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Investments (Mortgage Loans - Narrative) (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Financing Receivable, Recorded Investment [Line Items]      
Financing Receivable, Significant Purchases $ 4,700,000,000us-gaap_FinancingReceivableSignificantPurchases $ 2,200,000,000us-gaap_FinancingReceivableSignificantPurchases  
Average Recorded Investment 458,000,000us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment 693,000,000us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment  
Percentage of Mortgage Loans Classified as Performing 99.00%met_PercentageOfMortgageLoansClassifiedAsPerforming 99.00%met_PercentageOfMortgageLoansClassifiedAsPerforming  
Agricultural      
Financing Receivable, Recorded Investment [Line Items]      
Average Recorded Investment     204,000,000us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= met_AgriculturalPortfolioSegmentMember
Estimated fair value of mortgage loans held-for-investment 12,800,000,000us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= met_AgriculturalPortfolioSegmentMember
12,700,000,000us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= met_AgriculturalPortfolioSegmentMember
 
Residential      
Financing Receivable, Recorded Investment [Line Items]      
Average Recorded Investment     13,000,000us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
Estimated fair value of mortgage loans held-for-investment 6,600,000,000us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
2,800,000,000us-gaap_NotesReceivableNet
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialPortfolioSegmentMember
 
Commercial mortgage loans      
Financing Receivable, Recorded Investment [Line Items]      
Average Recorded Investment     $ 464,000,000us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialLoanMember