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Claim and Claim Adjustment Expense Reserves (Net prior year claim and allocated claim adjustment expense reserve development for CNA Hardy 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)
Hardy [Member] |  |  |  |
Marine and Aviation | 4 |  | 3 |
Non-Marine Property | 7 |  | 7 |
Property Treaty | 2 |  | 2 |
Specialty | (1) |  | (1) |
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | $ 12 |  | $ 11 |

X
- Definition Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Marine and Aviation + References No definition available. + Details
Name: | cna_NetPriorYearClaimandAllocatedClaimAdjustmentExpenseReserveDevelopmentMarineandAviation
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, Non-Marine Property + References No definition available. + Details
Name: | cna_NetPriorYearClaimandAllocatedClaimAdjustmentExpenseReserveDevelopmentNonMarineProperty
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, Property Treaty + References No definition available. + Details
Name: | cna_NetPriorYearClaimandAllocatedClaimAdjustmentExpenseReserveDevelopmentPropertyTreaty
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, Specialty + References No definition available. + Details
Name: | cna_NetPriorYearClaimandAllocatedClaimAdjustmentExpenseReserveDevelopmentSpecialty
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