Gains and Losses on Derivatives Not Designated as Hedging Instruments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	$ 0.3	$ (0.1)	$ (1.5)
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	(14.9)	30.7	40.6
Not Designated as Hedging Instrument | Credit Default Swaps
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	(0.3)	(0.4)	(0.9)
Not Designated as Hedging Instrument | Interest Rate Swaps
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	(0.3)	(0.1)	0.4
Not Designated as Hedging Instrument | Foreign Exchange Contracts
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	0.9	0.4	0.2
Not Designated as Hedging Instrument | Embedded Derivative in Modified Coinsurance Arrangement
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	$ (15.2)	$ 30.8	$ 40.9