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, 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 Commercial [Member]
Commercial Auto	4	9	1	11
General Liability	(18)	(21)	(24)	(26)
Workers' Compensation	26	24	96	13
Property and Other	(15)	(10)	(60)	(23)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (3)	$ 2	$ 13	$ (25)