Gains and Losses on Derivatives Not Designated as Hedging Instruments (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Derivative Instruments, Gain (Loss)
Gain (Loss) on Derivatives	$ 0.8	$ 12.7	$ (0.3)	$ 12.3
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Gain (Loss) on Derivatives	(1.2)	12.0	(6.2)	20.1
Not Designated as Hedging Instrument | Credit Default Swaps
Derivative Instruments, Gain (Loss)
Gain (Loss) on Derivatives	(0.3)	(0.4)	(0.6)	(0.8)
Not Designated as Hedging Instrument | Foreign Exchange Contracts
Derivative Instruments, Gain (Loss)
Gain (Loss) on Derivatives	1.1	0.0	0.3	0.0
Not Designated as Hedging Instrument | Embedded Derivative in Modified Coinsurance Arrangement
Derivative Instruments, Gain (Loss)
Gain (Loss) on Derivatives	$ (2.0)	$ 12.4	$ (5.9)	$ 20.9