Claim and Claim Adjustment Expense Reserves (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2012	Mar. 31, 2011
Catastrophe losses net of reinsurance	$ 28	$ 55
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, Life & Group	$ (1)	$ 7

Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development Life & Group

No definition available.

Name:	cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentLifeGroup
Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration