Amortization Expense Relating to Acquired Intangible Assets (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Finite Lived Intangible Assets [Line Items]
Amortization expense	$ 61.4	$ 34.2	$ 31.9