Claim and Claim Adjustment Expense Reserves Net prior year claim and allocated claim adjustment expense reserve development for International segment (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (98)	$ (5)	$ (150)	$ (12)
International [Member]
Medical Professional Liability	(1)	0	(1)	0
Other Professional Liability	18	(5)	17	(5)
Liability	(19)	(2)	(19)	(7)
Property & Marine	(3)	(8)	(7)	(14)
Other	(10)	7	(9)	14
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (15)	$ (8)	$ (19)	$ (12)

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

No definition available.

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

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

No definition available.

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

Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Liability

No definition available.

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

Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Other Professional Liability

No definition available.

Name:	cna_NetPriorYearClaimandAllocatedClaimAdjustmentExpenseReserveDevelopmentOtherProfessionalLiability
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 & Marine

No definition available.

Name:	cna_NetPriorYearClaimandAllocatedClaimAdjustmentExpenseReserveDevelopmentPropertyMarine
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_InternationalMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: