Claim and Claim Adjustment Expense Reserves (Net prior year claim and allocated claim adjustment expense reserve development for CNA Specialty 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 Specialty [Member]
Medical Professional Liability	(32)	(92)	(98)
Other Professional Liability	(22)	(78)	(129)
Surety	(63)	(47)	(103)
Warranty	(5)	(13)
Other	(13)	13	(11)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (135)	$ (217)	$ (341)