Business Segments (Revenues by Line of Business) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Revenues	$ 2,504	$ 2,466	$ 7,500	$ 7,113
CNA Specialty [Domain]
Revenues	996	957	2,906	2,798
International - Specialty [Member]
Revenues	59	53	177	163
Professional & Management Liability [Member]
Revenues	720	700	2,107	2,039
Surety [Member]
Revenues	126	124	365	364
Warranty & Alternative Risks [Member]
Revenues	91	80	257	232
CNA Commercial [Domain]
Revenues	1,062	1,091	3,267	3,163
Commercial Insurance [Member]
Revenues	776	826	2,429	2,396
International - Commercial [Member]
Revenues	93	93	276	274
Small Business [Member]
Revenues	193	172	562	493
Hardy [Domain]
Revenues	88	65	232	65
Life & Group Non-Core [Domain]
Revenues	346	340	1,055	1,050
Health [Member]
Revenues	290	274	875	852
Life & Annuity [Member]
Revenues	58	64	177	181
Other
Revenues	(2)	2	3	17
Corporate and Other Non-Core revenues [Member]
Revenues	13	14	43	39
Eliminations [Member]
Revenues	$ (1)	$ (1)	$ (3)	$ (2)