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Claim and Claim Adjustment Expense Reserves (Net prior year claim and allocated claim adjustment expense reserve development for CNA Commercial segment) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2013 | Jun. 30, 2012 | Jun. 30, 2013 | Jun. 30, 2012
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | $ (5) | $ (52) | $ (31) | $ (70)
CNA Commercial [Member] |  |  |  |
Commercial Auto | 2 | 2 | (3) | 2
General Liability | 15 | (13) | (6) | (5)
Workers' Compensation | 45 | 8 | 70 | (11)
Property and Other | (35) | (10) | (45) | (13)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | $ 27 | $ (13) | $ 16 | $ (27)

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- Definition Net prior year claim and allocated claim adjustment expense reserve development, Commercial Auto + References No definition available. + Details
Name: | cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentCommercialAuto
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Net prior year claim and allocated claim adjustment expense reserve development, General Liability + References No definition available. + Details
Name: | cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentGeneralLiability
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Net prior year claim and allocated claim adjustment expense reserve development, Property and Other + References No definition available. + Details
Name: | cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentPropertyAndOther
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Net prior year claim and allocated claim adjustment expense reserve development, Workers' Compensation + References No definition available. + Details
Name: | cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentWorkersCompensation
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group + References No definition available. + Details
Name: | cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration