Business Segments (Revenues by Line of Business) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenues	$ 2,378	$ 2,411	$ 2,440	$ 2,463	$ 2,569	$ 2,458	$ 2,450	$ 2,455	$ 9,692	$ 9,932	$ 9,364
Specialty [Domain]
Revenues									3,708	3,676	3,522
Management & Professional Liability [Member]
Revenues									2,818	2,836	2,723
Surety [Member]
Revenues									509	490	485
Warranty & Alternative Risks [Member]
Revenues									381	350	314
Commercial [Domain]
Revenues									3,683	3,984	3,869
Middle Markets [Member]
Revenues									1,631	1,642	1,560
Small Business [Member]
Revenues									709	754	669
Other Commercial Insurance [Member]
Revenues									1,343	1,588	1,640
International [Domain]
Revenues									973	981	712
Canada [Member]
Revenues									273	289	276
CNA Europe [Member]
Revenues									335	326	313
Hardy [Member]
Revenues									365	366	123
Life and Group Non-Core [Member]
Revenues									1,279	1,243	1,212
Corporate and Other Non-Core [Member]
Revenues									56	52	52
Eliminations [Member]
Revenues									$ (7)	$ (4)	$ (3)