Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenues	$ 2,535	$ 2,330
Net realized investment gains (losses)	12	36
Operating Segments | Specialty
Revenues	1,033	896
Operating Segments | Commercial
Revenues	900	853
Operating Segments | International
Revenues	250	209
Operating Segments | Life & Group revenues
Revenues	335	331
Operating Segments | Corporate & Other revenues
Revenues	6	5
Eliminations
Revenues	(1)	0
Operating segments and eliminations, excluding segment reconciling Items
Revenues	2,523	2,294
Management & Professional Liability | Operating Segments | Specialty
Revenues	624	638
Surety | Operating Segments | Specialty
Revenues	129	124
Warranty & Alternative Risks | Operating Segments | Specialty
Revenues	280	134
Middle Market | Operating Segments | Commercial
Revenues	504	468
Small Business | Operating Segments | Commercial
Revenues	119	96
Other Commercial Insurance | Operating Segments | Commercial
Revenues	277	289
Canada | Operating Segments | International
Revenues	58	51
CNA Europe | Operating Segments | International
Revenues	88	72
Hardy | Operating Segments | International
Revenues	$ 104	$ 86