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Investments (Fixed Maturity and Equity Securities Available-For-Sale by Sector) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Available-for-sale Securities [Abstract]      
Cost or Amortized Cost $ 334,780us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis $ 333,599us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis  
Cost or Amortized Cost 3,076us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis 3,012us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis  
Gross Unrealized OTTI Loss 112met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss 218met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss 361met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
Fixed maturity securities available-for-sale, at estimated fair value (amortized cost: $334,780 and $333,599, respectively; includes $4,266 and $4,005, respectively, relating to variable interest entities) 365,425us-gaap_AvailableForSaleSecuritiesDebtSecurities 350,187us-gaap_AvailableForSaleSecuritiesDebtSecurities  
Equity securities 3,631us-gaap_AvailableForSaleSecuritiesEquitySecurities 3,402us-gaap_AvailableForSaleSecuritiesEquitySecurities  
Fixed Maturity Securities      
Available-for-sale Securities [Abstract]      
Cost or Amortized Cost 334,780us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
333,599us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Gross Unrealized Gain 32,634met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
21,180met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Gross Unrealized Temporary Loss 1,877met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
4,374met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Gross Unrealized OTTI Loss 112met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
218met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Fixed maturity securities available-for-sale, at estimated fair value (amortized cost: $334,780 and $333,599, respectively; includes $4,266 and $4,005, respectively, relating to variable interest entities) 365,425us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
350,187us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
U.S. corporate      
Available-for-sale Securities [Abstract]      
Cost or Amortized Cost 96,235us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DomesticCorporateDebtSecuritiesMember
100,203us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DomesticCorporateDebtSecuritiesMember
 
Gross Unrealized Gain 10,343met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DomesticCorporateDebtSecuritiesMember
7,495met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DomesticCorporateDebtSecuritiesMember
 
Gross Unrealized Temporary Loss 624met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DomesticCorporateDebtSecuritiesMember
1,229met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DomesticCorporateDebtSecuritiesMember
 
Gross Unrealized OTTI Loss 0met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DomesticCorporateDebtSecuritiesMember
0met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DomesticCorporateDebtSecuritiesMember
 
Fixed maturity securities available-for-sale, at estimated fair value (amortized cost: $334,780 and $333,599, respectively; includes $4,266 and $4,005, respectively, relating to variable interest entities) 105,954us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DomesticCorporateDebtSecuritiesMember
106,469us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DomesticCorporateDebtSecuritiesMember
 
Foreign corporate      
Available-for-sale Securities [Abstract]      
Cost or Amortized Cost 57,695us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember
59,778us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ 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
 
Gross Unrealized Temporary Loss 664met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember
565met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember
 
Gross Unrealized OTTI Loss 7met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember
0met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember
 
Fixed maturity securities available-for-sale, at estimated fair value (amortized cost: $334,780 and $333,599, respectively; includes $4,266 and $4,005, respectively, relating to variable interest entities) 61,675us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember
63,152us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember
 
Foreign government      
Available-for-sale Securities [Abstract]      
Cost or Amortized Cost 47,327us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
50,717us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
 
Gross Unrealized Gain 5,500met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
4,107met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
 
Gross Unrealized Temporary Loss 161met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
387met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
 
Gross Unrealized OTTI Loss 0met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
0met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
 
Fixed maturity securities available-for-sale, at estimated fair value (amortized cost: $334,780 and $333,599, respectively; includes $4,266 and $4,005, respectively, relating to variable interest entities) 52,666us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
54,437us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
 
RMBS      
Available-for-sale Securities [Abstract]      
Cost or Amortized Cost 38,064us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
34,167us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
 
Gross Unrealized Gain 2,102met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
1,584met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
 
Gross Unrealized Temporary Loss 214met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
490met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
 
Gross Unrealized OTTI Loss 106met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
206met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
 
Fixed maturity securities available-for-sale, at estimated fair value (amortized cost: $334,780 and $333,599, respectively; includes $4,266 and $4,005, respectively, relating to variable interest entities) 39,846us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
35,055us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
 
U.S. Treasury and agency      
Available-for-sale Securities [Abstract]      
Cost or Amortized Cost 54,654us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
43,928us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
 
Gross Unrealized Gain 6,892met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
2,251met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
 
Gross Unrealized Temporary Loss 30met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
1,056met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
 
Gross Unrealized OTTI Loss 0met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
0met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
 
Fixed maturity securities available-for-sale, at estimated fair value (amortized cost: $334,780 and $333,599, respectively; includes $4,266 and $4,005, respectively, relating to variable interest entities) 61,516us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
45,123us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
 
CMBS (1)      
Available-for-sale Securities [Abstract]      
Cost or Amortized Cost 13,762us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
16,115us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ 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
 
Gross Unrealized Temporary Loss 46met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
170met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
 
Gross Unrealized OTTI Loss (1)met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
0met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
 
Fixed maturity securities available-for-sale, at estimated fair value (amortized cost: $334,780 and $333,599, respectively; includes $4,266 and $4,005, respectively, relating to variable interest entities) 14,332us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
16,550us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
 
ABS      
Available-for-sale Securities [Abstract]      
Cost or Amortized Cost 14,121us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
15,458us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
 
Gross Unrealized Gain 240met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
296met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
 
Gross Unrealized Temporary Loss 112met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
171met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
 
Gross Unrealized OTTI Loss 0met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
12met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
 
Fixed maturity securities available-for-sale, at estimated fair value (amortized cost: $334,780 and $333,599, respectively; includes $4,266 and $4,005, respectively, relating to variable interest entities) 14,249us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
15,571us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
 
State and political subdivision      
Available-for-sale Securities [Abstract]      
Cost or Amortized Cost 12,922us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
13,233us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
 
Gross Unrealized Gain 2,291met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
903met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
 
Gross Unrealized Temporary Loss 26met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
306met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
 
Gross Unrealized OTTI Loss 0met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
0met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
 
Fixed maturity securities available-for-sale, at estimated fair value (amortized cost: $334,780 and $333,599, respectively; includes $4,266 and $4,005, respectively, relating to variable interest entities) 15,187us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
13,830us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
 
Equity securities      
Available-for-sale Securities [Abstract]      
Cost or Amortized Cost 3,076us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
3,012us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Gross Unrealized Gain 622met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
507met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Gross Unrealized Temporary Loss 67met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
117met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Gross Unrealized OTTI Loss 0met_OtherThanTemporaryImpairmentsOnEquitySecuritiesTransferredToOtherComprehensiveLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
0met_OtherThanTemporaryImpairmentsOnEquitySecuritiesTransferredToOtherComprehensiveLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Equity securities 3,631us-gaap_AvailableForSaleSecuritiesEquitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
3,402us-gaap_AvailableForSaleSecuritiesEquitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Common stock      
Available-for-sale Securities [Abstract]      
Cost or Amortized Cost 1,990us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommonStockMember
1,927us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommonStockMember
 
Gross Unrealized Gain 554met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommonStockMember
431met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommonStockMember
 
Gross Unrealized Temporary Loss 28met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommonStockMember
5met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommonStockMember
 
Gross Unrealized OTTI Loss 0met_OtherThanTemporaryImpairmentsOnEquitySecuritiesTransferredToOtherComprehensiveLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommonStockMember
0met_OtherThanTemporaryImpairmentsOnEquitySecuritiesTransferredToOtherComprehensiveLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommonStockMember
 
Equity securities 2,516us-gaap_AvailableForSaleSecuritiesEquitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommonStockMember
2,353us-gaap_AvailableForSaleSecuritiesEquitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommonStockMember
 
Non-redeemable preferred stock      
Available-for-sale Securities [Abstract]      
Cost or Amortized Cost 1,086us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_NonredeemablePreferredStockMember
1,085us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_NonredeemablePreferredStockMember
 
Gross Unrealized Gain 68met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_NonredeemablePreferredStockMember
76met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_NonredeemablePreferredStockMember
 
Gross Unrealized Temporary Loss 39met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_NonredeemablePreferredStockMember
112met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_NonredeemablePreferredStockMember
 
Gross Unrealized OTTI Loss 0met_OtherThanTemporaryImpairmentsOnEquitySecuritiesTransferredToOtherComprehensiveLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_NonredeemablePreferredStockMember
0met_OtherThanTemporaryImpairmentsOnEquitySecuritiesTransferredToOtherComprehensiveLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_NonredeemablePreferredStockMember
 
Equity securities $ 1,115us-gaap_AvailableForSaleSecuritiesEquitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_NonredeemablePreferredStockMember
$ 1,049us-gaap_AvailableForSaleSecuritiesEquitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_NonredeemablePreferredStockMember