Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues	$ 2,269	$ 2,153	$ 2,327	$ 2,352	$ 2,378	$ 2,411	$ 2,440	$ 2,463	$ 9,101	$ 9,692	$ 9,932
Specialty [Domain]
Revenues									3,579	3,708	3,676
Management & Professional Liability [Member]
Revenues									2,617	2,818	2,836
Surety [Member]
Revenues									502	509	490
Warranty & Alternative Risks [Member]
Revenues									460	381	350
Commercial [Domain]
Revenues									3,371	3,683	3,984
Middle Markets [Member]
Revenues									1,623	1,631	1,642
Small Business [Member]
Revenues									616	709	754
Other Commercial Insurance [Member]
Revenues									1,132	1,343	1,588
International [Domain]
Revenues									856	973	981
Canada [Member]
Revenues									214	273	289
CNA Europe [Member]
Revenues									309	335	326
Hardy [Member]
Revenues									333	365	366
Life and Group Non-Core [Member]
Revenues									1,258	1,279	1,243
Corporate and Other Non-Core [Member]
Revenues									42	56	52
Eliminations [Member]
Revenues									$ (5)	$ (7)	$ (4)