Gains and Losses on Derivatives Not Designated as Hedging Instruments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	$ (2.9)	$ 3.8	$ (0.6)
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	3.4	16.8	11.8
Foreign Exchange Contracts | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	(5.7)	3.8	(0.6)
Embedded Derivative in Modified Coinsurance Arrangement | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	6.3	13.0	12.4
Total Return Swap | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	(15.7)	(12.5)	(13.6)
Total Return Swap | Not Designated as Hedging Instrument | Net Realized Investment Gain (Loss)
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	2.8	0.0	0.0
Total Return Swap | Not Designated as Hedging Instrument | Net Investment Income
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	$ 0.9	$ 0.0	$ 0.0