Realized Investment Gains and Losses Reported in Consolidated Statements of Income (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Fixed Maturity Securities
Gross Gains on Sales	$ 4.8	$ 4.3	$ 9.4	$ 8.6
Gross Losses on Sales	(3.8)	(1.2)	(8.3)	(3.5)
Other-Than-Temporary Impairment Loss on Fixed Maturity Securities	0.0	0.0	1.0	0.0
Mortgage Loans and Other Invested Assets
Gross Gains on Sales	0.1	0.8	0.2	3.4
Gross Losses on Sales	0.0	0.0	0.0	(0.2)
Provision for Loan and Lease Losses	0.6	0.9	0.8	0.9
Gain on Embedded Derivative	6.1	6.7	2.1	21.5
Derivative, Gain (Loss) on Derivative, Net	0.2	0.2	0.9	(0.3)
Foreign Currency Transactions	(0.1)	(0.1)	(0.6)	0.3
Net Realized Investment Gain	$ 6.7	$ 9.8	$ 1.9	$ 28.9