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Investments - Net Investment Income and Investment Gains (Losses) (Detail) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Net Investment Income [Line Items]      
Total investment income $ 2,221l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses $ 2,478l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses $ 2,229l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
Investment expenses (58)us-gaap_InvestmentIncomeInvestmentExpense (53)us-gaap_InvestmentIncomeInvestmentExpense (52)us-gaap_InvestmentIncomeInvestmentExpense
Net investment income 2,163l_NetInvestmentIncomeAndGainsLossesOnTradingSecurities 2,425l_NetInvestmentIncomeAndGainsLossesOnTradingSecurities 2,177l_NetInvestmentIncomeAndGainsLossesOnTradingSecurities
Investment gains 54us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet [1] 16us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet [1] 48us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet [1]
Fixed Maturity Securities [Member]      
Net Investment Income [Line Items]      
Total investment income 1,803l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember		 1,827l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember		 1,846l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
Investment gains 41us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember		 41us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember		 77us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
Short Term Investments [Member]      
Net Investment Income [Line Items]      
Total investment income 4l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember		 5l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember		 12l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Investment gains 13us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember		 6us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember		 (1)us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Limited Partnership Investments [Member]      
Net Investment Income [Line Items]      
Total investment income 304l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_LimitedPartnerMember		 519l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_LimitedPartnerMember		 283l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_LimitedPartnerMember
Equity Securities [Member]      
Net Investment Income [Line Items]      
Total investment income 12l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember		 12l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember		 12l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember
Investment gains 1us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember		 (22)us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember		 (23)us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember
Income from Trading Portfolio [Member]      
Net Investment Income [Line Items]      
Total investment income 64l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= l_IncomeLossFromTradingPortfolioMember		 [2] 90l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= l_IncomeLossFromTradingPortfolioMember		 [2] 52l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= l_IncomeLossFromTradingPortfolioMember		 [2]
Other [Member]      
Net Investment Income [Line Items]      
Total investment income 34l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= l_OtherInvestmentIncomeMember		 25l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= l_OtherInvestmentIncomeMember		 24l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= l_OtherInvestmentIncomeMember
Derivative Instruments [Member]      
Net Investment Income [Line Items]      
Investment gains $ (1)us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ us-gaap_InvestmentTypeAxis
= us-gaap_DerivativeMember		 $ (9)us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ us-gaap_InvestmentTypeAxis
= us-gaap_DerivativeMember		 $ (5)us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ us-gaap_InvestmentTypeAxis
= us-gaap_DerivativeMember
[1] Includes gross realized gains of $178, $198 and $241 and gross realized losses of $136, $179 and $187 on available-for-sale securities for the years ended December 31, 2014, 2013 and 2012.
[2] Includes net unrealized gains (losses) related to changes in fair value on trading securities still held of $42, $(2) and $6 for the years ended December 31, 2014, 2013 and 2012.