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, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ 24	$ (5)	$ 49	$ (31)
CNA Commercial [Member]
Commercial Auto	20	2	40	(3)
General Liability	30	15	25	(6)
Workers' Compensation	40	45	50	70
Property and Other	(11)	(35)	(19)	(45)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ 79	$ 27	$ 96	$ 16