Realized Investment Gains and Losses Reported in Consolidated Statements of Income (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Fixed Maturity Securities
Gross Gains on Sales	$ 2.5	$ 0.9
Gross Losses on Sales	(5.7)	(5.4)
Other-Than-Temporary Impairment Loss	(4.5)	0
Mortgage Loans and Other Invested Assets
Gross Gains on Sales	1.1unm_OtherInvestmentsGrossRealizedGains	7.4unm_OtherInvestmentsGrossRealizedGains
Gross Losses on Sales	0unm_OtherInvestmentsGrossRealizedLosses	(0.6)unm_OtherInvestmentsGrossRealizedLosses
Impairment Loss	(2.7)us-gaap_ImpairmentOfInvestments	(3.0)us-gaap_ImpairmentOfInvestments
Gain (Loss) on Embedded Derivative	(3.9)	8.5
Gain (Loss) on All Other Derivatives	(1.1)	(0.4)
Foreign Currency Transactions	(1.0)	(1.1)
Net Realized Investment Gain (Loss)	$ (15.3)us-gaap_GainLossOnInvestments	$ 6.3us-gaap_GainLossOnInvestments

This item represents the gross profit realized on the sale of investments not otherwise specified in the taxonomy

No definition available.

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- Definition

Other Investments Gross Realized Losses

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No definition available.

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- Definition

This item represents the gross profit realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 9

-Subparagraph (a)

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This item represents the gross loss realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

+ References

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- Definition

Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4A

-Subparagraph (b)

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- Definition

Net Increase or Decrease in the fair value of the embedded derivative or group of embedded derivatives included in earnings in the period.

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-Publisher FASB

-Name Accounting Standards Codification

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- Definition

Amount before tax of foreign currency transaction realized and unrealized gain (loss) recognized in the income statement.

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 830

-SubTopic 20

-Section 45

-Paragraph 1

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- Definition

This item represents the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain (loss) of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any gains (losses) realized during the period from the sale of investments accounted for under the cost method of accounting and losses recognized for other than temporary impairments (OTTI) of the subject investments.

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-Publisher FASB

-Name Accounting Standards Codification

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-SubTopic 10

-Section 45

-Paragraph 28

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-Name Accounting Standards Codification

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-Section S99

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-Subparagraph (SX 210.5-03.7(c),9(a))

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 7

-Article 5

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- Definition

The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

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-Topic 320

-SubTopic 10

-Section 35

-Paragraph 34

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- Definition

The amount of other than temporary impairment (OTTI) losses on equity securities, OTTI related to credit losses on debt securities, and OTTI losses on debt securities when the entity intends to sell the securities or it is more likely than not that the entity will be required to sell the securities before recovery of its amortized cost basis. Additionally, this item includes OTTI losses recognized during the period on investments accounted for under the cost method of accounting.

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-Name Accounting Standards Codification

-Topic 320

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