Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Total revenues	$ 2,866	$ 2,291
Total operating revenues	2,809	2,507
Net investment gains (losses)	57	(216)
Operating Segments | Specialty
Total revenues	1,190	1,043
Operating Segments | Commercial
Total revenues	1,008	868
Operating Segments | International
Total revenues	266	254
Operating Segments | Life & Group
Total revenues	340	335
Operating Segments | Corporate & Other
Total revenues	7	9
Eliminations
Total revenues	(2)	(2)
Management & Professional Liability | Operating Segments | Specialty
Total revenues	667	568
Surety | Operating Segments | Specialty
Total revenues	142	138
Warranty & Alternative Risks | Operating Segments | Specialty
Total revenues	381	337
Middle Market | Operating Segments | Commercial
Total revenues	375	335
Construction | Operating Segments | Commercial
Total revenues	309	250
Small Business | Operating Segments | Commercial
Total revenues	126	112
Other Commercial | Operating Segments | Commercial
Total revenues	198	171
Canada | Operating Segments | International
Total revenues	79	73
Europe | Operating Segments | International
Total revenues	111	92
Hardy | Operating Segments | International
Total revenues	$ 76	$ 89