Income Tax Expense (Benefit) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Tax [Abstract]
Income Tax Expense	$ 356.3	$ 342.8	$ 279.6
Total Income Tax Expense	381.6	836.7	592.0
Other Comprehensive Income (Loss), Tax
Change in Net Unrealized Gain on Securities Before Adjustment	300.6	(1,890.8)	(346.9)
OCI, Liability for Future Policy Benefit, Gain (Loss), after Reclassification Adjustment, Tax	(256.5)	2,385.1	622.8
Change in Net Gain on Hedges	(17.0)	(19.2)	(9.8)
Change in Foreign Currency Translation Adjustment	0.9	(0.1)	4.2
Change in Unrecognized Pension and Postretirement Benefit Costs	$ (2.7)	$ 18.9	$ 42.1