Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Revenues	$ 2,433	$ 2,153	$ 6,976	$ 6,832
Specialty [Domain]
Revenues	946	846	2,739	2,667
Management & Professional Liability [Member]
Revenues	677	605	1,954	1,976
Surety [Member]
Revenues	139	132	399	377
Warranty & Alternative Risks [Member]
Revenues	130	109	386	314
Commercial [Domain]
Revenues	913	767	2,591	2,545
Middle Markets [Member]
Revenues	463	386	1,298	1,207
Small Business [Member]
Revenues	154	144	448	467
Other Commercial Insurance [Member]
Revenues	296	237	845	871
International [Domain]
Revenues	229	216	658	642
Canada [Member]
Revenues	51	52	152	161
CNA Europe [Member]
Revenues	82	78	241	232
Hardy [Member]
Revenues	96	86	265	249
Life and Group Non-Core [Member]
Revenues	339	320	979	958
Corporate and Other Non-Core [Member]
Revenues	6	6	10	24
Eliminations [Member]
Revenues	$ 0	$ (2)	$ (1)	$ (4)