SEGMENT INFORMATION - Revenue and Long-Lived Assets (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue and Long-lived Assets by Geography
Revenue	$ 321,508	$ 357,358	$ 343,896	$ 303,443	$ 278,992	$ 303,116	$ 294,822	$ 255,311	$ 1,326,205	$ 1,132,241	$ 736,386
Long-lived assets (excluding goodwill and intangible assets)	103,361				70,859				103,361	70,859
United States
Revenue and Long-lived Assets by Geography
Revenue									1,234,755	1,050,641	672,159
Long-lived assets (excluding goodwill and intangible assets)	95,822				65,510				95,822	65,510
All other countries
Revenue and Long-lived Assets by Geography
Revenue									91,450	81,600	$ 64,227
Long-lived assets (excluding goodwill and intangible assets)	$ 7,539				$ 5,349				$ 7,539	$ 5,349