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Investments (Fixed Maturity and Equity Securities Available-For-Sale by Sector) (Details) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Available-for-sale Securities [Abstract]      
Amortized Cost $ 332,909us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis $ 334,780us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis  
Cost or Amortized Cost 3,133us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis 3,076us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis  
Gross Unrealized OTTI Loss 84met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss 112met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss 218met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
Fixed maturity securities available-for-sale, at estimated fair value (amortized cost: $332,909 and $334,780, respectively; includes $4,326 and $4,266, respectively, relating to variable interest entities) 366,275us-gaap_AvailableForSaleSecuritiesDebtSecurities 365,425us-gaap_AvailableForSaleSecuritiesDebtSecurities  
Equity securities 3,713us-gaap_AvailableForSaleSecuritiesEquitySecurities 3,631us-gaap_AvailableForSaleSecuritiesEquitySecurities  
Fixed Maturity Securities      
Available-for-sale Securities [Abstract]      
Amortized Cost 332,909us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
334,780us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Gross Unrealized Gain 35,574met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
32,634met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Gross Unrealized Temporary Loss 2,124met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
1,877met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Gross Unrealized OTTI Loss 84met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
112met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
Fixed maturity securities available-for-sale, at estimated fair value (amortized cost: $332,909 and $334,780, respectively; includes $4,326 and $4,266, respectively, relating to variable interest entities) 366,275us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
365,425us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
 
U.S. corporate      
Available-for-sale Securities [Abstract]      
Amortized Cost 95,661us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DomesticCorporateDebtSecuritiesMember
96,235us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DomesticCorporateDebtSecuritiesMember
 
Gross Unrealized Gain 11,231met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DomesticCorporateDebtSecuritiesMember
10,343met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DomesticCorporateDebtSecuritiesMember
 
Gross Unrealized Temporary Loss 557met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DomesticCorporateDebtSecuritiesMember
624met_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: $332,909 and $334,780, respectively; includes $4,326 and $4,266, respectively, relating to variable interest entities) 106,335us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DomesticCorporateDebtSecuritiesMember
105,954us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DomesticCorporateDebtSecuritiesMember
 
Foreign corporate      
Available-for-sale Securities [Abstract]      
Amortized Cost 56,728us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember
57,695us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember
 
Gross Unrealized Gain 4,781met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember
4,651met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember
 
Gross Unrealized Temporary Loss 1,008met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember
664met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember
 
Gross Unrealized OTTI Loss 7met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember
7met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember
 
Fixed maturity securities available-for-sale, at estimated fair value (amortized cost: $332,909 and $334,780, respectively; includes $4,326 and $4,266, respectively, relating to variable interest entities) 60,494us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember
61,675us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignCorporateDebtSecuritiesMember
 
Foreign government      
Available-for-sale Securities [Abstract]      
Amortized Cost 46,384us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
47,327us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
 
Gross Unrealized Gain 6,106met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
5,500met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
 
Gross Unrealized Temporary Loss 129met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
161met_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: $332,909 and $334,780, respectively; includes $4,326 and $4,266, respectively, relating to variable interest entities) 52,361us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
52,666us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ForeignGovernmentDebtSecuritiesMember
 
RMBS      
Available-for-sale Securities [Abstract]      
Amortized Cost 41,836us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
38,064us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
 
Gross Unrealized Gain 2,234met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
2,102met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
 
Gross Unrealized Temporary Loss 208met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
214met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
 
Gross Unrealized OTTI Loss 78met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
106met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
 
Fixed maturity securities available-for-sale, at estimated fair value (amortized cost: $332,909 and $334,780, respectively; includes $4,326 and $4,266, respectively, relating to variable interest entities) 43,784us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
39,846us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_ResidentialMortgageBackedSecuritiesMember
 
U.S. Treasury and agency      
Available-for-sale Securities [Abstract]      
Amortized Cost 52,560us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
54,654us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
 
Gross Unrealized Gain 7,946met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
6,892met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
 
Gross Unrealized Temporary Loss 34met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
30met_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: $332,909 and $334,780, respectively; includes $4,326 and $4,266, respectively, relating to variable interest entities) 60,472us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
61,516us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
 
CMBS (1)      
Available-for-sale Securities [Abstract]      
Amortized Cost 12,762us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
13,762us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
 
Gross Unrealized Gain 605met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
615met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
 
Gross Unrealized Temporary Loss 57met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
46met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
 
Gross Unrealized OTTI Loss (1)met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
(1)met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
 
Fixed maturity securities available-for-sale, at estimated fair value (amortized cost: $332,909 and $334,780, respectively; includes $4,326 and $4,266, respectively, relating to variable interest entities) 13,311us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
14,332us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommercialMortgageBackedSecuritiesMember
 
ABS      
Available-for-sale Securities [Abstract]      
Amortized Cost 13,685us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
14,121us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
 
Gross Unrealized Gain 226met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
240met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
 
Gross Unrealized Temporary Loss 103met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
112met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
 
Gross Unrealized OTTI Loss 0met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
0met_OtherThanTemporaryImpairmentsOnDebtSecuritiesTransferredToOtherComprehensiveIncomeLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
 
Fixed maturity securities available-for-sale, at estimated fair value (amortized cost: $332,909 and $334,780, respectively; includes $4,326 and $4,266, respectively, relating to variable interest entities) 13,808us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
14,249us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
 
State and political subdivision      
Available-for-sale Securities [Abstract]      
Amortized Cost 13,293us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
12,922us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
 
Gross Unrealized Gain 2,445met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
2,291met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
 
Gross Unrealized Temporary Loss 28met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
26met_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: $332,909 and $334,780, respectively; includes $4,326 and $4,266, respectively, relating to variable interest entities) 15,710us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
15,187us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
 
Equity securities      
Available-for-sale Securities [Abstract]      
Cost or Amortized Cost 3,133us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
3,076us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Gross Unrealized Gain 649met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
622met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Gross Unrealized Temporary Loss 69met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
67met_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 3,713us-gaap_AvailableForSaleSecuritiesEquitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
3,631us-gaap_AvailableForSaleSecuritiesEquitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_EquitySecuritiesMember
 
Common Stock      
Available-for-sale Securities [Abstract]      
Cost or Amortized Cost 2,037us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommonStockMember
1,990us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommonStockMember
 
Gross Unrealized Gain 563met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommonStockMember
554met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommonStockMember
 
Gross Unrealized Temporary Loss 25met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommonStockMember
28met_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 2,575us-gaap_AvailableForSaleSecuritiesEquitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommonStockMember
2,516us-gaap_AvailableForSaleSecuritiesEquitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CommonStockMember
 
Non-redeemable preferred stock      
Available-for-sale Securities [Abstract]      
Cost or Amortized Cost 1,096us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_NonredeemablePreferredStockMember
1,086us-gaap_AvailableForSaleEquitySecuritiesAmortizedCostBasis
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_NonredeemablePreferredStockMember
 
Gross Unrealized Gain 86met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_NonredeemablePreferredStockMember
68met_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_NonredeemablePreferredStockMember
 
Gross Unrealized Temporary Loss 44met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_NonredeemablePreferredStockMember
39met_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInvestments
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_NonredeemablePreferredStockMember
 
Gross Unrealized OTTI Loss 0met_OtherThanTemporaryImpairmentsOnEquitySecuritiesTransferredToOtherComprehensiveLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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0met_OtherThanTemporaryImpairmentsOnEquitySecuritiesTransferredToOtherComprehensiveLoss
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Equity securities $ 1,138us-gaap_AvailableForSaleSecuritiesEquitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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$ 1,115us-gaap_AvailableForSaleSecuritiesEquitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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