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, 2012	Dec. 31, 2011	Dec. 31, 2010
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (205)	$ (423)	$ (637)
CNA Commercial [Member]
Commercial Auto	27	(98)	(88)
General Liability	(64)	(39)	(59)
Workers' Compensation	15	36	47
Property and Other	(24)	(103)	(204)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (46)	$ (204)	$ (304)