Net Investment Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	$ 614.2	$ 627.6
Less Investment Expenses	8.5	8.0
Less Investment Income on PFA Assets	3.7unm_InvestmentIncomeOnPfaAssets	3.8unm_InvestmentIncomeOnPfaAssets
Net Investment Income	602.0us-gaap_NetInvestmentIncome	615.8us-gaap_NetInvestmentIncome
Fixed Maturity Securities
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	566.2	581.1
Derivatives
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	10.9	9.9
Mortgage Loans
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	28.3	28.4
Policy Loans
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	4.0	3.9
Other Long-term Investments
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	3.9	3.8
Short-term Investments
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	$ 0.9	$ 0.5

Investment Income Participation Fund Account Assets

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- Definition

Amount before accretion (amortization) of purchase discount (premium) of interest income and dividend income on nonoperating securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.7(a),(b))

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 7

-Subparagraph a, b

-Article 5

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Amount of expenses related to the generation of investment income.

+ References

Reference 1: 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 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 225

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SX 210.5-03.9)

-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

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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

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-Publisher SEC

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