Claim And Claim Adjustment Expense Reserves (Net Prior Year Claim And Allocated Claim Adjustment For CNA Commercial 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 Commercial [Member]
Claim And Claim Adjustment Expense Reserves [Line Items]
Commercial auto	(98)	(88)	(9)
General liability	(39)	(59)	(100)
Workers' compensation	36	47	69
Property and other	(103)	(204)	(190)
Total pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	$ (204)	$ (304)	$ (230)