SEGMENT INFORMATION - Revenue and Long-Lived Assets (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue and Long-lived Assets by Geography
Revenue	$ 278,992	$ 303,116	$ 294,822	$ 255,311	$ 223,213	$ 181,717	$ 180,711	$ 150,745	$ 1,132,241	$ 736,386	$ 498,890
Long-lived assets (excluding goodwill and intangible assets)	70,859				53,292				70,859	53,292
United States
Revenue and Long-lived Assets by Geography
Revenue									1,050,641	672,159	461,372
Long-lived assets (excluding goodwill and intangible assets)	65,510				49,356				65,510	49,356
All other countries
Revenue and Long-lived Assets by Geography
Revenue									81,600	64,227	$ 37,518
Long-lived assets (excluding goodwill and intangible assets)	$ 5,349				$ 3,936				$ 5,349	$ 3,936