Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Total revenues	$ 2,291	$ 2,695
Total operating revenues	2,507	2,664
Net investment gains (losses)	(216)	31
Operating Segments | Specialty
Total revenues	1,043	1,098
Operating Segments | Commercial
Total revenues	872	960
Operating Segments | International
Total revenues	254	265
Operating Segments | Life & Group
Total revenues	335	335
Operating Segments | Corporate & Other
Total revenues	5	9
Eliminations
Total revenues	(2)	(3)
Management & Professional Liability | Operating Segments | Specialty
Total revenues	568	636
Surety | Operating Segments | Specialty
Total revenues	138	139
Warranty & Alternative Risks | Operating Segments | Specialty
Total revenues	337	323
Middle Market | Operating Segments | Commercial
Total revenues	335	357
Construction | Operating Segments | Commercial
Total revenues	250	248
Small Business | Operating Segments | Commercial
Total revenues	112	131
Other Commercial | Operating Segments | Commercial
Total revenues	175	224
Canada | Operating Segments | International
Total revenues	73	66
Europe | Operating Segments | International
Total revenues	92	91
Hardy | Operating Segments | International
Total revenues	$ 89	$ 108