \| Specialty
Total revenues	446	459	891	920
Middle Market \| Operating Segments \| Commercial
Total revenues	481	427	947	859
Construction \| Operating Segments \| Commercial
Total revenues	517	483	1,027	938
Small Business \| Operating Segments \| Commercial
Total revenues	161	157	317	311
Other Commercial \| Operating Segments \| Commercial
Total revenues	459	365	892	710
Canada \| Operating Segments \| International
Total revenues	104	99	198	197
Europe \| Operating Segments \| International
Total revenues	156	147	300	290
Hardy \| Operating Segments \| International
Total revenues	$ 102	$ 97	$ 208	$ 202