Results for Reportable Segments and Other Activities (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Amortization associated with deferred acquisition costs	$ 1,261.7	$ 1,253.2	$ 1,024.5