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Stockholders' Equity and Earnings Per Common Share - 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
Stockholders Equity and Earnings Per Common Share Additional Information [Abstract] |  |  |
Exercise Price Floor on Outstanding Stock Options | $ 11.37 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit |  |
Exercise Price Ceiling on Outstanding Stock Options | $ 26.29 us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit |  |
Outstanding Restricted Stock Unit Grant Prices Lower Limit | $ 19.38 unm_OutstandingRestrictedStockUnitGrantPricesLowerLimit |  |
Outstanding Restricted Stock Unit Grant Prices Upper Limit | $ 35.47 unm_OutstandingRestrictedStockUnitGrantPricesUpperLimit |  |
Outstanding Performance Share Unit Awards Grant Price Range Lower Limit | $ 23.97 unm_OutstandingPerformanceShareUnitAwardsGrantPriceRangeLowerLimit |  |
Outstanding Performance Share Unit Awards Grant Price Range Upper Limit | $ 33.86 unm_OutstandingPerformanceShareUnitAwardsGrantPriceRangeUpperLimit |  |
Antidilutive Securities Excluded from Computation of Earnings Per Share | 100,000 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount | 100,000 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount | 2,500,000 us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
Shares of Treasury Stock Retired | 60,000,000 us-gaap_TreasuryStockSharesRetired |  |
Cost of Retired Treasury Stock | $ 1,451.7 us-gaap_TreasuryStockRetiredCostMethodAmount |  |
Authorized Shares of Preferred Stock | 25,000,000 us-gaap_PreferredStockSharesAuthorized |  |
Par Value Per Share of Preferred Stock | $ 0.10 us-gaap_PreferredStockParOrStatedValuePerShare |  |
Issued Shares of Preferred Stock | 0 us-gaap_PreferredStockSharesIssued |  |

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- Definition Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Antidilution -URI http://asc.fasb.org/extlink&oid=6505113 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Diluted Earnings Per Share -URI http://asc.fasb.org/extlink&oid=6510752 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Contingent Stock Agreement -URI http://asc.fasb.org/extlink&oid=6508534 + Details
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- Definition Face amount or stated value per share of preferred stock nonredeemable or redeemable solely at the option of the issuer. + 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 210.5-02.28) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29 -Article 5 + Details
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- Definition The maximum number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) permitted to be issued by an entity's charter and bylaws. + 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 210.5-02.28) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29 -Article 5 + Details
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- Definition Total number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) issued to shareholders (includes related preferred shares that were issued, repurchased, and remain in the treasury). May be all or portion of the number of preferred shares authorized. Excludes preferred shares that are classified as debt. + 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 210.5-02.28) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29 -Article 5 + Details
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- Definition The floor of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range. + 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 (g) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The ceiling of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range. + 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 (g) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Amount of decrease of par value, additional paid in capital (APIC) and retained earnings of common and preferred stock retired from treasury when treasury stock is accounted for under the cost method. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770 + Details
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- Definition Number of shares of common and preferred stock retired from treasury during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644 + Details
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