Net Investment Income (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2016	Mar. 31, 2015
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	$ 618.2	$ 614.2
Less Investment Expenses	8.2	8.5
Less Investment Income on PFA Assets	3.6	3.7
Net Investment Income	606.4	602.0
Fixed Maturity Securities
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	563.8	566.2
Derivatives
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	11.9	10.9
Mortgage Loans
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	26.9	28.3
Policy Loans
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	4.1	4.0
Other Long-term Investments
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	10.0	3.9
Short-term Investments
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	$ 1.5	$ 0.9

Investment Income Participation Fund Account Assets

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

+ References

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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 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.9)
-URI http://asc.fasb.org/extlink&oid=26872669&loc=d3e20235-122688

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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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 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 SEC
-Name Regulation S-X (SX)
-Number 210
-Section 16
-Article 12

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