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Stockholders' Equity and Statutory Accounting Practices (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Capital Required for Capital Adequacy to Risk Weighted Assets 270.00%us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets 265.00%us-gaap_CapitalRequiredForCapitalAdequacyToRiskWeightedAssets
Dividends payable without prior supervisory approval $ 466us-gaap_StatutoryAccountingPracticesStatutoryAmountAvailableForDividendPayments  
Hardy [Member]    
Capital provided by CCC $ 105cna_CapitalProvidedByCcc
/ dei_LegalEntityAxis
= cna_HardyMember