STOCK-BASED COMPENSATION - Summary of the Components of Total Stock-Based Compensation Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Allocation of Stock-based Compensation Expense Included in the Consolidated Statement of Operations [Abstract]
Total stock-based compensation expense	$ 4,051	$ 1,727	$ 9,911	$ 5,277
Cost of revenues
Allocation of Stock-based Compensation Expense Included in the Consolidated Statement of Operations [Abstract]
Total stock-based compensation expense	330	347	945	796
Research and development
Allocation of Stock-based Compensation Expense Included in the Consolidated Statement of Operations [Abstract]
Total stock-based compensation expense	878	607	2,645	1,994
Sales and marketing
Allocation of Stock-based Compensation Expense Included in the Consolidated Statement of Operations [Abstract]
Total stock-based compensation expense	1,151	226	2,509	889
General and administrative
Allocation of Stock-based Compensation Expense Included in the Consolidated Statement of Operations [Abstract]
Total stock-based compensation expense	$ 1,692	$ 547	$ 3,812	$ 1,598