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Claim and Claim Adjustment Expense Reserves (Narrative) (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
Claim and Claim Adjustment Expense Reserves [Abstract] |  |  |  |
Catastrophe losses net of reinsurance | $ 65 | $ 68 | $ 104 | $ 96

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- Definition Catastrophe losses net of reinsurance + References No definition available. + Details
Name: | cna_CatastropheLossesNetOfReinsurance
Namespace Prefix: | cna_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | cna_ClaimAndClaimAdjustmentExpenseReservesAbstract
Namespace Prefix: | cna_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration