Total Revenue of Major Clients (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenue, Major Customer [Line Items]
Total revenue	$ 179,815	$ 182,802	$ 601,042	$ 550,656
Credit Concentration Risk | Total Revenue | Client A
Revenue, Major Customer [Line Items]
Concentration risk, percentage	10.60%