Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Total revenues	$ 3,627	$ 3,444
Total operating revenues	3,636	3,466
Net investment gains (losses)	(9)	(22)
Operating Segments \| Specialty
Total revenues	1,379	1,372
Operating Segments \| Commercial
Total revenues	1,565	1,386
Operating Segments \| International
Total revenues	344	346
Operating Segments \| Life & Group
Total revenues	332	341
Operating Segments \| Corporate & Other
Total revenues	19	24
Eliminations
Total revenues	(3)	(3)
Management & Professional Liability \| Operating Segments \| Specialty
Total revenues	746	729
Surety \| Operating Segments \| Specialty
Total revenues	188	182
Warranty & Alternative Risks \| Operating Segments \| Specialty
Total revenues	445	461
Middle Market \| Operating Segments \| Commercial
Total revenues	466	432
Construction \| Operating Segments \| Commercial
Total revenues	510	455
Small Business \| Operating Segments \| Commercial
Total revenues	156	154
Other Commercial \| Operating Segments \| Commercial
Total revenues	433	345
Canada \| Operating Segments \| International
Total revenues	94	98
Europe \| Operating Segments \| International
Total revenues	144	143
Hardy \| Operating Segments \| International
Total revenues	$ 106	$ 105