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		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 Specialty [Member]
Medical Professional Liability	(17)	(9)	(20)	(15)
Other Professional Liability	(23)	(6)	(24)	(2)
Surety	1		2	1
Warranty				(1)
Other	(2)	(20)	(14)	(24)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (41)	$ (35)	$ (56)	$ (41)