Reconciliation of Beginning and Ending Amount of Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Balance at beginning of the year	$ 17.3	$ 19.1	$ 12.0
Additions for tax positions related to the current year	5.6	3.9	10.3
Additions for tax positions related to prior years		0.0	0.0
Reductions for tax positions related to prior years		(0.3)	(2.0)
Reductions related to settlements		(5.4)	0.0
Lapse of statute of limitations			(1.2)
Balance at the end of the year	$ 22.9	$ 17.3	$ 19.1