Gains and Losses on Derivatives Not Designated as Hedging Instruments (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Derivative Instruments, Gain (Loss)
Gain (Loss) on Investments	$ (853.0)	$ 128.0	$ (863.2)	$ (96.5)
Net Investment Income	522.4	476.8	1,484.2	1,550.7
Other Expenses	361.8	360.0	1,086.3	1,002.1
Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Gain (Loss) on Investments	(69.9)	3.3	(69.9)	(0.7)
Foreign Exchange Contracts | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Gain (Loss) on Investments	0.9	0.0	3.6	(5.9)
Embedded Derivative in Modified Coinsurance Arrangement | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Gain (Loss) on Investments	3.6	3.3	0.9	2.4
Total Return Swap | Not Designated as Hedging Instrument
Derivative Instruments, Gain (Loss)
Gain (Loss) on Investments	(27.2)	0.0	(27.2)	2.8
Net Investment Income	5.0	0.0	5.0	0.9
Other Expenses	$ 1.7	$ (6.8)	$ (10.6)	$ (13.8)