FAIR VALUE OF MEASUREMENTS AND INVESTMENT SECURITIES - Derivative Liabilities Reported at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrant derivative liability	$ 7,842	$ 10,345
Private placement option liability	2,800	7,803
Level 1
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrant derivative liability	0	0
Private placement option liability	0	0
Total fair value	0	0
Level 2
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrant derivative liability	0	0
Private placement option liability	0	0
Total fair value	0	0
Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrant derivative liability	7,842	10,345
Private placement option liability	2,800	7,803
Total fair value	$ 10,642	$ 18,148