Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Total revenues	$ 3,029	$ 2,766	$ 5,895	$ 5,057
Total operating revenues	2,991	2,697	5,800	5,204
Net investment gains (losses)	38	69	95	(147)
Operating Segments | Specialty
Total revenues	1,255	1,146	2,445	2,189
Operating Segments | Commercial
Total revenues	1,060	961	2,068	1,829
Operating Segments | International
Total revenues	281	238	547	492
Operating Segments | Life & Group
Total revenues	390	332	730	667
Operating Segments | Corporate & Other
Total revenues	6	21	13	30
Eliminations
Total revenues	(1)	(1)	(3)	(3)
Management & Professional Liability | Operating Segments | Specialty
Total revenues	693	645	1,360	1,213
Surety | Operating Segments | Specialty
Total revenues	155	153	297	291
Warranty & Alternative Risks | Operating Segments | Specialty
Total revenues	407	348	788	685
Middle Market | Operating Segments | Commercial
Total revenues	381	355	756	690
Construction | Operating Segments | Commercial
Total revenues	324	275	633	525
Small Business | Operating Segments | Commercial
Total revenues	143	114	269	226
Other Commercial | Operating Segments | Commercial
Total revenues	212	217	410	388
Canada | Operating Segments | International
Total revenues	87	68	166	141
Europe | Operating Segments | International
Total revenues	115	94	226	186
Hardy | Operating Segments | International
Total revenues	$ 79	$ 76	$ 155	$ 165