Gains and Losses on Derivatives Not Designated as Hedging Instruments (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Derivative Instruments, Gain (Loss)
Gain (Loss) on Derivatives	$ (0.3)	$ (1.1)	$ (0.5)	$ (3.6)
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Gain (Loss) on Derivatives	5.8	9.1	14.3	2.2
Not Designated as Hedging Instrument | Credit Default Swaps
Derivative Instruments, Gain (Loss)
Gain (Loss) on Derivatives	(0.1)	(0.4)	(0.4)	(0.4)
Not Designated as Hedging Instrument | Interest Rate Swaps
Derivative Instruments, Gain (Loss)
Gain (Loss) on Derivatives	(0.1)	0.0	(0.1)	0.0
Not Designated as Hedging Instrument | Foreign Exchange Contracts
Derivative Instruments, Gain (Loss)
Gain (Loss) on Derivatives	(0.2)	(0.7)	0.0	(2.0)
Not Designated as Hedging Instrument | Embedded Derivative in Modified Coinsurance Arrangement
Derivative Instruments, Gain (Loss)
Gain (Loss) on Derivatives	$ 6.2	$ 10.2	$ 14.8	$ 4.6