Claim and Claim Adjustment Expense Reserves (Net prior year claim and allocated claim adjustment expense reserve development for CNA commercial 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 Commercial [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development:
Commercial Auto	(98)	(88)	(9)
General Liability	(39)	(59)	(100)
Workers' Compensation	36	47	69
Property and Other	(103)	(204)	(190)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, P&C	$ (204)	$ (304)	$ (230)