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Unaudited Quarterly Results - Additional Information (Detail) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2013
Interim Period, Costs Not Allocable |  |  |  |
Reserve Increase for Long-term Care | $ 698.2 unm_ReserveIncreaseforLongtermCareBeforeTax |  | $ 698.2 unm_ReserveIncreaseforLongtermCareBeforeTax |
Reserve Charge for Long-term Care Product, After Tax | 453.8 unm_ReserveChargeforLongtermCareProductAfterTax |  |  |
Unclaimed Death Benefit Reserve Increase Before Taxes |  | 95.5 unm_UnclaimedDeathBenefitReserveIncreaseDecreaseBeforeTaxes |  | 95.5 unm_UnclaimedDeathBenefitReserveIncreaseDecreaseBeforeTaxes
Unclaimed Death Benefit Reserve Increase After Tax |  | 62.1 unm_UnclaimedDeathBenefitReserveIncreaseDecreaseAfterTax |  |
Waiver of Premium Benefit Reserve Decrease Before Tax |  | 85.0 unm_WaiverofPremiumBenefitReserveIncreaseDecreaseBeforeTax |  |
Waiver of Premium Benefit Reserve Decrease After Tax |  | 55.2 unm_WaiverofPremiumBenefitReserveIncreaseDecreaseAfterTax |  |
Pension Settlement Loss |  |  |  |
Interim Period, Costs Not Allocable |  |  |  |
Non-operating Retirement-related Loss | 64.4 unm_NonOperatingRetirementRelatedLoss / us-gaap_NatureOfExpenseAxis = unm_PensionSettlementLossMember |  |  |
Non-operating Retirement Related Loss, After Tax | $ 41.9 unm_NonoperatingRetirementRelatedLossAfterTax / us-gaap_NatureOfExpenseAxis = unm_PensionSettlementLossMember |  |  |

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- Definition Non-operating Retirement-related Loss + References No definition available. + Details
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- Definition Non-operating Retirement Related Loss, After Tax + References No definition available. + Details
Name: | unm_NonoperatingRetirementRelatedLossAfterTax
Namespace Prefix: | unm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition Reserve Charge for Long-term Care Product, After Tax + References No definition available. + Details
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Namespace Prefix: | unm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Reserve Increase for Long-term Care, Before Tax + References No definition available. + Details
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Namespace Prefix: | unm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Unclaimed Death Benefit Reserve Increase (Decrease) After Tax - the after tax increase (decrease) adjustment to claim reserves as a result of searching the Death Master File + References No definition available. + Details
Name: | unm_UnclaimedDeathBenefitReserveIncreaseDecreaseAfterTax
Namespace Prefix: | unm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Unclaimed Death Benefit Reserve Increase (Decrease) - the before tax increase (decrease) adjustment to reserves for unclaimed death benefits as a result of searching the Death Master File. + References No definition available. + Details
Name: | unm_UnclaimedDeathBenefitReserveIncreaseDecreaseBeforeTaxes
Namespace Prefix: | unm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Waiver of premium benefit reserve increase (decrease) after tax - the after tax adjustment increase (decrease) to reserves due to updated assumptions surrounding waiver of premium reserves + References No definition available. + Details
Name: | unm_WaiverofPremiumBenefitReserveIncreaseDecreaseAfterTax
Namespace Prefix: | unm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Waiver of premium benefit reserve increase (decrease) - the before tax adjustment increase (decrease) to reserves due to updated assumptions surrounding waiver of premium reserves + References No definition available. + Details
Name: | unm_WaiverofPremiumBenefitReserveIncreaseDecreaseBeforeTax
Namespace Prefix: | unm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Details
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- Details
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Period Type: |