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Investments in fixed maturity securities (Detail) (Fixed Maturities [Member], USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Summary of Investment Holdings [Line Items]    
Amortized cost of fixed maturity securities $ 26,023us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis $ 27,764us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
Unrealized gains on fixed maturity securities 1,754us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax 1,735us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Unrealized losses on fixed maturity securities (141)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax (146)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Fair value of fixed maturity securities 27,636us-gaap_AvailableForSaleSecuritiesDebtSecurities 29,353us-gaap_AvailableForSaleSecuritiesDebtSecurities
U.S. Treasury, U.S. government corporations and agencies [Member]
    
Summary of Investment Holdings [Line Items]    
Amortized cost of fixed maturity securities 2,921us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember		 2,650us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
Unrealized gains on fixed maturity securities 14us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember		 16us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
Unrealized losses on fixed maturity securities (5)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember		 (8)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
Fair value of fixed maturity securities 2,930us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember		 2,658us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
States, municipalities and political subdivisions [Member]
    
Summary of Investment Holdings [Line Items]    
Amortized cost of fixed maturity securities 1,820us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember		 2,221us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
Unrealized gains on fixed maturity securities 93us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember		 129us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
Unrealized losses on fixed maturity securities (1)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember		 (5)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
Fair value of fixed maturity securities 1,912us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember		 2,345us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
Foreign governments [Member]
    
Summary of Investment Holdings [Line Items]    
Amortized cost of fixed maturity securities 12,023us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember		 11,001us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
Unrealized gains on fixed maturity securities 373us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember		 182us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
Unrealized losses on fixed maturity securities (126)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember		 (110)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
Fair value of fixed maturity securities 12,270us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember		 11,073us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
Corporate bonds [Member]
    
Summary of Investment Holdings [Line Items]    
Amortized cost of fixed maturity securities 7,704us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember		 10,062us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
Unrealized gains on fixed maturity securities 1,072us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember		 1,190us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
Unrealized losses on fixed maturity securities (5)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember		 (15)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
Fair value of fixed maturity securities 8,771us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember		 11,237us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_CorporateBondSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
Mortgage-backed securities [Member]
    
Summary of Investment Holdings [Line Items]    
Amortized cost of fixed maturity securities 1,555us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_MortgageBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember		 1,830us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_MortgageBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
Unrealized gains on fixed maturity securities 202us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_MortgageBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember		 218us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_MortgageBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
Unrealized losses on fixed maturity securities (4)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_MortgageBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember		 (8)us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_MortgageBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
Fair value of fixed maturity securities $ 1,753us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_MortgageBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember		 $ 2,040us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InvestmentSecondaryCategorizationAxis
= us-gaap_MortgageBackedSecuritiesMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember