Realized Investment Gains and Losses Reported in Consolidated Statements of Income (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fixed Maturity Securities
Gross Gains on Sales									$ 9.7	$ 13.8	$ 34.3
Gross Losses on Sales									(15.2)	(4.4)	(17.2)
Other-Than-Temporary Impairment Loss									(17.5)	(8.1)	(30.5)
Mortgage Loans and Other Invested Assets
Gross Gains on Sales									0.5	9.4	5.5
Gross Losses on Sales									0.0	(0.2)	(0.7)
Impairment Loss									(1.4)	(0.9)	(5.7)
Gain (Loss) on Embedded Derivative									(15.2)	30.8	40.9
All Other Derivatives									0.3	(0.1)	(1.5)
Foreign Currency Transactions									(0.7)	0.0	(0.9)
Net Realized Investment Gain (Loss)	$ (41.4)	$ 6.7	$ (2.6)	$ (2.2)	$ 11.4	$ 9.8	$ 8.1	$ 11.0	$ (39.5)	$ 40.3	$ 24.2