Claim and Claim Adjustment Expense Reserves (Net prior year development) (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, excluding Life & Group	$ (52)	$ (111)	$ (70)	$ (130)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(23)	39	(48)	23
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(75)	(72)	(118)	(107)
CNA Specialty [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(35)	(52)	(41)	(67)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(5)	(1)	(14)	(8)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(40)	(53)	(55)	(75)
CNA Commercial [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(13)	(50)	(27)	(57)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(19)	40	(36)	32
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(32)	(10)	(63)	(25)
Corporate and Other Non-Core [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(4)	(9)	(2)	(6)
Pretax (favorable) unfavorable premium development, excluding Life & Group	1		2	(1)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	$ (3)	$ (9)		$ (7)