Business Segments (Revenues by Line of Business) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Revenues	$ 2,503	$ 2,401
CNA Specialty [Domain]
Revenues	956	945
International - Specialty [Member]
Revenues	59	57
Professional & Management Liability [Member]
Revenues	700	694
Surety [Member]
Revenues	116	119
Warranty & Alternative Risks [Member]
Revenues	81	75
CNA Commercial [Domain]
Revenues	1,101	1,088
Commercial Insurance [Member]
Revenues	826	835
International - Commercial [Member]
Revenues	93	91
Small Business [Member]
Revenues	182	162
Hardy [Domain]
Revenues	67
Life & Group Non-Core [Domain]
Revenues	363	350
Health [Member]
Revenues	298	291
Life & Annuity [Member]
Revenues	61	60
Other
Revenues	4	(1)
Corporate and Other Non-Core revenues [Member]
Revenues	17	18
Eliminations [Member]
Revenues	$ (1)