Gains and Losses on Derivatives Not Designated as Hedging Instruments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	$ (2.5)	$ (0.1)	$ 0.3
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	(19.5)	8.2	(14.9)
Credit Default Swaps | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	(0.5)	(0.1)	(0.3)
Interest Rate Swaps | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	0.0	0.0	(0.3)
Foreign Exchange Contracts | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	(2.0)	0.0	0.9
Embedded Derivative in Modified Coinsurance Arrangement | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	$ (17.0)	$ 8.3	$ (15.2)