Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Revenues	$ 2,153	$ 2,411	$ 6,832	$ 7,314
Specialty [Domain]
Revenues	846	930	2,667	2,776
Management & Professional Liability [Member]
Revenues	605	699	1,976	2,113
Surety [Member]
Revenues	132	133	377	380
Warranty & Alternative Risks [Member]
Revenues	109	98	314	283
Commercial [Domain]
Revenues	767	890	2,545	2,772
Middle Markets [Member]
Revenues	386	404	1,207	1,219
Small Business [Member]
Revenues	144	170	467	535
Other Commercial Insurance [Member]
Revenues	237	316	871	1,018
International [Domain]
Revenues	216	241	642	744
Canada [Member]
Revenues	52	69	161	210
CNA Europe [Member]
Revenues	78	83	232	254
Hardy [Member]
Revenues	86	89	249	280
Life & Group Non-Core [Domain]
Revenues	320	343	958	990
Corporate and Other [Domain]
Revenues	6	9	24	37
Intersegment Eliminations [Domain]
Revenues	$ (2)	$ (2)	$ (4)	$ (5)