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Mortgage Loans by Property Type and Geographic Region (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
Percent of Total 100.00%unm_PercentageOfLoanPortfolio 100.00%unm_PercentageOfLoanPortfolio
Apartment    
Mortgage Loans on Real Estate    
Mortgage Loans 110.1us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_ApartmentBuildingMember		 61.1us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_ApartmentBuildingMember
Percent of Total 5.90%unm_PercentageOfLoanPortfolio
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_ApartmentBuildingMember		 3.30%unm_PercentageOfLoanPortfolio
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_ApartmentBuildingMember
Industrial    
Mortgage Loans on Real Estate    
Mortgage Loans 542.9us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_IndustrialPropertyMember		 567.8us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_IndustrialPropertyMember
Percent of Total 29.20%unm_PercentageOfLoanPortfolio
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_IndustrialPropertyMember		 31.30%unm_PercentageOfLoanPortfolio
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_IndustrialPropertyMember
Office    
Mortgage Loans on Real Estate    
Mortgage Loans 794.0us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_OfficeBuildingMember		 776.5us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_OfficeBuildingMember
Percent of Total 42.80%unm_PercentageOfLoanPortfolio
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_OfficeBuildingMember		 42.80%unm_PercentageOfLoanPortfolio
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_OfficeBuildingMember
Retail    
Mortgage Loans on Real Estate    
Mortgage Loans 409.6us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_RetailSiteMember		 409.7us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_RetailSiteMember
Percent of Total 22.10%unm_PercentageOfLoanPortfolio
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_RetailSiteMember		 22.60%unm_PercentageOfLoanPortfolio
/ us-gaap_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis
= us-gaap_RetailSiteMember
New England    
Mortgage Loans on Real Estate    
Mortgage Loans 105.6us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ invest_InvestmentGeographicRegionAxis
= unm_NewEnglandMember		 100.9us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ invest_InvestmentGeographicRegionAxis
= unm_NewEnglandMember
Percent of Total 5.70%unm_PercentageOfLoanPortfolio
/ invest_InvestmentGeographicRegionAxis
= unm_NewEnglandMember		 5.60%unm_PercentageOfLoanPortfolio
/ invest_InvestmentGeographicRegionAxis
= unm_NewEnglandMember
Mid-Atlantic    
Mortgage Loans on Real Estate    
Mortgage Loans 179.4us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ invest_InvestmentGeographicRegionAxis
= unm_MiddleAtlanticMember		 191.5us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ invest_InvestmentGeographicRegionAxis
= unm_MiddleAtlanticMember
Percent of Total 9.70%unm_PercentageOfLoanPortfolio
/ invest_InvestmentGeographicRegionAxis
= unm_MiddleAtlanticMember		 10.50%unm_PercentageOfLoanPortfolio
/ invest_InvestmentGeographicRegionAxis
= unm_MiddleAtlanticMember
East North Central    
Mortgage Loans on Real Estate    
Mortgage Loans 210.6us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ invest_InvestmentGeographicRegionAxis
= unm_EastNorthCentralMember		 244.3us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ invest_InvestmentGeographicRegionAxis
= unm_EastNorthCentralMember
Percent of Total 11.40%unm_PercentageOfLoanPortfolio
/ invest_InvestmentGeographicRegionAxis
= unm_EastNorthCentralMember		 13.50%unm_PercentageOfLoanPortfolio
/ invest_InvestmentGeographicRegionAxis
= unm_EastNorthCentralMember
West North Central    
Mortgage Loans on Real Estate    
Mortgage Loans 166.2us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ invest_InvestmentGeographicRegionAxis
= unm_WestNorthCentralMember		 162.3us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ invest_InvestmentGeographicRegionAxis
= unm_WestNorthCentralMember
Percent of Total 8.90%unm_PercentageOfLoanPortfolio
/ invest_InvestmentGeographicRegionAxis
= unm_WestNorthCentralMember		 8.90%unm_PercentageOfLoanPortfolio
/ invest_InvestmentGeographicRegionAxis
= unm_WestNorthCentralMember
South Atlantic    
Mortgage Loans on Real Estate    
Mortgage Loans 453.6us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ invest_InvestmentGeographicRegionAxis
= unm_SouthAtlanticMember		 447.7us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ invest_InvestmentGeographicRegionAxis
= unm_SouthAtlanticMember
Percent of Total 24.40%unm_PercentageOfLoanPortfolio
/ invest_InvestmentGeographicRegionAxis
= unm_SouthAtlanticMember		 24.70%unm_PercentageOfLoanPortfolio
/ invest_InvestmentGeographicRegionAxis
= unm_SouthAtlanticMember
East South Central    
Mortgage Loans on Real Estate    
Mortgage Loans 75.3us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ invest_InvestmentGeographicRegionAxis
= unm_EastSouthCentralMember		 67.7us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ invest_InvestmentGeographicRegionAxis
= unm_EastSouthCentralMember
Percent of Total 4.10%unm_PercentageOfLoanPortfolio
/ invest_InvestmentGeographicRegionAxis
= unm_EastSouthCentralMember		 3.70%unm_PercentageOfLoanPortfolio
/ invest_InvestmentGeographicRegionAxis
= unm_EastSouthCentralMember
West South Central    
Mortgage Loans on Real Estate    
Mortgage Loans 215.6us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ invest_InvestmentGeographicRegionAxis
= unm_WestSouthCentralMember		 190.9us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ invest_InvestmentGeographicRegionAxis
= unm_WestSouthCentralMember
Percent of Total 11.60%unm_PercentageOfLoanPortfolio
/ invest_InvestmentGeographicRegionAxis
= unm_WestSouthCentralMember		 10.50%unm_PercentageOfLoanPortfolio
/ invest_InvestmentGeographicRegionAxis
= unm_WestSouthCentralMember
Mountain    
Mortgage Loans on Real Estate    
Mortgage Loans 116.0us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ invest_InvestmentGeographicRegionAxis
= unm_MountainMember		 101.9us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ invest_InvestmentGeographicRegionAxis
= unm_MountainMember
Percent of Total 6.20%unm_PercentageOfLoanPortfolio
/ invest_InvestmentGeographicRegionAxis
= unm_MountainMember		 5.60%unm_PercentageOfLoanPortfolio
/ invest_InvestmentGeographicRegionAxis
= unm_MountainMember
Pacific    
Mortgage Loans on Real Estate    
Mortgage Loans $ 334.3us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ invest_InvestmentGeographicRegionAxis
= unm_PacificMember		 $ 307.9us-gaap_MortgageLoansOnRealEstateCommercialAndConsumerNet
/ invest_InvestmentGeographicRegionAxis
= unm_PacificMember
Percent of Total 18.00%unm_PercentageOfLoanPortfolio
/ invest_InvestmentGeographicRegionAxis
= unm_PacificMember		 17.00%unm_PercentageOfLoanPortfolio
/ invest_InvestmentGeographicRegionAxis
= unm_PacificMember