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Investments (Variable Interest Entities - Narrative) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Variable Interest Entity [Line Items]      
Tax credits guaranteed by third parties that reduce maximum exposure to loss related to other invested assets $ 212mlic_TaxCreditsGuaranteedByThirdPartiesAmountThatReducesMaximumExposureToLossRelatedToOtherInvestedAssets $ 257mlic_TaxCreditsGuaranteedByThirdPartiesAmountThatReducesMaximumExposureToLossRelatedToOtherInvestedAssets  
Financial or other support to investees designated as VIEs 0us-gaap_VariableInterestEntityFinancialOrOtherSupportAmount 0us-gaap_VariableInterestEntityFinancialOrOtherSupportAmount 0us-gaap_VariableInterestEntityFinancialOrOtherSupportAmount
Real estate joint ventures      
Variable Interest Entity [Line Items]      
Variable interest, maximum exposure to loss in consolidated securitization entities   178us-gaap_VariableInterestEntityEntityMaximumLossExposureAmount
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= mlic_RealEstateJointVenturesMember
 
Debt Instrument, Interest Rate, Effective Percentage Rate Range, Minimum   1.39%us-gaap_DebtInstrumentInterestRateEffectivePercentageRateRangeMinimum
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= mlic_RealEstateJointVenturesMember
 
Debt Instrument, Interest Rate, Effective Percentage Rate Range, Maximum   4.45%us-gaap_DebtInstrumentInterestRateEffectivePercentageRateRangeMaximum
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= mlic_RealEstateJointVenturesMember
 
Consolidated Securitization Entities [Member]      
Variable Interest Entity [Line Items]      
Interest expense on long-term debt held by consolidated securitization entities 1us-gaap_InterestExpenseLongTermDebt
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_ConsolidatedEntitiesMember
3us-gaap_InterestExpenseLongTermDebt
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_ConsolidatedEntitiesMember
4us-gaap_InterestExpenseLongTermDebt
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_ConsolidatedEntitiesMember
Fixed Maturities [Member]      
Variable Interest Entity [Line Items]      
Interest expense on long-term debt held by consolidated securitization entities 2us-gaap_InterestExpenseLongTermDebt
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
2us-gaap_InterestExpenseLongTermDebt
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
1us-gaap_InterestExpenseLongTermDebt
/ us-gaap_InvestmentTypeAxis
= us-gaap_FixedMaturitiesMember
Maximum | Real estate joint ventures      
Variable Interest Entity [Line Items]      
Interest expense on long-term debt held by consolidated securitization entities   1us-gaap_InterestExpenseLongTermDebt
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= mlic_RealEstateJointVenturesMember
 
Maximum | Consolidated Securitization Entities [Member]      
Variable Interest Entity [Line Items]      
Variable interest, maximum exposure to loss in consolidated securitization entities $ 1us-gaap_VariableInterestEntityEntityMaximumLossExposureAmount
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_ConsolidatedEntitiesMember
$ 1us-gaap_VariableInterestEntityEntityMaximumLossExposureAmount
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_ConsolidatedEntitiesMember