DEFERRED REVENUE - Rollforward of deferred revenue (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($)
Movement in Deferred Revenue [Roll Forward]
Current deferred revenue, beginning balance	$ 9,988
Current deferred revenue, Add: Additions	1,273
Current deferred revenue, Less revenue recognized	(1,244)
Current deferred revenue, ending balance	10,017
Noncurrent deferred revenue, beginning balance	48,250
Noncurrent deferred revenue, Additions	667
Noncurrent deferred revenue, Less reclassification to current deferred revenue	(1,273)
Noncurrent deferred revenue, ending balance	$ 47,644