Gains and Losses on Derivatives Not Designated as Hedging Instruments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	$ 3.1	$ (2.5)	$ (0.1)
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	12.8	(19.5)	8.2
Credit Default Swaps | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	(0.3)	(0.5)	(0.1)
Foreign Exchange Contracts | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	3.4	(2.0)	0.0
Embedded Derivative in Modified Coinsurance Arrangement | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	9.7	(17.0)	8.3
Total Return Swap | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	$ (8.5)	$ 0.0	$ 0.0