Claim And Claim Adjustment Expense Reserves (Net Prior Year Development) (Details) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Claim And Claim Adjustment Expense Reserves [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	$ (36)	$ (89)	$ (166)	$ (439)
Pretax (favorable) unfavorable premium development	(36)	(4)	(13)	46
Total pretax (favorable) unfavorable net prior year development	(72)	(93)	(179)	(393)
CNA Specialty [Member]
Claim And Claim Adjustment Expense Reserves [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	(5)	(65)	(72)	(215)
Pretax (favorable) unfavorable premium development	(26)	(2)	(34)	(5)
Total pretax (favorable) unfavorable net prior year development	(31)	(67)	(106)	(220)
CNA Commercial [Member]
Claim And Claim Adjustment Expense Reserves [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	(42)	(26)	(99)	(229)
Pretax (favorable) unfavorable premium development	(11)	(2)	21	54
Total pretax (favorable) unfavorable net prior year development	(53)	(28)	(78)	(175)
Other Insurance [Member]
Claim And Claim Adjustment Expense Reserves [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	11	2	5	5
Pretax (favorable) unfavorable premium development	1			(3)
Total pretax (favorable) unfavorable net prior year development	$ 12	$ 2	$ 5	$ 2