Business Segments (Revenues by Line of Business) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Revenues	$ 2,348	$ 2,327	$ 4,543	$ 4,679
Specialty [Domain]
Revenues	928	904	1,793	1,821
Management & Professional Liability [Member]
Revenues	659	674	1,277	1,371
Surety [Member]
Revenues	133	125	260	245
Warranty & Alternative Risks [Member]
Revenues	136	105	256	205
Commercial [Domain]
Revenues	876	883	1,678	1,778
Middle Markets [Member]
Revenues	434	412	835	821
Small Business [Member]
Revenues	151	158	294	323
Other Commercial Insurance [Member]
Revenues	291	313	549	634
International [Domain]
Revenues	214	220	429	426
Canada [Member]
Revenues	51	54	101	109
CNA Europe [Member]
Revenues	81	77	159	154
Hardy [Member]
Revenues	82	89	169	163
Life and Group Non-Core [Member]
Revenues	325	311	640	638
Corporate and Other Non-Core [Member]
Revenues	6	10	4	18
Eliminations [Member]
Revenues	$ (1)	$ (1)	$ (1)	$ (2)