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	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 Commercial [Member]
Commercial Auto	9	(2)	11	(36)
General Liability	(21)	4	(26)	26
Workers' Compensation	24	3	13	39
Property and Other	(10)	(47)	(23)	(128)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ 2	$ (42)	$ (25)	$ (99)