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		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (5)	$ (52)	$ (31)	$ (70)
CNA Commercial [Member]
Commercial Auto	2	2	(3)	2
General Liability	15	(13)	(6)	(5)
Workers' Compensation	45	8	70	(11)
Property and Other	(35)	(10)	(45)	(13)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ 27	$ (13)	$ 16	$ (27)