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
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development:
Core (Non-A&EP)			$ (444)
A&EP			155
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, P&C	(423)	(637)	(289)
Pretax (favorable) unfavorable premium development	(8)	43	81
Total pretax (favorable) unfavorable net prior year development	(431)	(594)	(208)
CNA Specialty [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development:
Core (Non-A&EP)			(218)
A&EP
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, P&C	(217)	(341)	(218)
Pretax (favorable) unfavorable premium development	(28)	(3)	(6)
Total pretax (favorable) unfavorable net prior year development	(245)	(344)	(224)
CNA Commercial [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development:
Core (Non-A&EP)			(230)
A&EP
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, P&C	(204)	(304)	(230)
Pretax (favorable) unfavorable premium development	21	48	87
Total pretax (favorable) unfavorable net prior year development	(183)	(256)	(143)
Corporate and Other Non-Core [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development:
Core (Non-A&EP)			4
A&EP			155
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, P&C	(2)	8	159
Pretax (favorable) unfavorable premium development	(1)	(2)
Total pretax (favorable) unfavorable net prior year development	$ (3)	$ 6	$ 159