Business Segments (Revenues by Line of Business) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Revenues	$ 2,434	$ 2,466	$ 2,246	$ 2,401	$ 2,261	$ 2,175	$ 2,198	$ 2,315	$ 9,547	$ 8,949	$ 9,209
CNA Specialty [Domain]
Revenues									3,742	3,512	3,516
International - Specialty [Member]
Revenues									220	210	199
Professional & Management Liability [Member]
Revenues									2,723	2,541	2,551
Surety [Member]
Revenues									485	472	475
Warranty & Alternative Risks [Member]
Revenues									314	289	291
CNA Commercial [Domain]
Revenues									4,238	4,073	4,175
CNA Select Risk [Member]
Revenues									272	272	261
Commercial Insurance [Member]
Revenues									2,928	2,681	2,851
International - Commercial [Member]
Revenues									369	539	499
Small Business [Member]
Revenues									669	581	564
Hardy [Domain]
Revenues									123
Life & Group Non-Core [Domain]
Revenues									1,395	1,334	1,357
Health [Member]
Revenues									1,120	1,093	1,100
Life & Annuity [Member]
Revenues									239	229	249
Other
Revenues									36	12	8
Corporate and Other Non-Core revenues [Member]
Revenues									52	33	164
Eliminations [Member]
Revenues									$ (3)	$ (3)	$ (3)