FINANCIAL INSTRUMENTS AND SIGNIFICANT CONCENTRATIONS - Narrative (Details) $ in Thousands		3 Months Ended
	Mar. 07, 2019 USD ($) year	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Concentration Risk [Line Items]
Fair value of embedded derivative liability			$ 1,100		$ 1,600
Gain (loss) on embedded derivative, net		$ 0	$ 500
Cash and cash equivalents		32,264		$ 24,771
Single Financial Institution
Concentration Risk [Line Items]
Cash and cash equivalents		25,200		19,900
Current restricted cash		$ 400		$ 400
Private Placement
Concentration Risk [Line Items]
Temporary stock issued	$ 5,313
Series A Preferred Stock | Private Placement
Concentration Risk [Line Items]
Temporary stock issued	$ 5,313
Measurement Input, Expected Term | Series A Preferred Stock | Minimum
Concentration Risk [Line Items]
Measurement input (as a percent) | year	4
Measurement Input, Expected Term | Series A Preferred Stock | Maximum
Concentration Risk [Line Items]
Measurement input (as a percent) | year	5
Measurement Input, Implied Yield | Series A Preferred Stock
Concentration Risk [Line Items]
Measurement input (as a percent)	0.221
Measurement Input, Risk Free Interest Rate | Series A Preferred Stock
Concentration Risk [Line Items]
Measurement input (as a percent)	0.0244
Measurement Input, Price Volatility | Series A Preferred Stock
Concentration Risk [Line Items]
Measurement input (as a percent)		0.300