Income Tax Expense (Benefit) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax [Abstract]
Income Tax Expense	$ 171.0	$ 281.8	$ 104.4
Other Comprehensive Income (Loss), Tax
Change in Net Unrealized Gain on Securities Before Adjustment	250.2	757.0	(614.2)
Change in Adjustment to Deferred Acquisition Costs and Reserves for Future Policy and Contract Benefits, Net of Reinsurance	(138.2)	(511.7)	371.7
Change in Net Gain on Hedges	(23.8)	(17.0)	(8.2)
Change in Foreign Currency Translation Adjustment	(4.3)	0.2	(0.6)
Change in Unrecognized Pension and Postretirement Benefit Costs	(34.8)	(9.3)	17.0
Total	$ 220.1	$ 501.0	$ (129.9)