Realized Investment Gains and Losses Reported in Consolidated Statements of Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Fixed Maturity Securities
Gross Gains on Sales	$ 4.3	$ 1.6	$ 8.6	$ 29.4
Gross Losses on Sales	(1.2)	(0.4)	(3.5)	(14.0)
Other-Than-Temporary Impairment Loss	0.0	0.0	0.0	(30.5)
Mortgage Loans and Other Invested Assets
Gross Gains on Sales	0.8	0.2	3.4	4.0
Gross Losses on Sales	0.0	0.0	(0.2)	(0.7)
Other than Temporary Impairment Losses, Investments	0.9	0.0	0.9	3.1
Gain on Embedded Derivative	6.7	9.7	21.5	14.3
Gain (Loss) on All Other Derivatives	0.2	0.1	(0.3)	(3.5)
Foreign Currency Transactions	(0.1)	(0.2)	0.3	(0.1)
Net Realized Investment Gain (Loss)	$ 9.8	$ 11.0	$ 28.9	$ (4.2)