Business Segments (Revenues by Line of Business) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Revenues	$ 2,466	$ 2,175	$ 7,113	$ 6,688
CNA Specialty [Domain]
Revenues	957	874	2,798	2,640
International - Specialty [Member]
Revenues	53	54	163	159
Professional & Management Liability [Member]
Revenues	700	623	2,039	1,909
Surety [Member]
Revenues	124	123	364	353
Warranty & Alternative Risks [Member]
Revenues	80	74	232	219
CNA Commercial [Domain]
Revenues	1,091	962	3,163	3,040
CNA Select Risk [Member]
Revenues	70	65	208	203
Commercial Insurance [Member]
Revenues	756	622	2,188	2,015
International - Commercial [Member]
Revenues	93	133	274	390
Small Business [Member]
Revenues	172	142	493	432
Hardy [Domain]
Revenues	65		65
Life & Group Non-Core [Domain]
Revenues	340	334	1,050	993
Health [Member]
Revenues	274	274	852	818
Life & Annuity [Member]
Revenues	64	55	181	170
Other - Life & Group [Member]
Revenues	2	5	17	5
Corporate and Other Non-Core [Member]
Revenues	14	6	39	17
Eliminations [Member]
Revenues	$ (1)	$ (1)	$ (2)	$ (2)