Net Investment Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	$ 623.4	$ 622.0	$ 1,879.3	$ 1,892.7
Less Investment Expenses	7.7	7.0	23.5	21.9
Less Investment Income on PFA Assets	3.6	3.7	11.0	11.3
Net Investment Income	612.1	611.3	1,844.8	1,859.5
Fixed Maturity Securities
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	573.3	578.8	1,732.4	1,749.5
Derivatives
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	11.3	10.1	33.7	30.3
Mortgage Loans
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	26.8	25.8	82.5	80.8
Policy Loans
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	4.2	4.2	12.4	12.1
Other Long-term Investments
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	7.1	2.5	15.9	18.3
Short-term Investments
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	$ 0.7	$ 0.6	$ 2.4	$ 1.7