Gains and Losses on Derivatives Not Designated as Hedging Instruments (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	$ 1.5	$ (5.3)
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	(1.5)	(7.1)
Foreign Exchange Contracts | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	1.5	(2.0)
Embedded Derivative in Modified Coinsurance Arrangement | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	(3.0)	(1.9)
Total Return Swap | Not Designated as Hedging Instrument | Income Statement Location: us-gaap:GainLossOnInvestments
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	0.0	(3.2)
Total Return Swap | Not Designated as Hedging Instrument | Income Statement Location: us-gaap:NetInvestmentIncome
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	0.0	0.1
Total Return Swap | Not Designated as Hedging Instrument | Income Statement Location: us-gaap:OtherCostAndExpenseOperating
Derivative Instruments, Gain (Loss)
Net Gain (Loss) on Derivatives	$ 5.6	$ 4.3