INCOME TAXES - Deferred Tax Assets (Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Net operating loss carry-forwards	$ 60,718	$ 53,182
Allowance for doubtful accounts	26	18
Depreciation and amortization	(856)	(485)
Stock-based compensation	1,968	1,330
Change in fair value of convertible promissory notes	1,334	1,042
IRC Section 163(j) interest expense limitation	737	0
Other tax credit carry-forward	3,071	2,720
Other temporary differences	465	842
Valuation allowance	(67,463)	(58,649)	$ (84,483)	$ (59,789)
Net long-term deferred tax liability	$ 0	$ 0