Business Segments (Revenues by Line of Business) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Revenues	$ 2,246	$ 2,198	$ 4,647	$ 4,513
CNA Specialty [Domain]
Revenues	896	875	1,841	1,766
International - Specialty [Member]
Revenues	53	54	110	105
Professional & Management Liability [Member]
Revenues	645	626	1,339	1,286
Surety [Member]
Revenues	121	119	240	230
Warranty & Alternative Risks [Member]
Revenues	77	76	152	145
CNA Commercial [Domain]
Revenues	984	984	2,072	2,078
CNA Select Risk [Member]
Revenues	66	67	139	138
Commercial Insurance [Member]
Revenues	670	641	1,432	1,393
International - Commercial [Member]
Revenues	90	130	181	257
Small Business [Member]
Revenues	158	146	320	290
Life & Group Non-Core [Domain]
Revenues	360	333	710	659
Health [Member]
Revenues	287	274	578	544
Life & Annuity [Member]
Revenues	57	59	117	115
Other - Life & Group [Member]
Revenues	16		15
Corporate and Other Non-Core [Member]
Revenues	7	6	25	11
Eliminations [Member]
Revenues	$ (1)		$ (1)	$ (1)