Business Segments (Revenues by Line of Business) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Revenues	$ 2,411	$ 2,458	$ 7,314	$ 7,363
CNA Specialty [Domain]
Revenues	987	996	2,959	2,906
International - Specialty [Member]
Revenues	57	59	183	177
Management & Professional Liability [Member]
Revenues	699	720	2,113	2,107
Surety [Member]
Revenues	133	126	380	365
Warranty & Alternative Risks [Member]
Revenues	98	91	283	257
CNA Commercial [Domain]
Revenues	985	1,062	3,053	3,267
Commercial Insurance [Member]
Revenues	720	776	2,237	2,429
International - Commercial [Member]
Revenues	95	93	281	276
Small Business [Member]
Revenues	170	193	535	562
Hardy [Domain]
Revenues	89	88	280	232
Life and Group Non-Core [Member]
Revenues	343	300	990	918
Corporate and Other Non-Core [Member]
Revenues	9	13	37	43
Eliminations [Member]
Revenues	$ (2)	$ (1)	$ (5)	$ (3)