Total Trade Account Receivable of Major Clients (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Revenue, Major Customer [Line Items]
Total trade accounts receivable	$ 197,350	$ 269,154
Credit Concentration Risk | Trade accounts receivable | Client A
Revenue, Major Customer [Line Items]
Concentration risk, percentage	12.40%