Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Revenues	$ 2,695	$ 2,535
Net investment gains (losses)	31	12
Operating Segments | Specialty
Revenues	1,098	1,033
Operating Segments | Commercial
Revenues	960	900
Operating Segments | International
Revenues	265	250
Operating Segments | Life & Group
Revenues	335	335
Operating Segments | Corporate & Other
Revenues	9	6
Eliminations
Revenues	(3)	(1)
Total operating revenues
Revenues	2,664	2,523
Management & Professional Liability | Operating Segments | Specialty
Revenues	636	624
Surety | Operating Segments | Specialty
Revenues	139	129
Warranty & Alternative Risks | Operating Segments | Specialty
Revenues	323	280
Middle Market | Operating Segments | Commercial
Revenues	547	504
Small Business | Operating Segments | Commercial
Revenues	121	119
Other Commercial Insurance | Operating Segments | Commercial
Revenues	292	277
Canada | Operating Segments | International
Revenues	66	58
Europe | Operating Segments | International
Revenues	91	88
Hardy | Operating Segments | International
Revenues	$ 108	$ 104