Business Segments (Revenues by Line of Business) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Revenues	$ 2,401	$ 2,315
CNA Specialty [Domain]
Revenues	945	891
International - Specialty [Member]
Revenues	57	51
Professional & Management Liability [Member]
Revenues	694	660
Surety [Member]
Revenues	119	111
Warranty & Alternative Risks [Member]
Revenues	75	69
CNA Commercial [Domain]
Revenues	1,088	1,094
CNA Select Risk [Member]
Revenues	73	71
Commercial Insurance [Member]
Revenues	762	752
International - Commercial [Member]
Revenues	91	127
Small Business [Member]
Revenues	162	144
Life & Group Non-Core [Domain]
Revenues	350	326
Health [Member]
Revenues	291	270
Life & Annuity [Member]
Revenues	60	56
Other - Life & Group [Member]
Revenues	(1)
Corporate and Other Non-Core [Member]
Revenues	18	5
Eliminations [Member]
Revenues		$ (1)