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Investments - Additional Information (Detail) (USD $) | 12 Months Ended | 3 Months Ended
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Mar. 31, 2015
Schedule of Investments |  |  |  |
Fair value of fixed maturity securities | $ 45,064,900,000 us-gaap_AvailableForSaleSecuritiesDebtSecurities | $ 42,344,400,000 us-gaap_AvailableForSaleSecuritiesDebtSecurities |  |
Fixed Maturity Securities Other Than Temporary Impairments in Accumulated Other Comprehensive Income Loss | 0 unm_AvailableForSaleSecuritiesOtherThanTemporaryImpairmentsInAccumulatedOtherComprehensiveIncomeLoss | 0 unm_AvailableForSaleSecuritiesOtherThanTemporaryImpairmentsInAccumulatedOtherComprehensiveIncomeLoss |  |
Commitment to fund private placement fixed maturity securities | 15,000,000 unm_CommitmentToFundPrivatePlacementFixedMaturitySecurities |  |  |
Carrying amount of variable interest entity investments | 484,100,000 us-gaap_VariableInterestEntityNonconsolidatedCarryingAmountAssets |  |  |
Unfunded commitment to fund Investment Partnerships | 12,800,000 unm_UnfundedCommitmentToFundTaxCreditPartnershipInvestments |  |  |
Commitment to fund partnership Equity Investments | 161,600,000 unm_CommitmentToFundPartnershipEquityInvestments |  |  |
Other Long-term Investments | 591,900,000 us-gaap_OtherLongTermInvestments | 566,000,000 us-gaap_OtherLongTermInvestments |  |
Commitments to Fund Special Purpose Entities | 0 unm_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 | 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 |  |  |
Foreclosed Real Estate Expense | 0 us-gaap_GainsLossesOnSalesOfOtherRealEstate | 0 us-gaap_GainsLossesOnSalesOfOtherRealEstate | 0 us-gaap_GainsLossesOnSalesOfOtherRealEstate |
Financing Receivable, Recorder Investment, Nonaccrual status | 0 unm_CountOfFinancingReceivableRecordedInvestmentNonaccrualStatus | 0 unm_CountOfFinancingReceivableRecordedInvestmentNonaccrualStatus | 0 unm_CountOfFinancingReceivableRecordedInvestmentNonaccrualStatus |
Number of Changes to Accounting Policy for Estimating Credit Losses on Mortgage Loans | 0 unm_NumberOfChangesToAccountingPolicyForEstimatingCreditLossesOnMortgageLoans |  |  |
Average Recorded Investment of Impaired Mortgage Loans | 26,700,000 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment | 14,900,000 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment | 19,100,000 us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment |
Interest Income on Mortgage Loans Subsequent to Impairment | 1,000,000 us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod | 800,000 us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod | 800,000 us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod |
Committments to Fund Commercial Mortgage Loans | 49,300,000 unm_CommittmentsToFundCommercialMortgageLoans |  |  |
Repurchase agreements - Typical Days outstanding | 30 |  |  |
Minimum percent of the fair value of securities loaned or securities purchased under repurchase agreements be maintained as collateral | 102.00%unm_CollateralRequirementsMinimumPercentageCollateralToFairValueOfSecuritiesLoanedOrPurchasedUnderRepurchaseAgreements |  |  |
Gross Securities Borrowed | 176,500,000 us-gaap_SecuritiesBorrowedGross | 201,600,000 us-gaap_SecuritiesBorrowedGross |  |
Net Securities Borrowed Reported in Balance Sheet | 176,500,000 us-gaap_SecuritiesBorrowed | 201,600,000 us-gaap_SecuritiesBorrowed |  |
Outstanding Repurchase Agreements | 0 us-gaap_SecuritiesSoldUnderAgreementsToRepurchase | 0 us-gaap_SecuritiesSoldUnderAgreementsToRepurchase |  |
Off Balance Sheet Amount |  |  |  |
Schedule of Investments |  |  |  |
Cash Collateral for Borrowed Securities | 58,400,000 us-gaap_CashCollateralForBorrowedSecurities / us-gaap_BalanceSheetLocationAxis = unm_OffBalanceSheetAmountMember | 76,500,000 us-gaap_CashCollateralForBorrowedSecurities / us-gaap_BalanceSheetLocationAxis = unm_OffBalanceSheetAmountMember |  |
Securities Received as Collateral | 128,500,000 us-gaap_SecuritiesReceivedAsCollateral / us-gaap_BalanceSheetLocationAxis = unm_OffBalanceSheetAmountMember | 132,900,000 us-gaap_SecuritiesReceivedAsCollateral / us-gaap_BalanceSheetLocationAxis = unm_OffBalanceSheetAmountMember |  |
Commercial Real Estate Portfolio Segment |  |  |  |
Schedule of Investments |  |  |  |
Unpaid Principal Balance of Impaired Mortgage Loan | 14,600,000 us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember | 14,600,000 us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceUnpaidPrincipalBalance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember |  |
Impaired Financing Receivable, Related Allowance | 1,500,000 us-gaap_ImpairedFinancingReceivableRelatedAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember | 1,500,000 us-gaap_ImpairedFinancingReceivableRelatedAllowance / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember |  |
Impaired Financing Receivable, with Related Allowance, Recorded Investment | 13,100,000 us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember | 13,100,000 us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment / us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember |  |
Special Purpose Entity | Bonds |  |  |  |
Schedule of Investments |  |  |  |
Fair value of fixed maturity securities | 143,900,000 us-gaap_AvailableForSaleSecuritiesDebtSecurities / unm_EntityClassificationAxis = unm_SpecialPurposeEntityMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_BondsMember | 136,200,000 us-gaap_AvailableForSaleSecuritiesDebtSecurities / unm_EntityClassificationAxis = unm_SpecialPurposeEntityMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_BondsMember |  |
Special Purpose Entity | Partnership Interest |  |  |  |
Schedule of Investments |  |  |  |
Other Long-term Investments | 1,400,000 us-gaap_OtherLongTermInvestments / unm_EntityClassificationAxis = unm_SpecialPurposeEntityMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_PartnershipInterestMember | 4,400,000 us-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 | 289,000,000 us-gaap_VariableInterestEntityNonconsolidatedCarryingAmountAssets / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = unm_PartnershipInterestsInTaxCreditInvestmentsMember |  |  |
Equity Method Investments |  |  |  |
Schedule of Investments |  |  |  |
Carrying amount of variable interest entity investments | 195,100,000 us-gaap_VariableInterestEntityNonconsolidatedCarryingAmountAssets / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_EquityMethodInvestmentsMember |  |  |
Commercial Real Estate Portfolio Segment |  |  |  |
Schedule of Investments |  |  |  |
Recorded Investment in Mortgage Loan Modifications Due to Foreclosure | 18,100,000 us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember | 4,300,000 us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember | 17,300,000 us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1 / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember |
Troubled Debt Restructuring Write-down | 3,000,000 us-gaap_FinancingReceivablesImpairedTroubledDebtRestructuringWriteDown / us-gaap_FinancingReceivableTroubledDebtRestructuringAxis = us-gaap_CommercialRealEstatePortfolioSegmentMember |  |  |
External Credit Rating, Investment Grade |  |  |  |
Schedule of Investments |  |  |  |
Fair value of fixed maturity securities | 41,539,700,000 us-gaap_AvailableForSaleSecuritiesDebtSecurities / us-gaap_ExternalCreditRatingByGroupingAxis = us-gaap_ExternalCreditRatingInvestmentGradeMember |  |  |
Accumulated Gross Unrealized Gain on Securities | 6,238,000,000 us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_ExternalCreditRatingByGroupingAxis = us-gaap_ExternalCreditRatingInvestmentGradeMember |  |  |
Accumulated Gross Unrealized Loss on Securities | 47,500,000 us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_ExternalCreditRatingByGroupingAxis = us-gaap_ExternalCreditRatingInvestmentGradeMember |  |  |
Percent of Fixed Maturity Securities in Unrealized Loss Position | 38.10%unm_FixedMaturitySecuritiesInUnrealizedLossPositionPercentOfTotal / us-gaap_ExternalCreditRatingByGroupingAxis = us-gaap_ExternalCreditRatingInvestmentGradeMember |  |  |
Number of fixed maturity securities that were in an unrealized loss position | 75 us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1 / us-gaap_ExternalCreditRatingByGroupingAxis = us-gaap_ExternalCreditRatingInvestmentGradeMember |  |  |
Number of fixed maturity securities in an unrealized loss position continuously for over one year | 31 unm_NumberOfFixedMaturitySecuritiesInUnrealizedLossPositionContinuouslyForOverOneYear / us-gaap_ExternalCreditRatingByGroupingAxis = us-gaap_ExternalCreditRatingInvestmentGradeMember |  |  |
External Credit Rating, Non Investment Grade |  |  |  |
Schedule of Investments |  |  |  |
Fair value of fixed maturity securities | 3,525,200,000 us-gaap_AvailableForSaleSecuritiesDebtSecurities / us-gaap_ExternalCreditRatingByGroupingAxis = us-gaap_ExternalCreditRatingNonInvestmentGradeMember |  |  |
Accumulated Gross Unrealized Gain on Securities | 148,300,000 us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax / us-gaap_ExternalCreditRatingByGroupingAxis = us-gaap_ExternalCreditRatingNonInvestmentGradeMember |  |  |
Accumulated Gross Unrealized Loss on Securities | 77,300,000 us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax / us-gaap_ExternalCreditRatingByGroupingAxis = us-gaap_ExternalCreditRatingNonInvestmentGradeMember |  |  |
Percent of Fixed Maturity Securities in Unrealized Loss Position | 61.90%unm_FixedMaturitySecuritiesInUnrealizedLossPositionPercentOfTotal / us-gaap_ExternalCreditRatingByGroupingAxis = us-gaap_ExternalCreditRatingNonInvestmentGradeMember |  |  |
Number of fixed maturity securities that were in an unrealized loss position | 68 us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1 / us-gaap_ExternalCreditRatingByGroupingAxis = us-gaap_ExternalCreditRatingNonInvestmentGradeMember |  |  |
Number of fixed maturity securities in an unrealized loss position continuously for over one year | 15 unm_NumberOfFixedMaturitySecuritiesInUnrealizedLossPositionContinuouslyForOverOneYear / us-gaap_ExternalCreditRatingByGroupingAxis = us-gaap_ExternalCreditRatingNonInvestmentGradeMember |  |  |
Subsequent Event |  |  |  |
Schedule of Investments |  |  |  |
Federal Home Loan Bank Stock |  |  |  | 17,700,000 us-gaap_PaymentsToAcquireFederalHomeLoanBankStock / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember
Purchase Commitment |  |  |  |
Schedule of Investments |  |  |  |
Federal Home Loan Bank Stock | $ 12,500,000 us-gaap_PaymentsToAcquireFederalHomeLoanBankStock / us-gaap_PurchaseCommitmentExcludingLongtermCommitmentAxis = us-gaap_PurchaseCommitmentMember |  |  |

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- Definition Available-for-Sale Securities Other-Than-Temporary Impairments in Accumulated Other Comprehensive Income (Loss) + References No definition available. + Details
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- Definition Collateral Requirements, Minimum Percentage, Collateral To Fair Value Of Securities Loaned Or Purchased Under Repurchase Agreements + References No definition available. + Details
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- Definition Commitments to Fund Special Purpose Entities + References No definition available. + Details
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- Definition Commitment To Fund Partnership Equity Investments + References No definition available. + Details
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- Definition Committments to Fund Commercial Mortgage Loans + References No definition available. + Details
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- Definition Mortage Loan Policy Debt Service coverage ratio lower range + References No definition available. + Details
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- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563 + Details
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- Definition Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563 + Details
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- Definition Amount of investment related to financing receivables modified by troubled debt restructurings within the previous 12 months, for which there was a payment default. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 34 -URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953682-111524 + Details
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- Definition The net gain (loss) resulting from sales and other disposals of other real estate owned, increases (decreases) in the valuation allowance for foreclosed real estate, and write-downs of other real estate owned after acquisition or physical possession. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.14(d)) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 14 -Subparagraph d -Article 9 + Details
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- Definition Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5212-111524 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5258-111524 + Details
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- Definition Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5212-111524 + Details
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