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Investments (Evaluation of Available-For-Sale Securities for OTTI and Evaluating Temporarily Impaired AFS Securities - Narrative) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]    
Equity Securities Available For Sale With Gross Unrealized Loss Of Equal To Or Greater Than Stated Percentage 20.00%met_EquitySecuritiesAvailableForSaleWithGrossUnrealizedLossOfEqualToOrGreaterThanStatedPercentage  
Fixed Maturity Securities    
Schedule of Available-for-sale Securities [Line Items]    
Change in Gross Unrealized Temporary Loss $ 2,600us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss
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Gross Unrealized Temporary Loss 2,000us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLosses
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4,600us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLosses
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Gross Unrealized Temporary Loss 888us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses
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1,220us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses
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Equity securities    
Schedule of Available-for-sale Securities [Line Items]    
Change in Gross Unrealized Temporary Loss (50)us-gaap_AvailableForSaleSecuritiesChangeInNetUnrealizedHoldingGainLoss
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Gross Unrealized Temporary Loss 67us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLosses
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117us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLosses
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Gross Unrealized Temporary Loss 37us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses
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48us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses
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Non-redeemable preferred stock    
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Temporary Loss 37us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses
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47us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses
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20% or more [Member] | Six months or greater [Member] | Fixed Maturity Securities    
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Temporary Loss 143us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLosses
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Number of Securities 51us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
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20% or more [Member] | Six months or greater [Member] | Fixed Maturity Securities | Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Temporary Loss 61us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLosses
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Number of Securities 25us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
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Percentage of gross unrealized loss 43.00%met_PercentageOfGrossUnrealizedLoss
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20% or more [Member] | Six months or greater [Member] | Fixed Maturity Securities | Below Investment Grade [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Gross Unrealized Temporary Loss 82us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLosses
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Number of Securities 26us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
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Percentage of gross unrealized loss 57.00%met_PercentageOfGrossUnrealizedLoss
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20% or more [Member] | Twelve months or greater [Member] | Equity securities    
Schedule of Available-for-sale Securities [Line Items]    
Number of Securities 6us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
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Gross Unrealized Temporary Loss $ 27us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLosses
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20% or more [Member] | Twelve months or greater [Member] | Non-redeemable preferred stock | Aaa/Aa/A | Financial Services Industry [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Percentage of gross unrealized loss 22.00%met_PercentageOfGrossUnrealizedLoss
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