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Investments (Credit Quality of Commercial Mortgage Loans) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans $ 32,482us-gaap_LoansReceivableCommercialMortgage $ 33,072us-gaap_LoansReceivableCommercialMortgage
% of Total 100.00%mlic_LoansReceivableCommercialMortgagePercentage 100.00%mlic_LoansReceivableCommercialMortgagePercentage
Estimated Fair Value 34,039mlic_EstimatedFairValueCommercialMortgage 34,297mlic_EstimatedFairValueCommercialMortgage
% of Total 100.00%mlic_EstimatedFairValueCommercialMortgagePercentage 100.00%mlic_EstimatedFairValueCommercialMortgagePercentage
Less than 65%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 28,317us-gaap_LoansReceivableCommercialMortgage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeOneMember		 25,657us-gaap_LoansReceivableCommercialMortgage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeOneMember
% of Total 87.20%mlic_LoansReceivableCommercialMortgagePercentage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeOneMember		 77.60%mlic_LoansReceivableCommercialMortgagePercentage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeOneMember
Estimated Fair Value 29,860mlic_EstimatedFairValueCommercialMortgage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeOneMember		 26,900mlic_EstimatedFairValueCommercialMortgage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeOneMember
% of Total 87.70%mlic_EstimatedFairValueCommercialMortgagePercentage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeOneMember		 78.40%mlic_EstimatedFairValueCommercialMortgagePercentage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeOneMember
65% to 75%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 3,260us-gaap_LoansReceivableCommercialMortgage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeTwoMember		 5,761us-gaap_LoansReceivableCommercialMortgage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeTwoMember
% of Total 10.00%mlic_LoansReceivableCommercialMortgagePercentage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeTwoMember		 17.40%mlic_LoansReceivableCommercialMortgagePercentage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeTwoMember
Estimated Fair Value 3,322mlic_EstimatedFairValueCommercialMortgage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeTwoMember		 5,852mlic_EstimatedFairValueCommercialMortgage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeTwoMember
% of Total 9.80%mlic_EstimatedFairValueCommercialMortgagePercentage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeTwoMember		 17.10%mlic_EstimatedFairValueCommercialMortgagePercentage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeTwoMember
76% to 80%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 117us-gaap_LoansReceivableCommercialMortgage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeThreeMember		 790us-gaap_LoansReceivableCommercialMortgage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeThreeMember
% of Total 0.40%mlic_LoansReceivableCommercialMortgagePercentage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeThreeMember		 2.40%mlic_LoansReceivableCommercialMortgagePercentage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeThreeMember
Estimated Fair Value 121mlic_EstimatedFairValueCommercialMortgage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeThreeMember		 776mlic_EstimatedFairValueCommercialMortgage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeThreeMember
% of Total 0.30%mlic_EstimatedFairValueCommercialMortgagePercentage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeThreeMember		 2.30%mlic_EstimatedFairValueCommercialMortgagePercentage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeThreeMember
Greater than 80%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 788us-gaap_LoansReceivableCommercialMortgage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeFourMember		 864us-gaap_LoansReceivableCommercialMortgage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeFourMember
% of Total 2.40%mlic_LoansReceivableCommercialMortgagePercentage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeFourMember		 2.60%mlic_LoansReceivableCommercialMortgagePercentage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeFourMember
Estimated Fair Value 736mlic_EstimatedFairValueCommercialMortgage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeFourMember		 769mlic_EstimatedFairValueCommercialMortgage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeFourMember
% of Total 2.20%mlic_EstimatedFairValueCommercialMortgagePercentage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeFourMember		 2.20%mlic_EstimatedFairValueCommercialMortgagePercentage
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeFourMember
Greater than 1.20x [Member]    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 30,086us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeOneMember		 30,831us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeOneMember
Greater than 1.20x [Member] | Less than 65%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 26,810us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeOneMember
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeOneMember		 24,585us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeOneMember
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeOneMember
Greater than 1.20x [Member] | 65% to 75%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 2,783us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeOneMember
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeTwoMember		 5,219us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeOneMember
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeTwoMember
Greater than 1.20x [Member] | 76% to 80%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 109us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeOneMember
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeThreeMember		 444us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeOneMember
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeThreeMember
Greater than 1.20x [Member] | Greater than 80%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 384us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeOneMember
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeFourMember		 583us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeOneMember
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeFourMember
1.00x - 1.20x    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 1,393us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeTwoMember		 1,276us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeTwoMember
1.00x - 1.20x | Less than 65%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 746us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeTwoMember
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeOneMember		 476us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeTwoMember
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeOneMember
1.00x - 1.20x | 65% to 75%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 391us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeTwoMember
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeTwoMember		 438us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeTwoMember
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeTwoMember
1.00x - 1.20x | 76% to 80%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 0us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeTwoMember
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeThreeMember		 157us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeTwoMember
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeThreeMember
1.00x - 1.20x | Greater than 80%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 256us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeTwoMember
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeFourMember		 205us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeTwoMember
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeFourMember
Less than 1.00x [Member]    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 1,003us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeThreeMember		 965us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeThreeMember
Less than 1.00x [Member] | Less than 65%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 761us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeThreeMember
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeOneMember		 596us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeThreeMember
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeOneMember
Less than 1.00x [Member] | 65% to 75%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 86us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeThreeMember
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeTwoMember		 104us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeThreeMember
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeTwoMember
Less than 1.00x [Member] | 76% to 80%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans 8us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeThreeMember
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeThreeMember		 189us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeThreeMember
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeThreeMember
Less than 1.00x [Member] | Greater than 80%    
Mortgage Loans on Real Estate [Line Items]    
Commercial mortgage loans $ 148us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeThreeMember
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeFourMember		 $ 76us-gaap_LoansReceivableCommercialMortgage
/ mlic_DebtServiceCoverageRatioByRangeAxis
= mlic_DebtServiceCoverageRatioRangeThreeMember
/ us-gaap_InternalCreditAssessmentAxis
= mlic_MortgageLoansByCreditQualityIndicatorRangeFourMember