Gains and Losses on Derivatives Not Designated as Hedging Instruments (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative Instruments, Gain (Loss)
Gain (Loss) on Derivatives	$ 0.0	$ (0.3)	$ 0.7	$ (0.5)
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Gain (Loss) on Derivatives	(2.3)	5.8	(3.3)	14.3
Not Designated as Hedging Instrument | Credit Default Swaps
Derivative Instruments, Gain (Loss)
Gain (Loss) on Derivatives	0.0	(0.1)	0.0	(0.4)
Not Designated as Hedging Instrument | Interest Rate Swaps
Derivative Instruments, Gain (Loss)
Gain (Loss) on Derivatives	(0.3)	(0.1)	(0.3)	(0.1)
Not Designated as Hedging Instrument | Foreign Exchange Contracts
Derivative Instruments, Gain (Loss)
Gain (Loss) on Derivatives	0.3	(0.2)	1.0	0.0
Not Designated as Hedging Instrument | Embedded Derivative in Modified Coinsurance Arrangement
Derivative Instruments, Gain (Loss)
Gain (Loss) on Derivatives	$ (2.3)	$ 6.2	$ (4.0)	$ 14.8