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Insurance (Insurance Liabilities Assumptions and Ratios - Narrative) (Details)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Liability for Future Policy Benefits and Policyholder Account Balances [Abstract]      
Participating business as a percentage of gross life insurance policies in-force 5.00%us-gaap_ParticipatingPoliciesAsPercentageOfGrossInsuranceInForce 5.00%us-gaap_ParticipatingPoliciesAsPercentageOfGrossInsuranceInForce  
Participating business as a percentage of the gross life insurance premiums 18.00%us-gaap_LifePremiumsAsPercentageOfGrossPremiums 19.00%us-gaap_LifePremiumsAsPercentageOfGrossPremiums 20.00%us-gaap_LifePremiumsAsPercentageOfGrossPremiums
Domestic Business [Member] | Minimum      
Liability for Future Policy Benefit, by Product Segment [Line Items]      
Interest rate assumptions for the aggregate future policy benefit liabilities for individual and group traditional fixed annuities after annuitization 1.00%met_IndividualAndGroupTraditionalFixedAnnuitiesInterestRate
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_StatementGeographicalAxis
= country_US
   
Interest rate assumptions for the aggregate future policy benefit liabilities for non-medical health insurance 4.00%met_NonMedicalHealthInsuranceInterestRate
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_StatementGeographicalAxis
= country_US
   
Interest rate assumptions for the aggregate future policy benefit liabilities for disabled lives 3.00%met_DisabilityInsuranceInterestRate
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_StatementGeographicalAxis
= country_US
   
Interest rate range credited to policyholder account balances 1.00%met_LiabilityForPolicyholderContractDepositsCreditedInterestRateAllApplicableProducts
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_StatementGeographicalAxis
= country_US
   
Domestic Business [Member] | Maximum      
Liability for Future Policy Benefit, by Product Segment [Line Items]      
Interest rate assumptions for the aggregate future policy benefit liabilities for individual and group traditional fixed annuities after annuitization 11.00%met_IndividualAndGroupTraditionalFixedAnnuitiesInterestRate
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_StatementGeographicalAxis
= country_US
   
Interest rate assumptions for the aggregate future policy benefit liabilities for non-medical health insurance 7.00%met_NonMedicalHealthInsuranceInterestRate
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_StatementGeographicalAxis
= country_US
   
Interest rate assumptions for the aggregate future policy benefit liabilities for disabled lives 8.00%met_DisabilityInsuranceInterestRate
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_StatementGeographicalAxis
= country_US
   
Interest rate range credited to policyholder account balances 13.00%met_LiabilityForPolicyholderContractDepositsCreditedInterestRateAllApplicableProducts
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_StatementGeographicalAxis
= country_US
   
Domestic Business [Member] | Participating Life Insurance Policies [Member]      
Liability for Future Policy Benefit, by Product Segment [Line Items]      
Interest rate assumptions for the aggregate future policy benefit liabilities for traditional life insurance policies, low end 3.00%us-gaap_TraditionalLifeInterestRateLowEnd
/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis
= met_ParticipatingLifeInsurancePoliciesMember
/ us-gaap_StatementGeographicalAxis
= country_US
   
Interest rate assumptions for the aggregate future policy benefit liabilities for traditional life insurance policies, high end 7.00%us-gaap_TraditionalLifeInterestRateHighEnd
/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis
= met_ParticipatingLifeInsurancePoliciesMember
/ us-gaap_StatementGeographicalAxis
= country_US
   
Domestic Business [Member] | Nonparticipating Life Insurance Policies [Member]      
Liability for Future Policy Benefit, by Product Segment [Line Items]      
Interest rate assumptions for the aggregate future policy benefit liabilities for traditional life insurance policies, low end 2.00%us-gaap_TraditionalLifeInterestRateLowEnd
/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis
= met_NonparticipatingLifeInsurancePoliciesMember
/ us-gaap_StatementGeographicalAxis
= country_US
   
Interest rate assumptions for the aggregate future policy benefit liabilities for traditional life insurance policies, high end 11.00%us-gaap_TraditionalLifeInterestRateHighEnd
/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis
= met_NonparticipatingLifeInsurancePoliciesMember
/ us-gaap_StatementGeographicalAxis
= country_US
   
International Business [Member] | Minimum      
Liability for Future Policy Benefit, by Product Segment [Line Items]      
Interest rate assumptions for the aggregate future policy benefit liabilities for individual and group traditional fixed annuities after annuitization 1.00%met_IndividualAndGroupTraditionalFixedAnnuitiesInterestRate
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_StatementGeographicalAxis
= met_NonUSMember
   
Interest rate assumptions for the aggregate future policy benefit liabilities for disabled lives 1.00%met_DisabilityInsuranceInterestRate
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_StatementGeographicalAxis
= met_NonUSMember
   
Interest rate range credited to policyholder account balances 1.00%met_LiabilityForPolicyholderContractDepositsCreditedInterestRateAllApplicableProducts
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_StatementGeographicalAxis
= met_NonUSMember
   
International Business [Member] | Maximum      
Liability for Future Policy Benefit, by Product Segment [Line Items]      
Interest rate assumptions for the aggregate future policy benefit liabilities for individual and group traditional fixed annuities after annuitization 12.00%met_IndividualAndGroupTraditionalFixedAnnuitiesInterestRate
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_StatementGeographicalAxis
= met_NonUSMember
   
Interest rate assumptions for the aggregate future policy benefit liabilities for disabled lives 9.00%met_DisabilityInsuranceInterestRate
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_StatementGeographicalAxis
= met_NonUSMember
   
Interest rate range credited to policyholder account balances 12.00%met_LiabilityForPolicyholderContractDepositsCreditedInterestRateAllApplicableProducts
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_StatementGeographicalAxis
= met_NonUSMember
   
International Business [Member] | Participating Life Insurance Policies [Member]      
Liability for Future Policy Benefit, by Product Segment [Line Items]      
Interest rate assumptions for the aggregate future policy benefit liabilities for traditional life insurance policies, low end 1.00%us-gaap_TraditionalLifeInterestRateLowEnd
/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis
= met_ParticipatingLifeInsurancePoliciesMember
/ us-gaap_StatementGeographicalAxis
= met_NonUSMember
   
Interest rate assumptions for the aggregate future policy benefit liabilities for traditional life insurance policies, high end 11.00%us-gaap_TraditionalLifeInterestRateHighEnd
/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis
= met_ParticipatingLifeInsurancePoliciesMember
/ us-gaap_StatementGeographicalAxis
= met_NonUSMember
   
International Business [Member] | Nonparticipating Life Insurance Policies [Member]      
Liability for Future Policy Benefit, by Product Segment [Line Items]      
Interest rate assumptions for the aggregate future policy benefit liabilities for traditional life insurance policies, low end 1.00%us-gaap_TraditionalLifeInterestRateLowEnd
/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis
= met_NonparticipatingLifeInsurancePoliciesMember
/ us-gaap_StatementGeographicalAxis
= met_NonUSMember
   
Interest rate assumptions for the aggregate future policy benefit liabilities for traditional life insurance policies, high end 13.00%us-gaap_TraditionalLifeInterestRateHighEnd
/ us-gaap_ReinsurancePremiumsForInsuranceCompaniesByProductSegmentAxis
= met_NonparticipatingLifeInsurancePoliciesMember
/ us-gaap_StatementGeographicalAxis
= met_NonUSMember