Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (40)	$ (36)	$ (110)	$ (166)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(7)	(36)	(55)	(13)
Total pretax favorable unfavorable net prior year development, excluding Life & Group	(47)	(72)	(165)	(179)
CNA Specialty [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(39)	(5)	(80)	(72)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(1)	(26)	(15)	(34)
Total pretax favorable unfavorable net prior year development, excluding Life & Group	(40)	(31)	(95)	(106)
CNA Commercial [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	2	(42)	(25)	(99)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(5)	(11)	(41)	21
Total pretax favorable unfavorable net prior year development, excluding Life & Group	(3)	(53)	(66)	(78)
Hardy [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(6)		(6)
Pretax (favorable) unfavorable premium development, excluding Life & Group
Total pretax favorable unfavorable net prior year development, excluding Life & Group	(6)		(6)
Corporate and Other Non-Core [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	3	11	1	5
Pretax (favorable) unfavorable premium development, excluding Life & Group	(1)	1	1
Total pretax favorable unfavorable net prior year development, excluding Life & Group	$ 2	$ 12	$ 2	$ 5