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, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development:
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (52)	$ (111)	$ (70)	$ (130)
CNA Specialty [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development:
Medical Professional Liability	(9)	(20)	(15)	(34)
Other Professional Liability	(6)	(27)	(2)	(21)
Surety		(3)	1	(3)
Warranty		(2)	(1)	(12)
Other	(20)		(24)	3
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (35)	$ (52)	$ (41)	$ (67)