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	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (81)	$ (40)	$ (112)	$ (110)
CNA Specialty [Member]
Medical Professional Liability	9	9	(11)	(6)
Other Professional Liability	(4)	1	(28)	(1)
Surety	(76)	(60)	(74)	(59)
Warranty				(1)
Other	(3)	11	(17)	(13)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (74)	$ (39)	$ (130)	$ (80)