Net Investment Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018		Mar. 31, 2017
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	$ 615.3		$ 614.8
Less Investment Expenses	9.6		8.8
Less Investment Income on PFA Assets	3.4		3.6
Net Investment Income	602.3		602.4
Fixed Maturity Securities
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	557.1		563.7
Derivatives
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	15.4		13.4
Mortgage Loans
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	27.3		25.4
Policy Loans
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	4.4		4.3
Equity Securities
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	(0.2)	[1]	0.3
Unrealized Loss on Securities	2.0
Partnership Interest
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	5.6	[1]	5.1
Unrealized Loss on Securities	1.6
Other Long-term Investments
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	2.1		0.7
Short-term Investments
Schedule of Investment Income, Reported Amounts, by Category
Investment Income, Interest and Dividend	$ 3.6		$ 1.9

[1] 1 The net unrealized loss recognized in net investment income for the first three months of 2018 related to equity securities and private equity partnerships still held at March 31, 2018 was $2.0 million and $1.6 million, respectively.