Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Revenues	$ 2,327	$ 2,440	$ 4,679	$ 4,903
Specialty [Domain]
Revenues	904	931	1,821	1,846
Management & Professional Liability [Member]
Revenues	674	710	1,371	1,414
Surety [Member]
Revenues	125	125	245	247
Warranty & Alternative Risks [Member]
Revenues	105	96	205	185
Commercial [Domain]
Revenues	883	936	1,778	1,882
Middle Markets [Member]
Revenues	412	412	821	815
Small Business [Member]
Revenues	158	181	323	365
Other Commercial Insurance [Member]
Revenues	313	343	634	702
International [Domain]
Revenues	220	244	426	503
Canada [Member]
Revenues	54	70	109	141
CNA Europe [Member]
Revenues	77	83	154	171
Hardy [Member]
Revenues	89	91	163	191
Life & Group Non-Core [Domain]
Revenues	311	316	638	647
Corporate and Other [Domain]
Revenues	10	15	18	28
Intersegment Eliminations [Domain]
Revenues	$ (1)	$ (2)	$ (2)	$ (3)