Claim And Claim Adjustment Expense Reserves (Net Prior Year Claim And Allocated Claim Adjustment Expense Reserve Development For CNA Commercial Segment) (Details) (CNA Commercial [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
CNA Commercial [Member]
Claim And Claim Adjustment Expense Reserves [Line Items]
Commercial auto		$ 10
General liability	8	22
Workers' compensation	(19)	8
Property and other	(3)	(47)
Total pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	$ (14)	$ (7)