Accounts Receivable, Net - Accounts Receivables Net (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounts Receivable Net Current [Abstract]
Trade accounts receivable	$ 139,730	$ 132,880
Unbilled revenue, at net realizable value	62,397	55,982
Total accounts receivable	202,127	188,862
Allowance for doubtful accounts	(13,827)	(13,508)	$ (12,659)	$ (11,915)
Accounts receivable, net	$ 188,300	$ 175,354