Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Total revenues	$ 14,270	$ 13,299	$ 11,879
Total operating revenues	14,351	13,398	12,078
Net investment gains (losses)	(81)	(99)	(199)
Operating Segments | Specialty
Total revenues	5,598	5,490	5,209
Operating Segments | Commercial
Total revenues	5,920	5,221	4,441
Operating Segments | International
Total revenues	1,387	1,279	1,134
Operating Segments | Life & Group
Total revenues	1,377	1,346	1,276
Operating Segments | Corporate & Other
Total revenues	80	73	24
Eliminations
Total revenues	(11)	(11)	(6)
Management & Professional Liability | Operating Segments | Specialty
Total revenues	2,998	2,931	2,771
Surety | Operating Segments | Specialty
Total revenues	785	731	652
Warranty & Alternative Risks | Operating Segments | Specialty
Total revenues	1,815	1,828	1,786
Middle Market | Operating Segments | Commercial
Total revenues	1,775	1,696	1,532
Construction | Operating Segments | Commercial
Total revenues	1,991	1,678	1,421
Small Business | Operating Segments | Commercial
Total revenues	637	631	581
Other Commercial | Operating Segments | Commercial
Total revenues	1,517	1,216	907
Canada | Operating Segments | International
Total revenues	401	383	366
Europe | Operating Segments | International
Total revenues	593	532	466
Hardy | Operating Segments | International
Total revenues	$ 393	$ 364	$ 302