Results for Reportable Segments and Corporate Activities (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting [Abstract]
Amortization associated with deferred acquisition costs	$ 1,457.0	$ 1,392.8	$ 1,271.4