Business Segments (Revenues by Line of Business) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Revenues	$ 2,259	$ 2,175	$ 2,198	$ 2,315	$ 2,298	$ 2,363	$ 2,233	$ 2,315	$ 8,947	$ 9,209	$ 8,472
CNA Specialty [Domain]
Revenues									3,512	3,516	3,243
International - Specialty [Member]
Revenues									210	199	171
Professional & Management Liability [Member]
Revenues									2,541	2,551	2,339
Surety [Member]
Revenues									472	475	474
Warranty & Alternative Risks [Member]
Revenues									289	291	259
CNA Commercial [Domain]
Revenues									4,071	4,175	4,068
CNA Select Risk [Member]
Revenues									272	261	210
Commercial Insurance [Member]
Revenues									2,681	2,851	2,692
International - Commercial [Member]
Revenues									537	499	617
Small Business [Member]
Revenues									581	564	549
Life & Group Non-Core [Domain]
Revenues									1,334	1,357	1,035
Health [Member]
Revenues									1,093	1,100	811
Life & Annuity [Member]
Revenues									229	249	211
Other - Life & Group [Member]
Revenues									12	8	13
Corporate and Other Non-Core [Member]
Revenues									33	164	129
Eliminations [Member]
Revenues									$ (3)	$ (3)	$ (3)