Business Segments (Revenues by line of business) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Total revenues	$ 2,885	$ 2,866
Total operating revenues	2,896	2,809
Net investment gains (losses)	(11)	57
Operating Segments | Specialty
Total revenues	1,258	1,190
Operating Segments | Commercial
Total revenues	1,030	1,008
Operating Segments | International
Total revenues	277	266
Operating Segments | Life & Group
Total revenues	331	340
Operating Segments | Corporate & Other
Total revenues	2	7
Eliminations
Total revenues	(2)	(2)
Management & Professional Liability | Operating Segments | Specialty
Total revenues	673	667
Surety | Operating Segments | Specialty
Total revenues	148	142
Warranty & Alternative Risks | Operating Segments | Specialty
Total revenues	437	381
Middle Market | Operating Segments | Commercial
Total revenues	362	375
Construction | Operating Segments | Commercial
Total revenues	324	309
Small Business | Operating Segments | Commercial
Total revenues	138	126
Other Commercial | Operating Segments | Commercial
Total revenues	206	198
Canada | Operating Segments | International
Total revenues	88	79
Europe | Operating Segments | International
Total revenues	120	111
Hardy | Operating Segments | International
Total revenues	$ 69	$ 76