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Claim and Claim Adjustment Expense Reserves (Net prior year development) (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)
Pretax (favorable) unfavorable premium development, excluding Life & Group | (8) | (23) | (21) | (48)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group | (13) | (75) | (52) | (118)
CNA Specialty [Member] |  |  |  |
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (41) | (35) | (56) | (41)
Pretax (favorable) unfavorable premium development, excluding Life & Group | (5) | (5) | (13) | (14)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group | (46) | (40) | (69) | (55)
CNA Commercial [Member] |  |  |  |
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | 27 | (13) | 16 | (27)
Pretax (favorable) unfavorable premium development, excluding Life & Group | (5) | (19) | (15) | (36)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group | 22 | (32) | 1 | (63)
Hardy [Member] |  |  |  |
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | 12 |  | 11 |
Pretax (favorable) unfavorable premium development, excluding Life & Group | 2 |  | 6 |
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group | 14 |  | 17 |
Corporate and Other [Member] |  |  |  |
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group | (3) | (4) | (2) | (2)
Pretax (favorable) unfavorable premium development, excluding Life & Group |  | 1 | 1 | 2
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group | $ (3) | $ (3) | $ (1) |

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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
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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

X
- 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