XML 31 R30.htm IDEA: XBRL DOCUMENT v2.4.1.9
Investments (Net investment income) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Net Investment Income [Line Items]    
Gross investment income $ 572cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses $ 539cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
Investment expense (14)us-gaap_InvestmentIncomeInvestmentExpense (13)us-gaap_InvestmentIncomeInvestmentExpense
Net investment income 558cna_NetInvestmentIncomeAndGainsLossesOnTradingSecurities 526cna_NetInvestmentIncomeAndGainsLossesOnTradingSecurities
Fixed maturity securities [Member]    
Net Investment Income [Line Items]    
Gross investment income 443cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember		 452cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
Short term investments [Member]    
Net Investment Income [Line Items]    
Gross investment income 2cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember		 1cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Limited partnership investments [Member]    
Net Investment Income [Line Items]    
Gross investment income 114cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_LimitedPartnerMember		 73cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_LimitedPartnerMember
Equity securities [Member]    
Net Investment Income [Line Items]    
Gross investment income 3cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember		 2cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember
Mortgage loans [Member]    
Net Investment Income [Line Items]    
Gross investment income 8cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageLoansOnRealEstateMember		 6cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_MortgageLoansOnRealEstateMember
Trading portfolio [Member]    
Net Investment Income [Line Items]    
Gross investment income 2cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_TradingAccountAssetsMember		 3cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_TradingAccountAssetsMember
Other [Member]    
Net Investment Income [Line Items]    
Gross investment income $ 0cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= cna_OtherInvestmentIncomeMember		 $ 2cna_InvestmentIncomeIncludingRealizedGainsLossesOnTradingSecuritiesBeforeExpenses
/ us-gaap_InvestmentTypeAxis
= cna_OtherInvestmentIncomeMember