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Mortgage Loans Sorted by Applicable Internal Credit Ratings and Loan-to-Value Ratios (Detail) (USD $)
In Millions, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Financing Receivable, Recorded Investment    
Mortgage Loans $ 1,896.5us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet $ 1,856.6us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
Loan to Value Ratio Below or Equal to 65 Percent    
Financing Receivable, Recorded Investment    
Mortgage Loans 916.7us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_LoanToValueRatioBelowOrEqualTo65PercentMember		 898.7us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_LoanToValueRatioBelowOrEqualTo65PercentMember
Loan To Value Ratio Above 65 To 75 Percent    
Financing Receivable, Recorded Investment    
Mortgage Loans 839.1us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_LoanToValueRatioAbove65To75PercentMember		 818.0us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_LoanToValueRatioAbove65To75PercentMember
Loan To Value Ratio Above 75 To 85 Percent    
Financing Receivable, Recorded Investment    
Mortgage Loans 86.6us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_LoanToValueRatioAbove75To85PercentMember		 102.3us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_LoanToValueRatioAbove75To85PercentMember
Loan To Value Ratio Above 85 Percent    
Financing Receivable, Recorded Investment    
Mortgage Loans 54.1us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_LoanToValueRatioAbove85PercentMember		 37.6us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_LoanToValueRatioAbove85PercentMember
Aa Credit Rating    
Financing Receivable, Recorded Investment    
Mortgage Loans 1.7us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_AaCreditRatingMember		 7.7us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_AaCreditRatingMember
A Credit Rating    
Financing Receivable, Recorded Investment    
Mortgage Loans 615.1us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_CreditRatingMember		 666.0us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_CreditRatingMember
Baa Credit Rating    
Financing Receivable, Recorded Investment    
Mortgage Loans 1,253.6us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_BaaCreditRatingMember		 1,156.7us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_BaaCreditRatingMember
Ba Credit Rating    
Financing Receivable, Recorded Investment    
Mortgage Loans 13.0us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_BaCreditRatingMember		 13.1us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_BaCreditRatingMember
B Credit Rating    
Financing Receivable, Recorded Investment    
Mortgage Loans $ 13.1us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_BCreditRatingMember		 $ 13.1us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_InternalCreditAssessmentAxis
= unm_BCreditRatingMember