Business Segments (Revenues by Line of Business) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Revenues	$ 2,613	$ 2,504	$ 2,493	$ 2,503	$ 2,434	$ 2,466	$ 2,246	$ 2,401	$ 10,113	$ 9,547	$ 8,949
CNA Specialty [Domain]
Revenues									3,915	3,742	3,512
International - Specialty [Member]
Revenues									239	220	210
Management & Professional Liability [Member]
Revenues									2,836	2,723	2,541
Surety [Member]
Revenues									490	485	472
Warranty & Alternative Risks [Member]
Revenues									350	314	289
CNA Commercial [Domain]
Revenues									4,360	4,238	4,073
Commercial Insurance [Member]
Revenues									3,230	3,200	2,953
International - Commercial [Member]
Revenues									376	369	539
Small Business [Member]
Revenues									754	669	581
Hardy [Domain]
Revenues									366	123
Life & Group Non-Core [Domain]
Revenues									1,424	1,395	1,334
Health [Member]
Revenues									1,192	1,120	1,093
Life & Annuity [Member]
Revenues									232	239	229
Other [Member]
Revenues										36	12
Corporate and Other Non-Core [Member]
Revenues									52	52	33
Eliminations [Member]
Revenues									$ (4)	$ (3)	$ (3)