Claim And Claim Adjustment Expense Reserves (Net Prior Year Development) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Claim And Claim Adjustment Expense Reserves [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development - Core (Non-A&EP)	$ (423)	$ (637)	$ (444)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development - A&EP			155
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development			(289)
Pretax (favorable) unfavorable premium development	(8)	43	81
Total pretax (favorable) unfavorable net prior year development	(431)	(594)	(208)
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 - Core (Non-A&EP)	(217)	(341)	(218)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development			(218)
Pretax (favorable) unfavorable premium development	(28)	(3)	(6)
Total pretax (favorable) unfavorable net prior year development	(245)	(344)	(224)
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 - Core (Non-A&EP)	(204)	(304)	(230)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development			(230)
Pretax (favorable) unfavorable premium development	21	48	87
Total pretax (favorable) unfavorable net prior year development	(183)	(256)	(143)
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 - Core (Non-A&EP)	(2)	8	4
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development - A&EP			155
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development			159
Pretax (favorable) unfavorable premium development	(1)	(2)
Total pretax (favorable) unfavorable net prior year development	$ (3)	$ 6	$ 159