Claim and Claim Adjustment Expense Reserves Net prior year claim and allocated claim adjustment expense reserve development for Hardy segment (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2014	Mar. 31, 2013
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ 25	$ (26)
Hardy [Member]
Marine and Aviation	5	(1)
Non-Marine Property	(2)
Property Treaty	(2)
Specialty	(1)
Commutations	10
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ 10	$ (1)

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

No definition available.

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Marine and Aviation

No definition available.

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Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Non-Marine Property

No definition available.

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Property Treaty

No definition available.

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

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

No definition available.

Name:	cna_NetPriorYearClaimandAllocatedClaimAdjustmentExpenseReserveDevelopmentSpecialty
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