Income Taxes - Changes in Uncertain Tax Positions (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unrecognized tax benefits:
Amount beginning of year	$ 35.6	$ 28.8	$ 30.1
Net increase (decrease):
Tax positions taken in prior periods	(13.3)	(0.6)	(0.4)
Tax positions taken in current period	4.5	11.0	6.4
Lapse due to applicable statute of limitations	(8.1)	(1.6)	(6.0)
Settlement with taxing authorities	(2.3)	(2.3)
Changes in currency exchange rates	0.7	0.3	(1.3)
Amount at end of year	$ 17.1	$ 35.6	$ 28.8