SEGMENT INFORMATION - Revenue and Long-Lived Assets by Geographic Areas (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Revenue and Long-lived Assets by Geography
Revenue	$ 357,358	$ 303,116	$ 1,004,697	$ 853,249
Long-lived assets (excluding goodwill and intangible assets)	101,972		101,972		$ 70,859
United States
Revenue and Long-lived Assets by Geography
Revenue	335,230	283,672	934,409	791,932
Long-lived assets (excluding goodwill and intangible assets)	95,466		95,466		65,510
All other countries
Revenue and Long-lived Assets by Geography
Revenue	22,128	$ 19,444	70,288	$ 61,317
Long-lived assets (excluding goodwill and intangible assets)	$ 6,506		$ 6,506		$ 5,349