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Stock-Based Compensation - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award |  |  |
Cash Received Under Share-based Payment Arrangements | $ 12.3 us-gaap_ProceedsFromIssuanceOfSharesUnderIncentiveAndShareBasedCompensationPlansIncludingStockOptions | $ 11.4 us-gaap_ProceedsFromIssuanceOfSharesUnderIncentiveAndShareBasedCompensationPlansIncludingStockOptions | $ 4.9 us-gaap_ProceedsFromIssuanceOfSharesUnderIncentiveAndShareBasedCompensationPlansIncludingStockOptions
Stock Incentive Plan 2012 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award |  |  |
Number of Shares Authorized | 20,000,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / us-gaap_PlanNameAxis = unm_StockIncentivePlan2012Domain |  |
Number of Shares for Each Full Value Award | 1.76 unm_NumberofSharesforEachFullValueAward / us-gaap_PlanNameAxis = unm_StockIncentivePlan2012Domain |  |
Total Duration of the Stock Incentive Plan | 8 years |  |
Award Vesting Period | 3 years |  |
Number of Shares Available for Grant | 17,050,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant / us-gaap_PlanNameAxis = unm_StockIncentivePlan2012Domain |  |
Stock Incentive Plan 2007 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award |  |  |
Number of Shares Authorized | 35,000,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / us-gaap_PlanNameAxis = unm_StockIncentivePlan2007Member |  |
Number of Shares for Each Full Value Award | 2.7 unm_NumberofSharesforEachFullValueAward / us-gaap_PlanNameAxis = unm_StockIncentivePlan2007Member |  |
Total Duration of the Stock Incentive Plan | 8 years |  |
Award Vesting Period | 3 years |  |
Performance Share Units |  |  |
Share-based Compensation Arrangement by Share-based Payment Award |  |  |
Award Vesting Period | 3 years |  |
Weighted Average Grant Date Fair Value of Shares Granted | $ 34.72 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember | $ 25.26 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |
Compensation Cost Not yet Recognized | 5.2 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |
Period for Recognition of Compensation Cost Not Yet Recognized | 1 year 272 days |  |
Percentage Stock-based Awards Expected to Vest | 100.00%unm_PercentageofSharesExpectedtoVestforUnrecognizedCompensationCostandCompensationExpense / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember |  |
Restricted Stock Units (RSUs) |  |  |
Share-based Compensation Arrangement by Share-based Payment Award |  |  |
Weighted Average Grant Date Fair Value of Shares Granted | $ 33.77 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember | $ 24.68 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember | $ 22.96 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember
Compensation Cost Not yet Recognized | 10.9 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember |  |
Period for Recognition of Compensation Cost Not Yet Recognized | 292 days |  |
Fair Value of Awards Vested During the Period | 19.8 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember | 18.3 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember | 19.5 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember
Cash Settled Awards |  |  |
Share-based Compensation Arrangement by Share-based Payment Award |  |  |
Weighted Average Grant Date Fair Value of Shares Granted | $ 33.85 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = unm_CashSettledAwardsMember | $ 24.22 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = unm_CashSettledAwardsMember | $ 23.23 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = unm_CashSettledAwardsMember
Compensation Cost Not yet Recognized | 0 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized / us-gaap_AwardTypeAxis = unm_CashSettledAwardsMember |  |
Fair Value of Awards Vested During the Period | 2.1 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = unm_CashSettledAwardsMember | 2.4 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = unm_CashSettledAwardsMember | 1.5 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue / us-gaap_AwardTypeAxis = unm_CashSettledAwardsMember
Stock Options |  |  |
Share-based Compensation Arrangement by Share-based Payment Award |  |  |
Period for Recognition of Compensation Cost Not Yet Recognized | 0 years 110 days |  |
Stock Option Compensation Not yet Recognized | $ 0.2 us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember |  |
Options Granted | 0.00 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockOptionMember |  |
Minimum | Performance Share Units |  |  |
Share-based Compensation Arrangement by Share-based Payment Award |  |  |
Percentage of Stock-based Awards Possible to be Earned | 0.00%unm_PercentageofStockbasedAwardsPossibletobeEarned / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / us-gaap_RangeAxis = us-gaap_MinimumMember |  |
Minimum | Restricted Stock Units (RSUs) |  |  |
Share-based Compensation Arrangement by Share-based Payment Award |  |  |
Award Vesting Period | 1 year |  |
Minimum | Cash Settled Awards |  |  |
Share-based Compensation Arrangement by Share-based Payment Award |  |  |
Award Vesting Period | 1 year |  |
Minimum | Stock Options |  |  |
Share-based Compensation Arrangement by Share-based Payment Award |  |  |
Award Vesting Period | 1 year |  |
Maximum | Performance Share Units |  |  |
Share-based Compensation Arrangement by Share-based Payment Award |  |  |
Percentage of Stock-based Awards Possible to be Earned | 180.00%unm_PercentageofStockbasedAwardsPossibletobeEarned / us-gaap_AwardTypeAxis = us-gaap_PerformanceSharesMember / us-gaap_RangeAxis = us-gaap_MaximumMember |  |
Maximum | Restricted Stock Units (RSUs) |  |  |
Share-based Compensation Arrangement by Share-based Payment Award |  |  |
Award Vesting Period | 3 years |  |
Maximum | Cash Settled Awards |  |  |
Share-based Compensation Arrangement by Share-based Payment Award |  |  |
Award Vesting Period | 3 years |  |
Maximum | Stock Options |  |  |
Share-based Compensation Arrangement by Share-based Payment Award |  |  |
Award Vesting Period | 3 years |  |

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- Definition The percentage of shares expected to vest in the calculation for unrecognized compensation cost and compensation expense. + References No definition available. + Details
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- Definition Percentage of Stock-based Awards Possible to be Earned + References No definition available. + Details
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- Definition Total Duration of the Stock Incentive Plan + References No definition available. + Details
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- Definition Unrecognized cost of unvested share-based compensation awards. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Unrecognized cost of unvested options awarded to employees as compensation. + References No definition available. + Details
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- Definition The total cash inflow associated with the amount received from holders to acquire the entity's shares under incentive and share awards, including stock option exercises. This item inherently excludes any excess tax benefit, which the entity may have realized and reported separately. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Financing Activities -URI http://asc.fasb.org/extlink&oid=6513228 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3255-108585 + Details
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- Definition Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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