Realized Investment Gains and Losses Reported in Consolidated Statements of Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Fixed Maturity Securities
Gross Gains on Sales	$ 2.0	$ 2.4
Gross Losses on Sales	(1.4)	(1.1)
Other-Than-Temporary Impairment Loss	(1.0)	0.0
Mortgage Loans and Other Invested Assets
Gross Gains on Sales	0.0	0.7
Gross Losses on Sales	0.0	(0.2)
Gain (Loss) on Embedded Derivative	(1.7)	8.6
Gain (Loss) on All Other Derivatives	0.7	(0.2)
Foreign Currency Transactions	(0.8)	0.8
Net Realized Investment Gain (Loss)	$ (2.2)	$ 11.0