Schedule of Changes in Carrying Amount of Goodwill (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Line Items]
Beginning balance	$ 491.5	$ 420.9
Acquisitions	22.2	70.0
Currency translation adjustment	3.0	0.6
Ending balance	516.7	491.5
North America
Goodwill [Line Items]
Beginning balance	432.2	361.0
Acquisitions	22.2	70.0
Currency translation adjustment	2.8	1.2
Ending balance	457.2	432.2
Rest of World
Goodwill [Line Items]
Beginning balance	59.3	59.9
Currency translation adjustment	0.2	(0.6)
Ending balance	$ 59.5	$ 59.3