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Common Equity Units (Common Equity Units) (Details) (USD $)
 12 Months Ended 1 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2010
Oct. 31, 2014
Sep. 30, 2013
Oct. 31, 2012
Dec. 31, 2011
Oct. 07, 2014
Sep. 11, 2013
Oct. 10, 2012
Oct. 04, 2012
Temporary Equity Disclosure [Abstract]                        
Effective date of acquisition       Nov. 01, 2010                
Equity Units, Shares Transferred       40,000,000us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued                
Equity Units Stated Interest Percentage       5.00%met_EquityUnitsStatedInterestPercentage                
Equity Units, Aggregate Purchase Price per Unit       $ 75met_EquityUnitsAggregatePurchasePricePerUnit                
Common Stock, Par, Common Equity Units $ 0.01us-gaap_CommonStockParOrStatedValuePerShare $ 0.01us-gaap_CommonStockParOrStatedValuePerShare   $ 0.01us-gaap_CommonStockParOrStatedValuePerShare                
Equity Units, Purchase Price Per Contract Settlement       $ 25us-gaap_ForwardContractIndexedToIssuersEquityForwardRate                
Equity Units, Beneficial Interest Stated Percentage               2.50%met_EquityUnitsBeneficialInterestStatedPercentage        
Weighted Average Share Price, Trading Day Periods       20 days                
Stock Purchase Contracts, Value at Issuance       247,000,000met_StockPurchaseContractsValueAtIssuance                
Stock Purchase Contracts, Payments Made 21,000,000met_StockPurchaseContractsPaymentsMade 48,000,000met_StockPurchaseContractsPaymentsMade 84,000,000met_StockPurchaseContractsPaymentsMade                  
Stock Purchase Contracts, Average Stated Interest Rate       3.02%met_StockPurchaseContractsAverageStatedInterestRate                
Issuance of stock purchase contracts related to common equity units       (69,000,000)met_IssuanceOfStockPurchaseContractsRelatedToCommonEquityUnits                
Class of Stock [Line Items]                        
Value of securities issued for business acquisition       3,000,000,000us-gaap_BusinessAcquisitionEquityInterestIssuedOrIssuableValueAssigned                
Proceeds from issuance of common stock 1,000,000,000us-gaap_ProceedsFromIssuanceOfCommonStock 1,000,000,000us-gaap_ProceedsFromIssuanceOfCommonStock 1,000,000,000us-gaap_ProceedsFromIssuanceOfCommonStock                  
Common stock, shares issued 1,153,998,144us-gaap_CommonStockSharesIssued 1,125,224,024us-gaap_CommonStockSharesIssued                    
Common Stock [Member]                        
Class of Stock [Line Items]                        
Proceeds from issuance of common stock         1,000,000,000us-gaap_ProceedsFromIssuanceOfCommonStock
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember		 1,000,000,000us-gaap_ProceedsFromIssuanceOfCommonStock
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember		 1,000,000,000us-gaap_ProceedsFromIssuanceOfCommonStock
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember		          
Common stock, shares issued 22,907,960us-gaap_CommonStockSharesIssued
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember		               22,907,960us-gaap_CommonStockSharesIssued
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember		 22,679,955us-gaap_CommonStockSharesIssued
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember		 28,231,956us-gaap_CommonStockSharesIssued
/ us-gaap_StatementClassOfStockAxis
= us-gaap_CommonClassAMember		  
Debt Remarketing Series E Tranche One [Member]                        
Class of Stock [Line Items]                        
Debt Instrument, Interest Rate, Stated Percentage 1.903%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= met_DebtRemarketingSeriesETrancheOneMember		                      
Debt Instrument, Maturity Date Dec. 15, 2017                      
Debt Remarketing Series E Tranche Two [Member]                        
Class of Stock [Line Items]                        
Debt Instrument, Interest Rate, Stated Percentage 4.721%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= met_DebtRemarketingSeriesETrancheTwoMember		                      
Debt Instrument, Maturity Date Dec. 15, 2044                      
Senior notes                        
Class of Stock [Line Items]                        
Debt Instrument, Interest Rate, Stated Percentage 5.875%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		                      
Senior notes | Debt Remarketing Series D [Member]                        
Class of Stock [Line Items]                        
Debt Instrument, Interest Rate, Stated Percentage   4.368%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= met_DebtRemarketingSeriesDMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		               4.368%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= met_DebtRemarketingSeriesDMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		    
Debt Instrument, Maturity Date   Sep. 15, 2023       Sep. 15, 2023            
Senior notes | Debt Remarketing Series C Tranche 1 [Member]                        
Class of Stock [Line Items]                        
Debt Instrument, Interest Rate, Stated Percentage                       1.756%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= met_DebtRemarketingSeriesCTrancheOneMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
Debt Instrument, Maturity Date             Dec. 15, 2017          
Senior notes | Debt Remarketing Series C Tranche 2 [Member]                        
Class of Stock [Line Items]                        
Debt Instrument, Interest Rate, Stated Percentage                       3.048%us-gaap_DebtInstrumentInterestRateStatedPercentage
/ us-gaap_DebtInstrumentAxis
= met_DebtRemarketingSeriesCTrancheTwoMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember
Debt Instrument, Maturity Date             Dec. 15, 2022          
Senior notes | Debt Remarketing Series C [Member]                        
Class of Stock [Line Items]                        
Proceeds from remarketing 0met_ProceedsFromRemarketing
/ us-gaap_DebtInstrumentAxis
= met_DebtRemarketingSeriesCMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		 0met_ProceedsFromRemarketing
/ us-gaap_DebtInstrumentAxis
= met_DebtRemarketingSeriesCMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		 0met_ProceedsFromRemarketing
/ us-gaap_DebtInstrumentAxis
= met_DebtRemarketingSeriesCMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_SeniorNotesMember		                  
Estimated Fair Value [Member]                        
Class of Stock [Line Items]                        
Value of securities issued for business acquisition       3,200,000,000us-gaap_BusinessAcquisitionEquityInterestIssuedOrIssuableValueAssigned
/ us-gaap_FairValueByMeasurementBasisAxis
= us-gaap_EstimateOfFairValueFairValueDisclosureMember		                
Alico Transaction [Member]                        
Class of Stock [Line Items]                        
Debt Security, Principle Amount       $ 1,000,000,000us-gaap_DebtInstrumentFaceAmount
/ met_EquityUnitsTransactionAxis
= met_AlicoTransactionMember