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Investments - Additional Information (Detail) (USD $)
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Schedule of Investments      
Fair Value of Fixed Maturity Securities $ 45,852,700,000us-gaap_AvailableForSaleSecuritiesDebtSecurities   $ 45,064,900,000us-gaap_AvailableForSaleSecuritiesDebtSecurities
Fixed Maturity Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss 0unm_AvailableForSaleSecuritiesOtherThanTemporaryImpairmentsInAccumulatedOtherComprehensiveIncomeLoss   0unm_AvailableForSaleSecuritiesOtherThanTemporaryImpairmentsInAccumulatedOtherComprehensiveIncomeLoss
Commitment to Fund Private Placement Fixed Maturity Securities 59,000,000unm_CommitmentToFundPrivatePlacementFixedMaturitySecurities    
Carrying Amount of Variable Interest Entity Investments 452,400,000us-gaap_VariableInterestEntityNonconsolidatedCarryingAmountAssets    
Unfunded Commitment to Fund Investment Partnerships 8,800,000unm_UnfundedCommitmentToFundTaxCreditPartnershipInvestments    
Commitment to Fund Partnership Equity Investments 159,000,000unm_CommitmentToFundPartnershipEquityInvestments    
Other Long-term Investments 603,100,000us-gaap_OtherLongTermInvestments 530,800,000us-gaap_OtherLongTermInvestments 545,000,000us-gaap_OtherLongTermInvestments
Commitments to Fund Special Purpose Entities 0unm_CommitmentsToFundSpecialPurposeEntities    
Amount Funded to Special Purpose Entity Partnerships nor did we fund any amounts to the partnership nor did we fund any amounts to the partnership  
Mortgage Loan Policy is not to Exceed a Loan-to-value Ratio, Percent 75.00%unm_LoanToValueRatioMaximum    
Loan to Value Ratio Update Frequency 3 years    
Mortgage Loan Inspection Frequency 2 years    
Mortage Loan Policy Debt Service Coverage Ratio Lower Range 125.00%unm_MortageLoanPolicyDebtServiceCoverageRatioLowerRange    
Mortgage Loans Issuance, Term Years 25 years    
Number of Financing Receivables on Nonaccrual Status 0unm_CountOfFinancingReceivableRecordedInvestmentNonaccrualStatus   0unm_CountOfFinancingReceivableRecordedInvestmentNonaccrualStatus
Number of Changes to Accounting Policy for Estimating Credit Losses on Mortgage Loans 0unm_NumberOfChangesToAccountingPolicyForEstimatingCreditLossesOnMortgageLoans    
Average Recorded Investment in Impaired Mortgage Loans 13,100,000us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment 19,100,000us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment  
Interest Income on Mortgage Loans Subsequent to Impairment 200,000us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod 300,000us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod  
Committments to Fund Commercial Mortgage Loans 55,800,000unm_CommittmentsToFundCommercialMortgageLoans    
Repurchase agreements - Typical Days Outstanding 30 days    
Minimum Percent of Fair Value of Securities Loaned or Securities Purchased Under Repurchase Agreements to be Maintained as Collateral 102.00%unm_CollateralRequirementsMinimumPercentageCollateralToFairValueOfSecuritiesLoanedOrPurchasedUnderRepurchaseAgreements    
Net Securities Borrowed Reported in Balance Sheet 181,100,000us-gaap_SecuritiesBorrowed   176,500,000us-gaap_SecuritiesBorrowed
Outstanding Repurchase Agreements 0us-gaap_SecuritiesSoldUnderAgreementsToRepurchase   0us-gaap_SecuritiesSoldUnderAgreementsToRepurchase
Federal Home Loan Bank Stock 18,200,000us-gaap_FederalHomeLoanBankStock    
Federal Home Loan Bank (FHLB) Funding Agreements 26,000,000us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankAmountOfAdvancesByBranch    
Fixed Maturity Securities      
Schedule of Investments      
Fair Value of Fixed Maturity Securities 45,852,700,000us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		   45,064,900,000us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Accumulated Gross Unrealized Gain on Securities 6,916,700,000us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		   6,386,300,000us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Accumulated Gross Unrealized Loss on Securities 89,900,000us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		   124,800,000us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember
Collateral Pledged to Federal Home Loan Bank (FHLB) 49,800,000us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_DebtSecuritiesMember		    
Mortgage Loans as Collateral      
Schedule of Investments      
Collateral Pledged to Federal Home Loan Bank (FHLB) 113,300,000us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MortgageBackedSecuritiesMember		    
Off Balance Sheet Amount      
Schedule of Investments      
Cash Collateral for Borrowed Securities 25,300,000us-gaap_CashCollateralForBorrowedSecurities
/ us-gaap_BalanceSheetLocationAxis
= unm_OffBalanceSheetAmountMember		   58,400,000us-gaap_CashCollateralForBorrowedSecurities
/ us-gaap_BalanceSheetLocationAxis
= unm_OffBalanceSheetAmountMember
Securities Received as Collateral 161,500,000us-gaap_SecuritiesReceivedAsCollateral
/ us-gaap_BalanceSheetLocationAxis
= unm_OffBalanceSheetAmountMember		   128,500,000us-gaap_SecuritiesReceivedAsCollateral
/ us-gaap_BalanceSheetLocationAxis
= unm_OffBalanceSheetAmountMember
Commercial Real Estate Portfolio Segment      
Schedule of Investments      
Unpaid Principal Balance of Impaired Mortgage Loan 14,600,000us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember		   14,600,000us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
Impaired Mortgage Loans, Related Allowance 1,500,000us-gaap_ImpairedFinancingReceivableRelatedAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember		   1,500,000us-gaap_ImpairedFinancingReceivableRelatedAllowance
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
Impaired Mortgage Loans, with Related Allowance, Recorded Investment 13,100,000us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember		   13,100,000us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
Commercial Real Estate Portfolio Segment      
Schedule of Investments      
Number of Loan Modifications due to Loan Foreclosure 0us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember		    
Recorded Investment in Mortgage Loan Modifications Due to Foreclosure 18,100,000us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember		    
Troubled Debt Restructuring Write-Down 3,000,000us-gaap_FinancingReceivablesImpairedTroubledDebtRestructuringWriteDown
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember		    
Special Purpose Entity | Bonds      
Schedule of Investments      
Fair Value of Fixed Maturity Securities 146,700,000us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ unm_EntityClassificationAxis
= unm_SpecialPurposeEntityMember
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_BondsMember		    
Special Purpose Entity | Partnership Interest      
Schedule of Investments      
Other Long-term Investments 1,600,000us-gaap_OtherLongTermInvestments
/ unm_EntityClassificationAxis
= unm_SpecialPurposeEntityMember
/ us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis
= us-gaap_PartnershipInterestMember		    
Partnership Interests In Tax Credit Investments      
Schedule of Investments      
Carrying Amount of Variable Interest Entity Investments 230,000,000us-gaap_VariableInterestEntityNonconsolidatedCarryingAmountAssets
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= unm_PartnershipInterestsInTaxCreditInvestmentsMember		    
Equity Method Investments      
Schedule of Investments      
Carrying Amount of Variable Interest Entity Investments 222,400,000us-gaap_VariableInterestEntityNonconsolidatedCarryingAmountAssets
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_EquityMethodInvestmentsMember		    
External Credit Rating, Investment Grade      
Schedule of Investments      
Fair Value of Fixed Maturity Securities 42,085,700,000us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_ExternalCreditRatingByGroupingAxis
= us-gaap_ExternalCreditRatingInvestmentGradeMember		    
Accumulated Gross Unrealized Gain on Securities 6,709,700,000us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_ExternalCreditRatingByGroupingAxis
= us-gaap_ExternalCreditRatingInvestmentGradeMember		    
Accumulated Gross Unrealized Loss on Securities 17,500,000us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_ExternalCreditRatingByGroupingAxis
= us-gaap_ExternalCreditRatingInvestmentGradeMember		    
Percent of Fixed Maturity Securities in Unrealized Loss Position 19.50%unm_FixedMaturitySecuritiesInUnrealizedLossPositionPercentOfTotal
/ us-gaap_ExternalCreditRatingByGroupingAxis
= us-gaap_ExternalCreditRatingInvestmentGradeMember		    
Number of Fixed Maturity Securities in an Unrealized Loss Position 42us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_ExternalCreditRatingByGroupingAxis
= us-gaap_ExternalCreditRatingInvestmentGradeMember		    
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One year 9unm_NumberOfFixedMaturitySecuritiesInUnrealizedLossPositionContinuouslyForOverOneYear
/ us-gaap_ExternalCreditRatingByGroupingAxis
= us-gaap_ExternalCreditRatingInvestmentGradeMember		    
External Credit Rating, Non Investment Grade      
Schedule of Investments      
Fair Value of Fixed Maturity Securities 3,767,000,000us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_ExternalCreditRatingByGroupingAxis
= us-gaap_ExternalCreditRatingNonInvestmentGradeMember		    
Accumulated Gross Unrealized Gain on Securities 207,000,000us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_ExternalCreditRatingByGroupingAxis
= us-gaap_ExternalCreditRatingNonInvestmentGradeMember		    
Accumulated Gross Unrealized Loss on Securities $ 72,400,000us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_ExternalCreditRatingByGroupingAxis
= us-gaap_ExternalCreditRatingNonInvestmentGradeMember		    
Percent of Fixed Maturity Securities in Unrealized Loss Position 80.50%unm_FixedMaturitySecuritiesInUnrealizedLossPositionPercentOfTotal
/ us-gaap_ExternalCreditRatingByGroupingAxis
= us-gaap_ExternalCreditRatingNonInvestmentGradeMember		    
Number of Fixed Maturity Securities in an Unrealized Loss Position 52us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
/ us-gaap_ExternalCreditRatingByGroupingAxis
= us-gaap_ExternalCreditRatingNonInvestmentGradeMember		    
Number of Fixed Maturity Securities in an Unrealized Loss Position Continuously for Over One year 13unm_NumberOfFixedMaturitySecuritiesInUnrealizedLossPositionContinuouslyForOverOneYear
/ us-gaap_ExternalCreditRatingByGroupingAxis
= us-gaap_ExternalCreditRatingNonInvestmentGradeMember