Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Total revenues	$ 2,766	$ 2,610	$ 5,057	$ 5,305
Total operating revenues	2,697	2,628	5,204	5,292
Net investment gains (losses)	69	(18)	(147)	13
Operating Segments | Specialty
Total revenues	1,146	1,106	2,189	2,204
Operating Segments | Commercial
Total revenues	977	920	1,849	1,880
Operating Segments | International
Total revenues	238	259	492	524
Operating Segments | Life & Group
Total revenues	332	335	667	670
Operating Segments | Corporate & Other
Total revenues	5	9	10	18
Eliminations
Total revenues	(1)	(1)	(3)	(4)
Management & Professional Liability | Operating Segments | Specialty
Total revenues	645	629	1,213	1,265
Surety | Operating Segments | Specialty
Total revenues	153	151	291	290
Warranty & Alternative Risks | Operating Segments | Specialty
Total revenues	348	326	685	649
Middle Market | Operating Segments | Commercial
Total revenues	355	351	690	708
Construction | Operating Segments | Commercial
Total revenues	275	241	525	489
Small Business | Operating Segments | Commercial
Total revenues	114	123	226	254
Other Commercial | Operating Segments | Commercial
Total revenues	233	205	408	429
Canada | Operating Segments | International
Total revenues	68	68	141	134
Europe | Operating Segments | International
Total revenues	94	88	186	179
Hardy | Operating Segments | International
Total revenues	$ 76	$ 103	$ 165	$ 211