Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues	$ 2,390	$ 2,433	$ 2,348	$ 2,195	$ 2,269	$ 2,153	$ 2,327	$ 2,352	$ 9,366	$ 9,101	$ 9,692
Specialty [Domain]
Revenues									3,662	3,579	3,708
Management & Professional Liability [Member]
Revenues									2,620	2,617	2,818
Surety [Member]
Revenues									532	502	509
Warranty & Alternative Risks [Member]
Revenues									510	460	381
Commercial [Domain]
Revenues									3,478	3,371	3,683
Middle Markets [Member]
Revenues									1,769	1,623	1,631
Small Business [Member]
Revenues									583	616	709
Other Commercial Insurance [Member]
Revenues									1,126	1,132	1,343
International [Domain]
Revenues									875	856	973
Canada [Member]
Revenues									203	214	273
CNA Europe [Member]
Revenues									319	309	335
Hardy [Member]
Revenues									353	333	365
Life and Group Non-Core [Member]
Revenues									1,326	1,258	1,279
Corporate and Other [Member]
Revenues									26	42	56
Eliminations [Member]
Revenues									$ (1)	$ (5)	$ (7)