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Income Tax Additional Information (Detail) (USD $) | 12 Months Ended
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Dec. 31, 2014 Rate | Dec. 31, 2013 | Dec. 31, 2012
Federal Income Tax Note |  |  |
2013 UK Tax Rate | 23.00%unm_A2013UKTaxRate |  |
2014 UK Tax Rate | 21.00%unm_A2014UKTaxRate |  |
2015 UK Tax Rate | 20.00%unm_A2015UKTaxRate |  |
Income Tax Reconciliation, Change in Enacted Tax Rate |  | $ 6,300,000 us-gaap_IncomeTaxReconciliationChangeInEnactedTaxRate | $ 9,300,000 us-gaap_IncomeTaxReconciliationChangeInEnactedTaxRate
Excess of Financial Reporting Carrying Amount Over the Tax Basis of Investments in Foreign Subsidiaries, Provision Not Practicable | 1,000,000,000 unm_ExcessofFinancialReportingCarryingAmountOvertheTaxBasisofInvestmentsinForeignSubsidiariesProvisionNotPracticable |  |
Liability for unrecognized tax benefits | 10,400,000 us-gaap_LiabilityForUncertainTaxPositionsNoncurrent | 10,200,000 us-gaap_LiabilityForUncertainTaxPositionsNoncurrent | 15,000,000 us-gaap_LiabilityForUncertainTaxPositionsNoncurrent
Unrecognized Tax Benefits, Interest on Income Taxes Expense | 200,000 us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesExpense | (1,100,000)us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesExpense | (10,400,000)us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesExpense
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued | 1,000,000 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued | 800,000 us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued |
Tax Adjustments, Settlements, and Unusual Provisions | 17,500,000 us-gaap_TaxAdjustmentsSettlementsAndUnusualProvisions |  | 11,000,000 us-gaap_TaxAdjustmentsSettlementsAndUnusualProvisions
Impact of Reinstatement of Exemption of Active Finance Income | 900,000 unm_ImpactofReinstatementofExemptionofActiveFinanceIncome |  |
Operating Loss Carryforwards | 0 us-gaap_OperatingLossCarryforwards | 0 us-gaap_OperatingLossCarryforwards |
Income Taxes Paid, Net | 155,700,000 us-gaap_IncomeTaxesPaidNet | 398,100,000 us-gaap_IncomeTaxesPaidNet | 185,000,000 us-gaap_IncomeTaxesPaidNet
Maximum |  |  |
Federal Income Tax Note |  |  |
Liability (Refund) Adjustment from Settlement with Taxing Authority | $ 19,300,000 us-gaap_IncomeTaxExaminationLiabilityRefundAdjustmentFromSettlementWithTaxingAuthority / us-gaap_RangeAxis = us-gaap_MaximumMember |  |

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- Definition Excess of financial reporting carrying amount over the tax basis of investments in foreign subsidiaries where calculation of a provision for deferred tax was not practicable and a provision was not made. + References No definition available. + Details
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- Definition The amount of cash paid during the current period to foreign, federal, state, and local authorities as taxes on income, net of any cash received during the current period as refunds for the overpayment of taxes. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4297-108586 + Details
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- Definition The amount of the additional liability or refund received or expected based on a final settlement with a taxing authority. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319 + Details
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- Definition Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations, attributable to increase (decrease) in the income tax rates. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32687-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32698-109319 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 45 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=37586315&loc=d3e32059-109318 + Details
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- Definition The noncurrent portion of the amount recognized for uncertain tax positions as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.24) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Amount of operating loss carryforward, before tax effects, available to reduce future taxable income under enacted tax laws. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32559-109319 + Details
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- Definition Amount of increase (decrease) to previously recorded tax expense. Includes, but is not limited to, significant settlements of income tax disputes, and unusual tax positions or infrequent actions taken by the entity, including tax assessment reversal, and IRS tax settlement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 25 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=29652012&loc=d3e28446-109314 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 25 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=29652012&loc=d3e28511-109314 + Details
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- Definition Amount accrued for interest on an underpayment of income taxes and penalties related to a tax position claimed or expected to be claimed in the tax return. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32718-109319 + Details
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