Net Investment Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	$ 619.6	$ 622.3	$ 1,866.5	$ 1,875.3
Less Investment Expenses	7.1	7.4	24.0	23.5
Less Investment Income on PFA Assets	3.5	3.5	10.6	10.7
Net Investment Income	609.0	611.4	1,831.9	1,841.1
Fixed Maturity Securities
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	564.8	573.2	1,700.5	1,716.6
Derivatives
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	14.4	12.6	41.8	36.9
Mortgage Loans
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	25.8	30.6	76.8	87.6
Policy Loans
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	4.5	4.4	13.3	12.8
Other Long-term Investments
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	6.6	(0.9)	26.1	15.4
Short-term Investments
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	$ 3.5	$ 2.4	$ 8.0	$ 6.0