Amortization Expense Relating to Acquired Intangible Assets (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Finite Lived Intangible Assets [Line Items]
Amortization expense	$ 7.6	$ 6.9	$ 24.1	$ 20.5