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Investments (Net investment income) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Net Investment Income [Line Items]      
Gross investment income $ 2,125cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses $ 2,335cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses $ 2,162cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
Investment expense (58)us-gaap_InvestmentIncomeInvestmentExpense (53)us-gaap_InvestmentIncomeInvestmentExpense (52)us-gaap_InvestmentIncomeInvestmentExpense
Net investment income 2,067cna_NetInvestmentIncomeAndGainsLossesOnTradingSecurities 2,282cna_NetInvestmentIncomeAndGainsLossesOnTradingSecurities 2,110cna_NetInvestmentIncomeAndGainsLossesOnTradingSecurities
Fixed maturity securities [Member]      
Net Investment Income [Line Items]      
Gross investment income 1,803cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember		 1,827cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember		 1,846cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
Short term investments [Member]      
Net Investment Income [Line Items]      
Gross investment income 3cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember		 3cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember		 5cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Limited partnership investments [Member]      
Net Investment Income [Line Items]      
Gross investment income 263cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_LimitedPartnerMember		 451cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_LimitedPartnerMember		 251cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_LimitedPartnerMember
Equity securities [Member]      
Net Investment Income [Line Items]      
Gross investment income 12cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember		 12cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember		 12cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember
Mortgage loans [Member]      
Net Investment Income [Line Items]      
Gross investment income 31cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageLoansOnRealEstateMember		 23cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageLoansOnRealEstateMember		 17cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageLoansOnRealEstateMember
Trading portfolio [Member]      
Net Investment Income [Line Items]      
Gross investment income 10cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_TradingAccountAssetsMember		 17cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_TradingAccountAssetsMember		 24cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_TradingAccountAssetsMember
Other [Member]      
Net Investment Income [Line Items]      
Gross investment income $ 3cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= cna_OtherInvestmentIncomeMember		 $ 2cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= cna_OtherInvestmentIncomeMember		 $ 7cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= cna_OtherInvestmentIncomeMember