INCOME TAXES - Deferred Tax Assets (Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Net operating loss carry-forwards	$ 92,387	$ 64,348
Allowance for credit losses	94	28
Depreciation and amortization	(13,601)	(1,321)
Stock-based compensation	3,769	2,727
Change in fair value of convertible promissory notes	1,462	2,275
IRC Section 163(j) interest expense limitation	89	436
Other tax credit carry-forward	1,771	1,419
Other temporary differences	20	319
Valuation allowance	(85,991)	(70,231)	$ (67,463)	$ (58,649)
Net long-term deferred tax asset	$ 0	$ 0