SEGMENT INFORMATION - Revenue and Long-Lived Assets by Geographic Areas (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Revenue and Long-lived Assets by Geography
Revenue	$ 461,565	$ 389,913	$ 1,269,582	$ 1,108,624
Long-lived assets (excluding goodwill and intangible assets):	116,825		116,825		$ 108,842
United States
Revenue and Long-lived Assets by Geography
Revenue	437,872	368,082	1,188,854	1,041,903
Long-lived assets (excluding goodwill and intangible assets):	108,496		108,496		97,841
All other countries
Revenue and Long-lived Assets by Geography
Revenue	23,693	$ 21,831	80,728	$ 66,721
Long-lived assets (excluding goodwill and intangible assets):	$ 8,329		$ 8,329		$ 11,001