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Unrecognized tax benefits (Detail) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Income Tax [Abstract] |  |  |
Balance at Beginning of Year | $ 18.4 us-gaap_UnrecognizedTaxBenefits | $ 17.5 us-gaap_UnrecognizedTaxBenefits | $ 86.9 us-gaap_UnrecognizedTaxBenefits
Tax Positions Related to Prior Years - Additions | 1.7 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions | 5.7 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions | 13.3 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions
Tax Positions Related to Prior Years - Subtractions | 0 us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions | 0 us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions | (0.6)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions
Settlements with Tax Authorities | (0.6)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities | (4.8)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities | (23.5)us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities
Lapses of Statute of Limitations | 0 us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations | 0 us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations | (61.1)us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations
Tax Positions Taken During Current Year | 0.3 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions | 0 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions | 2.5 us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions
Balance at End of Year | 19.8 us-gaap_UnrecognizedTaxBenefits | 18.4 us-gaap_UnrecognizedTaxBenefits | 17.5 us-gaap_UnrecognizedTaxBenefits
Less Tax Attributable to Temporary Items Included Above | (10.4)unm_TaxAttributableToTemporaryItemsIncludedAbove | (10.2)unm_TaxAttributableToTemporaryItemsIncludedAbove | (15.0)unm_TaxAttributableToTemporaryItemsIncludedAbove
Total Unrecognized Tax Benefits that if Recognized Would Affect the Effective Tax Rate | $ 9.4 us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate | $ 8.2 us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate | $ 2.5 us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate

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- Definition Unrecognized Tax Benefits Increases (Decreases) Resulting From Other Deductions + References No definition available. + Details
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- Definition Amount of unrecognized tax benefits pertaining to uncertain tax positions taken in tax returns. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Unrecognized Tax Benefit -URI http://asc.fasb.org/extlink&oid=6527854 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 + Details
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- Definition Amount of decrease in unrecognized tax benefits resulting from tax positions taken in prior period tax returns. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 + Details
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- Definition Amount of decrease in unrecognized tax benefits resulting from settlements with taxing authorities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 + Details
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- Definition Amount of increase in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period 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 15A -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 + Details
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- Definition Amount of increase in unrecognized tax benefits resulting from tax positions taken in prior period tax returns. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 + Details
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- Definition Amount of decrease in unrecognized tax benefits resulting from lapses of applicable statutes of limitations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (a)(4) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 + Details
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- Definition The total amount of unrecognized tax benefits that, if recognized, would affect the effective tax rate. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 15A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6907707&loc=SL6600010-109319 + Details
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