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Employee Benefit Plans (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]        
Percentage of benefits calculated with the traditional formula 89.00%mlic_PercentageOfBenefitsCalculatedWithTraditionalFormula      
Total revenue from annuity and life insurance contracts recognized $ 50mlic_DefinedBenefitPlanRevenueFromAnnuityAndLifeInsuranceContracts $ 49mlic_DefinedBenefitPlanRevenueFromAnnuityAndLifeInsuranceContracts $ 54mlic_DefinedBenefitPlanRevenueFromAnnuityAndLifeInsuranceContracts  
Total investment income (loss) from annuity and life insurance contracts 1,200mlic_DefinedBenefitPlanInvestmentIncomeLossFromAnnuityAndLifeInsuranceContracts 20mlic_DefinedBenefitPlanInvestmentIncomeLossFromAnnuityAndLifeInsuranceContracts 867mlic_DefinedBenefitPlanInvestmentIncomeLossFromAnnuityAndLifeInsuranceContracts  
Defined Contribution Plan, Cost Recognized 68us-gaap_DefinedContributionPlanCostRecognized 84us-gaap_DefinedContributionPlanCostRecognized 83us-gaap_DefinedContributionPlanCostRecognized  
Pension Benefits U.S. Plans [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Aggregate projected benefit obligations 10,262us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
8,130us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
8,937us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
 
Estimated net actuarial losses amortized into net periodic benefit cost over the next year 200us-gaap_DefinedBenefitPlanFutureAmortizationOfGainLoss
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
     
Estimated prior service cost amortized into net periodic benefit cost over the next year (1)us-gaap_DefinedBenefitPlanFutureAmortizationOfPriorServiceCostCredit
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
     
Weighted average expected rate of return on plan assets 6.25%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
6.24%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
7.00%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
 
Other Postretirement Benefits U.S. Plans [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Aggregate projected benefit obligations 2,129us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPostretirementBenefitPlansOfUSEntityDefinedBenefitMember
1,861us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPostretirementBenefitPlansOfUSEntityDefinedBenefitMember
2,402us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPostretirementBenefitPlansOfUSEntityDefinedBenefitMember
 
Expected future discretionary contributions 50us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPostretirementBenefitPlansOfUSEntityDefinedBenefitMember
     
Estimated net actuarial losses amortized into net periodic benefit cost over the next year 31us-gaap_DefinedBenefitPlanFutureAmortizationOfGainLoss
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPostretirementBenefitPlansOfUSEntityDefinedBenefitMember
     
Estimated prior service cost amortized into net periodic benefit cost over the next year (4)us-gaap_DefinedBenefitPlanFutureAmortizationOfPriorServiceCostCredit
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPostretirementBenefitPlansOfUSEntityDefinedBenefitMember
     
Weighted average expected rate of return on plan assets 5.70%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPostretirementBenefitPlansOfUSEntityDefinedBenefitMember
5.76%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPostretirementBenefitPlansOfUSEntityDefinedBenefitMember
6.26%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPostretirementBenefitPlansOfUSEntityDefinedBenefitMember
 
United States Pension Plan of US Entity, Qualified [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Expected future discretionary contributions 300us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mlic_UnitedStatesPensionPlanofUSEntityQualifiedMember
     
United States Pension Plan of US Entity, Non Qualified [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Aggregate projected benefit obligations 1,300us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mlic_UnitedStatesPensionPlanofUSEntityNonQualifiedMember
1,000us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mlic_UnitedStatesPensionPlanofUSEntityNonQualifiedMember
   
Expected future discretionary contributions $ 70us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInNextFiscalYear
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= mlic_UnitedStatesPensionPlanofUSEntityNonQualifiedMember
     
Scenario, Forecast [Member] | Pension Benefits U.S. Plans [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Weighted average expected rate of return on plan assets       6.24%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPensionPlansOfUSEntityDefinedBenefitMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember
Scenario, Forecast [Member] | Other Postretirement Benefits U.S. Plans [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Weighted average expected rate of return on plan assets       5.65%us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_UnitedStatesPostretirementBenefitPlansOfUSEntityDefinedBenefitMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember