Claim and Claim Adjustment Expense Reserves (Net prior year claim and allocated claim adjustment expense reserve development for Commercial segment) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2016	Mar. 31, 2015
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (52)	$ (7)
Commercial [Member]
Commercial Auto	(15)	0
General Liability	(15)	4
Workers' Compensation	4	(1)
Property and Other	12	(8)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (14)	$ (5)

Net prior year claim and allocated claim adjustment expense reserve development, Commercial Auto

No definition available.

Name:	cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentCommercialAuto
Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Net prior year claim and allocated claim adjustment expense reserve development, General Liability

No definition available.

Name:	cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentGeneralLiability
Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Net prior year claim and allocated claim adjustment expense reserve development, Property and Other

No definition available.

Name:	cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentPropertyAndOther
Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Net prior year claim and allocated claim adjustment expense reserve development, Workers' Compensation

No definition available.

Name:	cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentWorkersCompensation
Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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

No definition available.

Name:	cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup
Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Name:	us-gaap_StatementBusinessSegmentsAxis=cna_CommercialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: