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Fair Value of Financial Instruments Additional Information (Detail) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Ceded Policy Loans | $ 3,068.4 unm_PolicyLoansCededtoReinsurers | $ 3,043.7 unm_PolicyLoansCededtoReinsurers |
Long-term Debt | 2,628.7 us-gaap_LongTermDebtAndCapitalLeaseObligations | 2,612.0 us-gaap_LongTermDebtAndCapitalLeaseObligations |
Gain (Loss) on Embedded Derivative | 3.3 us-gaap_EmbeddedDerivativeGainLossOnEmbeddedDerivativeNet | 30.7 us-gaap_EmbeddedDerivativeGainLossOnEmbeddedDerivativeNet | 51.8 us-gaap_EmbeddedDerivativeGainLossOnEmbeddedDerivativeNet
Other Than Fair Value Inputs Level 1 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Percentage of Total Fair Value of Fixed Maturities Securities | 93.40%unm_AvailableForSaleSecuritiesPercentOfTotalFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = unm_FairValueInputsOtherThanLevel1Member |  |
Fair Value, Inputs, Level 1 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Long-term Debt | 849.7 us-gaap_LongTermDebtAndCapitalLeaseObligations / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 1,329.2 us-gaap_LongTermDebtAndCapitalLeaseObligations / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |
Percentage of Total Fair Value of Fixed Maturities Securities | 6.60%unm_AvailableForSaleSecuritiesPercentOfTotalFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |  |
Fair Value, Inputs, Level 2 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Long-term Debt | $ 2,062.9 us-gaap_LongTermDebtAndCapitalLeaseObligations / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | $ 1,495.2 us-gaap_LongTermDebtAndCapitalLeaseObligations / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |
Fair Value, Inputs, Level 2 | Pricing Service |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Percentage of Total Fair Value of Fixed Maturities Securities | 78.10%unm_AvailableForSaleSecuritiesPercentOfTotalFairValue / unm_FairValueAssetsAndLiabilitiesComponentsAxis = unm_PricingServiceMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |  |
Fair Value, Inputs, Level 2 | Other Observable Market Data |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Percentage of Total Fair Value of Fixed Maturities Securities | 3.50%unm_AvailableForSaleSecuritiesPercentOfTotalFairValue / unm_FairValueAssetsAndLiabilitiesComponentsAxis = unm_OtherObservableMarketDataMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |  |
Fair Value Inputs Level 2 Or Level 3 |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Percentage of Total Fair Value of Fixed Maturities Securities | 11.80%unm_AvailableForSaleSecuritiesPercentOfTotalFairValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = unm_FairValueInputsLevel2OrLevel3Member |  |
Minimum |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Private Equity Partnership Future Liquidation Term | 1 year |  |
Maximum |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |
Private Equity Partnership Future Liquidation Term | 13 years |  |

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- Definition Available For Sale Securities, Percent Of Total Fair Value + References No definition available. + Details
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- Definition Loans ceded to reinsurers. These loans use insurance policy funds such as cash surrender values as collateral. + References No definition available. + Details
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- Definition Net Increase or Decrease in the fair value of the embedded derivative or group of embedded derivatives included in earnings in the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624171-113959 + Details
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