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Stock-Based Compensation Expense (Detail) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award      
Allocated Share-based Compensation Expense $ 22.6us-gaap_AllocatedShareBasedCompensationExpense $ 23.6us-gaap_AllocatedShareBasedCompensationExpense $ 24.2us-gaap_AllocatedShareBasedCompensationExpense
Allocated Share-based Compensation Expense, Net of Tax 14.9us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax 15.6us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax 15.6us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax
Performance Share Units      
Share-based Compensation Arrangement by Share-based Payment Award      
Allocated Share-based Compensation Expense 2.5us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		 1.1us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember		 0us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_PerformanceSharesMember
Restricted Stock Units and Cash-Settled Awards      
Share-based Compensation Arrangement by Share-based Payment Award      
Allocated Share-based Compensation Expense 19.0us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= unm_RestrictedstockunitsandcashsettledawardsDomain		 21.0us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= unm_RestrictedstockunitsandcashsettledawardsDomain		 20.9us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= unm_RestrictedstockunitsandcashsettledawardsDomain
Stock Options      
Share-based Compensation Arrangement by Share-based Payment Award      
Allocated Share-based Compensation Expense 0.6us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		 1.0us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember		 2.7us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
Other Awards      
Share-based Compensation Arrangement by Share-based Payment Award      
Allocated Share-based Compensation Expense $ 0.5us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOwnershipPlanESOPPlanDomain		 $ 0.5us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOwnershipPlanESOPPlanDomain		 $ 0.6us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOwnershipPlanESOPPlanDomain