v3.25.1
Gains and Losses on Derivatives Not Designated as Hedging Instruments (Detail) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Derivative Instruments, Gain (Loss)    
Net Gain (Loss) on Derivatives $ (5.3) $ 1.6
Not Designated as Hedging Instrument    
Derivative Instruments, Gain (Loss)    
Net Gain (Loss) on Derivatives (7.1) 7.7
Foreign Exchange Contracts | Not Designated as Hedging Instrument    
Derivative Instruments, Gain (Loss)    
Net Gain (Loss) on Derivatives (2.0) 1.6
Embedded Derivative in Modified Coinsurance Arrangement | Not Designated as Hedging Instrument    
Derivative Instruments, Gain (Loss)    
Net Gain (Loss) on Derivatives (1.9) 6.1
Total Return Swap | Not Designated as Hedging Instrument | Net Investment Gain (Loss)    
Derivative Instruments, Gain (Loss)    
Net Gain (Loss) on Derivatives (3.2) 0.0
Total Return Swap | Not Designated as Hedging Instrument | Investment Income [Member]    
Derivative Instruments, Gain (Loss)    
Net Gain (Loss) on Derivatives 0.1 0.0
Total Return Swap | Not Designated as Hedging Instrument | Other Expense    
Derivative Instruments, Gain (Loss)    
Net Gain (Loss) on Derivatives $ 4.3 $ (7.9)