Gains and Losses on Derivatives Not Designated as Hedging Instruments (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	$ (2.2)	$ 2.0	$ (0.2)	$ 2.0
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	(3.0)	11.2	5.9	16.7
Foreign Exchange Contracts | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	(2.2)	2.0	(0.2)	2.0
Embedded Derivative in Modified Coinsurance Arrangement | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	(0.8)	9.2	6.1	14.7
Total Return Swap | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	$ (5.4)	$ 4.1	$ (14.1)	$ (4.7)