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Investments (Net change in unrealized gains (losses) on investments) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Net change in unrealized gains (losses) on investments $ 1,517us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss $ (2,556)us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss $ 1,875us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss
Fixed maturity securities [Member]      
Net change in unrealized gains (losses) on investments 1,511us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedIncomeSecuritiesMember		 (2,541)us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedIncomeSecuritiesMember		 1,871us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_FixedIncomeSecuritiesMember
Equity securities [Member]      
Net change in unrealized gains (losses) on investments 6us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember		 (15)us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember		 5us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
Other [Member]      
Net change in unrealized gains (losses) on investments     $ (1)us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= cna_OtherInvestedAssetsMember