Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Revenues	$ 2,195	$ 2,352
Specialty [Domain]
Revenues	865	917
Management & Professional Liability [Member]
Revenues	618	697
Surety [Member]
Revenues	127	120
Warranty & Alternative Risks [Member]
Revenues	120	100
Commercial [Domain]
Revenues	802	895
Middle Markets [Member]
Revenues	401	409
Small Business [Member]
Revenues	143	165
Other Commercial Insurance [Member]
Revenues	258	321
International [Domain]
Revenues	215	206
Canada [Member]
Revenues	50	55
CNA Europe [Member]
Revenues	78	77
Hardy [Member]
Revenues	87	74
Life and Group Non-Core [Member]
Revenues	315	327
Corporate and Other Non-Core [Member]
Revenues	(2)	8
Eliminations [Member]
Revenues	$ 0	$ (1)