Net Investment Income (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	$ 632.1	$ 634.8	$ 1,246.9	$ 1,253.0
Less Investment Expenses	8.1	7.9	16.9	16.1
Less Investment Income on PFA Assets	3.5	3.6	7.1	7.2
Net Investment Income	620.5	623.3	1,222.9	1,229.7
Fixed Maturity Securities
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	572.0	579.6	1,135.7	1,143.4
Derivatives
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	14.0	12.4	27.4	24.3
Mortgage Loans
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	25.6	30.1	51.0	57.0
Policy Loans
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	4.5	4.3	8.8	8.4
Other Long-term Investments
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	13.4	6.3	19.5	16.3
Short-term Investments
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	$ 2.6	$ 2.1	$ 4.5	$ 3.6

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=63488584&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 FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 235
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-16(Column G))
-URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574960-122915

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