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Investments (Fixed Maturity and Equity Securities Available-For-Sale - Narrative) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Summary of Certain Fixed Maturity Securities      
Gross Unrealized OTTI Loss $ 112met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss $ 218met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss $ 361met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
Available-for-sale Securities, Debt Securities 365,425us-gaap_AvailableForSaleSecuritiesDebtSecurities 350,187us-gaap_AvailableForSaleSecuritiesDebtSecurities  
CMBS (1)      
Summary of Certain Fixed Maturity Securities      
Gross Unrealized OTTI Loss (1)met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		 0met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		  
Available-for-sale Securities, Debt Securities 14,332us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		 16,550us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		  
Gross Unrealized Gain 615met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		 605met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember		  
Foreign corporate      
Summary of Certain Fixed Maturity Securities      
Gross Unrealized OTTI Loss 7met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember		 0met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember		  
Available-for-sale Securities, Debt Securities 61,675us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember		 63,152us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember		  
Gross Unrealized Gain 4,651met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember		 3,939met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember		  
Fixed Maturity Securities      
Summary of Certain Fixed Maturity Securities      
Available-for-sale Securities, Debt Securities 64us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DebtSecuritiesMember		 74us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DebtSecuritiesMember		  
Gross Unrealized Gain $ 28met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DebtSecuritiesMember		 $ 23met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_DebtSecuritiesMember