Claim And Claim Adjustment Expense Reserves (Net Prior Year Claim And Allocated Claim Adjustment For CNA Specialty Segment) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Claim And Claim Adjustment Expense Reserves [Line Items]
Total pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	$ (423)	$ (637)	$ (444)
CNA Specialty [Member]
Claim And Claim Adjustment Expense Reserves [Line Items]
Medical professional liability	(92)	(98)	(62)
Other professional liability	(78)	(129)	(98)
Surety	(47)	(103)	(51)
Warranty	(13)
Other	13	(11)	(7)
Total pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	$ (217)	$ (341)	$ (218)