Claim and Claim Adjustment Expense Reserves (Net prior year claim and allocated claim adjustment expense reserve development for CNA specialty segment) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development:
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, P&C	$ (423)	$ (637)	$ (289)
CNA Specialty [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development:
Medical Professional Liability	(92)	(98)	(62)
Other Professional Liability	(78)	(129)	(98)
Surety	(47)	(103)	(51)
Warranty	(13)
Other	13	(11)	(7)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, P&C	$ (217)	$ (341)	$ (218)