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Investments Investment Income (Detail) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2014 | Sep. 30, 2014 | Jun. 30, 2014 | Mar. 31, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Net Investment Income |  |  |  |  |  |  |  |  |  |  |
Investment Income, Interest and Dividend |  |  |  |  |  |  |  |  | $ 2,536.3 us-gaap_InvestmentIncomeInterestAndDividend | $ 2,552.1 us-gaap_InvestmentIncomeInterestAndDividend | $ 2,574.3 us-gaap_InvestmentIncomeInterestAndDividend
Less Investment Expenses |  |  |  |  |  |  |  |  | 29.0 us-gaap_InvestmentIncomeInvestmentExpense | 29.5 us-gaap_InvestmentIncomeInvestmentExpense | 26.9 us-gaap_InvestmentIncomeInvestmentExpense
Less Investment Income on PFA Assets |  |  |  |  |  |  |  |  | 15.1 unm_InvestmentIncomeOnPfaAssets | 15.7 unm_InvestmentIncomeOnPfaAssets | 16.1 unm_InvestmentIncomeOnPfaAssets
Less Amortization of Tax Credit Partnerships |  |  |  |  |  |  |  |  | 14.8 us-gaap_IncomeTaxCreditsAndAdjustments | 14.8 us-gaap_IncomeTaxCreditsAndAdjustments | 16.1 us-gaap_IncomeTaxCreditsAndAdjustments
Net Investment Income | 629.4 us-gaap_NetInvestmentIncome | 606.4 us-gaap_NetInvestmentIncome | 629.1 us-gaap_NetInvestmentIncome | 612.5 us-gaap_NetInvestmentIncome | 629.4 us-gaap_NetInvestmentIncome | 615.5 us-gaap_NetInvestmentIncome | 626.1 us-gaap_NetInvestmentIncome | 621.1 us-gaap_NetInvestmentIncome | 2,477.4 us-gaap_NetInvestmentIncome | 2,492.1 us-gaap_NetInvestmentIncome | 2,515.2 us-gaap_NetInvestmentIncome
Fixed Maturity Securities |  |  |  |  |  |  |  |  |  |  |
Net Investment Income |  |  |  |  |  |  |  |  |  |  |
Investment Income, Interest and Dividend |  |  |  |  |  |  |  |  | 2,344.4 us-gaap_InvestmentIncomeInterestAndDividend / us-gaap_InvestmentTypeAxis = us-gaap_DebtSecuritiesMember | 2,371.6 us-gaap_InvestmentIncomeInterestAndDividend / us-gaap_InvestmentTypeAxis = us-gaap_DebtSecuritiesMember | 2,404.0 us-gaap_InvestmentIncomeInterestAndDividend / us-gaap_InvestmentTypeAxis = us-gaap_DebtSecuritiesMember
Derivatives |  |  |  |  |  |  |  |  |  |  |
Net Investment Income |  |  |  |  |  |  |  |  |  |  |
Investment Income, Interest and Dividend |  |  |  |  |  |  |  |  | 40.4 us-gaap_InvestmentIncomeInterestAndDividend / us-gaap_InvestmentTypeAxis = us-gaap_DerivativeMember | 35.2 us-gaap_InvestmentIncomeInterestAndDividend / us-gaap_InvestmentTypeAxis = us-gaap_DerivativeMember | 28.9 us-gaap_InvestmentIncomeInterestAndDividend / us-gaap_InvestmentTypeAxis = us-gaap_DerivativeMember
Mortgage Loans |  |  |  |  |  |  |  |  |  |  |
Net Investment Income |  |  |  |  |  |  |  |  |  |  |
Investment Income, Interest and Dividend |  |  |  |  |  |  |  |  | 109.8 us-gaap_InvestmentIncomeInterestAndDividend / us-gaap_InvestmentTypeAxis = us-gaap_MortgageReceivablesMember | 109.2 us-gaap_InvestmentIncomeInterestAndDividend / us-gaap_InvestmentTypeAxis = us-gaap_MortgageReceivablesMember | 107.1 us-gaap_InvestmentIncomeInterestAndDividend / us-gaap_InvestmentTypeAxis = us-gaap_MortgageReceivablesMember
Policy Loans |  |  |  |  |  |  |  |  |  |  |
Net Investment Income |  |  |  |  |  |  |  |  |  |  |
Investment Income, Interest and Dividend |  |  |  |  |  |  |  |  | 16.3 us-gaap_InvestmentIncomeInterestAndDividend / us-gaap_InvestmentTypeAxis = us-gaap_PolicyLoansMember | 15.7 us-gaap_InvestmentIncomeInterestAndDividend / us-gaap_InvestmentTypeAxis = us-gaap_PolicyLoansMember | 14.8 us-gaap_InvestmentIncomeInterestAndDividend / us-gaap_InvestmentTypeAxis = us-gaap_PolicyLoansMember
Other Long-term Investments |  |  |  |  |  |  |  |  |  |  |
Net Investment Income |  |  |  |  |  |  |  |  |  |  |
Investment Income, Interest and Dividend |  |  |  |  |  |  |  |  | 23.0 us-gaap_InvestmentIncomeInterestAndDividend / us-gaap_InvestmentTypeAxis = us-gaap_OtherLongTermInvestmentsMember | 18.0 us-gaap_InvestmentIncomeInterestAndDividend / us-gaap_InvestmentTypeAxis = us-gaap_OtherLongTermInvestmentsMember | 15.2 us-gaap_InvestmentIncomeInterestAndDividend / us-gaap_InvestmentTypeAxis = us-gaap_OtherLongTermInvestmentsMember
Short-term Investments |  |  |  |  |  |  |  |  |  |  |
Net Investment Income |  |  |  |  |  |  |  |  |  |  |
Investment Income, Interest and Dividend |  |  |  |  |  |  |  |  | $ 2.4 us-gaap_InvestmentIncomeInterestAndDividend / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember | $ 2.4 us-gaap_InvestmentIncomeInterestAndDividend / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember | $ 4.3 us-gaap_InvestmentIncomeInterestAndDividend / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember

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- Definition Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 16 -Article 12 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.2) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 2 -Article 7 + Details
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