Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Total revenues	$ 11,908	$ 10,808	$ 10,767
Total operating revenues	11,788	10,862	10,738
Net investment gains (losses)	120	(54)	29
Operating Segments | Specialty
Total revenues	5,004	4,585	4,491
Operating Segments | Commercial
Total revenues	4,199	3,861	3,796
Operating Segments | International
Total revenues	1,114	998	1,037
Operating Segments | Life & Group
Total revenues	1,457	1,355	1,340
Operating Segments | Corporate & Other
Total revenues	21	69	81
Eliminations
Total revenues	(7)	(6)	(7)
Management & Professional Liability | Operating Segments | Specialty
Total revenues	2,776	2,577	2,572
Surety | Operating Segments | Specialty
Total revenues	604	596	596
Warranty & Alternative Risks | Operating Segments | Specialty
Total revenues	1,624	1,412	1,323
Middle Market | Operating Segments | Commercial
Total revenues	1,508	1,444	1,439
Construction | Operating Segments | Commercial
Total revenues	1,322	1,120	1,043
Small Business | Operating Segments | Commercial
Total revenues	558	482	504
Other Commercial | Operating Segments | Commercial
Total revenues	811	815	810
Canada | Operating Segments | International
Total revenues	344	291	277
Europe | Operating Segments | International
Total revenues	473	389	363
Hardy | Operating Segments | International
Total revenues	$ 297	$ 318	$ 397