Income Tax Expense (Benefit) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax [Abstract]
Income Tax Expense	$ 281.8	$ 104.4	$ 409.8
Other Comprehensive Income (Loss), Tax
Change in Net Unrealized Gain on Securities Before Adjustment	757.0	(614.2)	328.1
Change in Adjustment to Deferred Acquisition Costs and Reserves for Future Policy and Contract Benefits, Net of Reinsurance, Tax Expense (Benefit)	(511.7)	371.7	(245.0)
Change in Net Gain on Hedges, Tax Expense (Benefit)	(17.0)	(8.2)	(24.4)
Change in Foreign Currency Translation Adjustment, Tax Expense (Benefit)	0.2	(0.6)	0.0
Change in Unrecognized Pension and Postretirement Benefit Costs, Tax Expense (Benefit)	(9.3)	17.0	(8.3)
Total	$ 501.0	$ (129.9)	$ 460.2