FAIR VALUE OF FINANCIAL INSTRUMENTS - Cost Method Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Investment balance before cumulative impairment as of January 1,	$ 3,305	$ 3,305	$ 3,305	$ 11,205
Accumulated impairment as of January 1,			$ 0	$ 0
Investment beginning balance after cumulative impairment as of January 1,	3,305	11,205
Removal of investment in connection with step acquisition	0	(9,700)
Acquisition of interest in a privately held company	0	1,800
Ending period balance	$ 3,305	$ 3,305