Gains and Losses on Derivatives Not Designated as Hedging Instruments (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Derivative Instruments, Gain (Loss)
Gain (Loss) on Derivatives	$ 0.7	$ (0.2)
Net Realized Investment Gain (Loss) | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Gain (Loss) on Derivatives	(1.0)	8.5
Net Realized Investment Gain (Loss) | Not Designated as Hedging Instrument | Credit Default Swaps
Derivative Instruments, Gain (Loss)
Gain (Loss) on Derivatives	0.0	(0.3)
Net Realized Investment Gain (Loss) | Not Designated as Hedging Instrument | Foreign Exchange Contracts
Derivative Instruments, Gain (Loss)
Gain (Loss) on Derivatives	0.7	0.2
Net Realized Investment Gain (Loss) | Not Designated as Hedging Instrument | Embedded Derivative in Modified Coinsurance Arrangement
Derivative Instruments, Gain (Loss)
Gain (Loss) on Derivatives	$ (1.7)	$ 8.6