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Investments - Net Investment Income and Investment Gains (Losses) (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Net Investment Income [Line Items]    
Total investment income $ 602l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses $ 590l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
Investment expenses (14)us-gaap_InvestmentIncomeInvestmentExpense (13)us-gaap_InvestmentIncomeInvestmentExpense
Net investment income 588l_NetInvestmentIncomeAndGainsLossesOnTradingSecurities 577l_NetInvestmentIncomeAndGainsLossesOnTradingSecurities
Investment gains 10us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet [1] 42us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet [1]
Fixed Maturity Securities [Member]    
Net Investment Income [Line Items]    
Total investment income 443l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember		 452l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
Investment gains 12us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember		 38us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedIncomeSecuritiesMember
Short Term Investments [Member]    
Net Investment Income [Line Items]    
Total investment income 3l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember		 1l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_ShortTermInvestmentsMember
Investment gains (1)us-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 160l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_PartnershipInterestMember		 87l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_PartnershipInterestMember
Equity Securities [Member]    
Net Investment Income [Line Items]    
Total investment income 3l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember		 2l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember
Investment gains   5us-gaap_AvailableForSaleSecuritiesGrossRealizedGainLossNet
/ us-gaap_InvestmentTypeAxis
= us-gaap_EquitySecuritiesMember
Income (Loss) from Trading Portfolio [Member]    
Net Investment Income [Line Items]    
Total investment income (15)l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= l_IncomeLossFromTradingPortfolioMember		 [2] 40l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= l_IncomeLossFromTradingPortfolioMember		 [2]
Other [Member]    
Net Investment Income [Line Items]    
Total investment income 8l_NetInvestmentIncomeAndGainsLossesOnTradingSecuritiesBeforeInvestmentExpenses
/ us-gaap_InvestmentTypeAxis
= l_OtherInvestmentIncomeMember		 8l_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		  
[1] Includes gross realized gains of $34 and $58 and gross realized losses of $22 and $15 on available-for-sale securities for the three months ended March 31, 2015 and 2014.
[2] Includes net unrealized gains (losses) related to changes in fair value on trading securities still held of $(14) and $13 for the three months ended March 31, 2015 and 2014.