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Claim and Claim Adjustment Expense Reserves (Net prior year claim and allocated claim adjustment expense reserve development for CNA Specialty 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 Specialty [Member] |  |  |  |
Medical Professional Liability | (17) | (9) | (20) | (15)
Other Professional Liability | (23) | (6) | (24) | (2)
Surety | 1 |  | 2 | 1
Warranty |  |  |  | (1)
Other | (2) | (20) | (14) | (24)
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | $ (41) | $ (35) | $ (56) | $ (41)

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

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

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

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

X
- Definition Net prior year claim and allocated claim adjustment expense reserve development, Warranty + References No definition available. + Details
Name: | cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentWarranty
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