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, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (40)	$ (36)	$ (110)	$ (166)
CNA Specialty [Member]
Medical Professional Liability	9	(18)	(6)	(52)
Other Professional Liability	1	1	(1)	(20)
Surety	(60)	1	(59)	(2)
Warranty			(1)	(12)
Other	11	11	(13)	14
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (39)	$ (5)	$ (80)	$ (72)