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Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2013 | Mar. 31, 2012
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | $ (26) | $ (18)
Pretax (favorable) unfavorable premium development, excluding Life & Group | (13) | (25)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group | (39) | (43)
CNA Specialty [Member] |  |
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (15) | (6)
Pretax (favorable) unfavorable premium development, excluding Life & Group | (8) | (9)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group | (23) | (15)
CNA Commercial [Member] |  |
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (11) | (14)
Pretax (favorable) unfavorable premium development, excluding Life & Group | (10) | (17)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group | (21) | (31)
Hardy [Member] |  |
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (1) |
Pretax (favorable) unfavorable premium development, excluding Life & Group | 4 |
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group | 3 |
Corporate and Other [Member] |  |
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | 1 | 2
Pretax (favorable) unfavorable premium development, excluding Life & Group | 1 | 1
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group | $ 2 | $ 3

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- 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

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- Definition Pretax Favorable Unfavorable Premium Development, excluding Life & Group + References No definition available. + Details
Name: | cna_PretaxFavorableUnfavorablePremiumDevelopmentExcludingLifeGroup
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Total Pretax Favorable Unfavorable Net Prior Year Development, excluding Life & Group + References No definition available. + Details
Name: | cna_TotalPretaxFavorableUnfavorableNetPriorYearDevelopmentExcludingLifeGroup
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration