Income Tax Expense (Benefit) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Income Tax [Abstract]
Income Tax Expense	$ 472.2	$ 356.3	$ 342.8
Total Income Tax Expense	696.1	381.6	836.7
Other Comprehensive Income (Loss), Tax
Income Tax Expense (Benefit), Change in Net Unrealized Gain on Securities	(226.6)	300.6	(1,890.8)
Income Tax Expense (Benefit), Change in the Effect of Discount Rate Assumptions on the Liability for Future Policy and Contract Benefits	488.9	(256.5)	2,385.1
Income Tax Benefit, Change in Net Gain (Loss) on Derivatives	(51.5)	(17.0)	(19.2)
Income Tax Expense (Benefit), Change in Foreign Currency Translation Adjustment	1.1	0.9	(0.1)
Income Tax Expense (Benefit), Change in Unrecognized Pension and Postretirement Benefit Costs	$ 12.0	$ (2.7)	$ 18.9