REVENUE RECOGNITION - Summary of Significant Changes in Unbilled Receivables and Deferred Liabilities Balances (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Deferred Revenue (Increase) Decrease
Balance at December 31, 2019	$ (144,862)	$ (122,567)
Revenue recognized that was previously included in deferred revenue	157,704	133,394
Increases due to cash received	(192,369)	(157,321)
Cumulative catch-up adjustment arising from changes in estimates of transaction price	17,226	1,632
Net change	(17,439)	(22,295)
Balance at December 31, 2020	(162,301)	(144,862)
Unbilled Receivables Increase (Decrease)
Balance at December 31, 2019	1,362	1,020
Unbilled receivables recognized	9,067	1,834
Transferred to receivables from unbilled receivables		(1,492)
Net change	9,067	342
Balance at December 31, 2020	$ 10,429	$ 1,362