Net Investment Income (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	$ 634.8	$ 641.7	$ 1,253.0	$ 1,255.9
Less Investment Expenses	7.9	7.3	16.1	15.8
Less Investment Income on PFA Assets	3.6	3.7	7.2	7.4
Net Investment Income	623.3	630.7	1,229.7	1,232.7
Fixed Maturity Securities
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	579.6	592.9	1,143.4	1,159.1
Derivatives
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	12.4	11.5	24.3	22.4
Mortgage Loans
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	30.1	27.4	57.0	55.7
Policy Loans
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	4.3	4.2	8.4	8.2
Other Long-term Investments
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	6.3	4.9	16.3	8.8
Short-term Investments
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	$ 2.1	$ 0.8	$ 3.6	$ 1.7