Income Taxes - Schedule of Unrecognized Tax Benefit (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Beginning balance	$ 14.2	$ 12.2	$ 14.9
Increases in tax positions for prior years	1.7	2.0	0.0
Decreases in tax positions for prior years		(1.4)	(4.3)
Increases in tax positions for current year	11.9	1.9	1.9
Settlements		(0.2)	0.0
Lapse in statutes of limitation	(0.9)	(0.3)	(0.3)
Ending balance	$ 26.9	$ 14.2	$ 12.2