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Mortgage Loans, Sorted by Applicable Credit Quality Indicators (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Mortgage Loans on Real Estate    
Mortgage Loans $ 1,856.6us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet $ 1,815.1us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
Loan to Value Ratio Below or Equal to 65 Percent    
Mortgage Loans on Real Estate    
Mortgage Loans 898.7us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_LoanToValueRatioBelowOrEqualTo65PercentMember		 777.4us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_LoanToValueRatioBelowOrEqualTo65PercentMember
Loan To Value Ratio Above 65 To 75 Percent    
Mortgage Loans on Real Estate    
Mortgage Loans 818.0us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_LoanToValueRatioAbove65To75PercentMember		 867.5us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_LoanToValueRatioAbove65To75PercentMember
Loan To Value Ratio Above 75 To 85 Percent    
Mortgage Loans on Real Estate    
Mortgage Loans 102.3us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_LoanToValueRatioAbove75To85PercentMember		 107.6us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_LoanToValueRatioAbove75To85PercentMember
Loan To Value Ratio Above 85 Percent    
Mortgage Loans on Real Estate    
Mortgage Loans 37.6us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_LoanToValueRatioAbove85PercentMember		 62.6us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_LoanToValueRatioAbove85PercentMember
AA Credit Rating    
Mortgage Loans on Real Estate    
Mortgage Loans 7.7us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_AaCreditRatingMember		 10.8us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_AaCreditRatingMember
A Credit Rating    
Mortgage Loans on Real Estate    
Mortgage Loans 666.0us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_CreditRatingMember		 683.1us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_CreditRatingMember
Baa Credit Rating    
Mortgage Loans on Real Estate    
Mortgage Loans 1,156.7us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_BaaCreditRatingMember		 1,094.6us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_BaaCreditRatingMember
Ba Credit Rating    
Mortgage Loans on Real Estate    
Mortgage Loans 13.1us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_BaCreditRatingMember		 13.5us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_BaCreditRatingMember
B Credit Rating    
Mortgage Loans on Real Estate    
Mortgage Loans $ 13.1us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_BCreditRatingMember		 $ 13.1us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_BCreditRatingMember