Gains and Losses on Derivatives Not Designated as Hedging Instruments (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	$ (0.1)	$ (1.0)	$ 1.6	$ (1.7)
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	1.7	40.9	20.2	(46.7)
Credit Default Swaps | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	(0.1)	(0.9)	(0.2)	(0.1)
Foreign Exchange Contracts | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	0.1	(0.1)	1.8	(1.6)
Embedded Derivative in Modified Coinsurance Arrangement | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	1.7	41.9	18.6	(45.0)
Total Return Swap | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	$ (5.0)	$ 0.0	$ (3.7)	$ 0.0