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Claim and Claim Adjustment Expense Reserves (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2013 | Mar. 31, 2012
Catastrophe losses net of reinsurance | $ 39 | $ 28

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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