Stock Based Compensation - Costs Related to Company's Stock-Based Compensation Plan (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expenses	$ 6,270	$ 5,708	$ 5,107	$ 5,956	$ 5,027	$ 4,484	$ 4,450	$ 5,809	$ 23,041	$ 19,770	$ 16,047
Cost of revenue
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expenses									4,600	3,664	2,895
General and administrative expenses
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expenses									10,363	8,372	6,077
Selling and marketing expenses
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Stock-based compensation expenses									$ 8,078	$ 7,734	$ 7,075