FAIR VALUE OF FINANCIAL INSTRUMENTS - Schedule of Fair Value of Financial Liabilities (Details) - Fair Value Measurement on a Recurring Basis - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Warrant liabilities	$ 10,088	$ 4,864
Level 1
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Warrant liabilities	0	0
Level 2
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Warrant liabilities	0	0
Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Warrant liabilities	10,088	4,864
2017 Placement Warrants
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Warrant liabilities	7,449	3,675
2017 Placement Warrants | Level 1
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Warrant liabilities	0	0
2017 Placement Warrants | Level 2
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Warrant liabilities	0	0
2017 Placement Warrants | Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Warrant liabilities	7,449	3,675
2016 Placement Warrants
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Warrant liabilities	2,639	1,189
2016 Placement Warrants | Level 1
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Warrant liabilities	0	0
2016 Placement Warrants | Level 2
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Warrant liabilities	0	0
2016 Placement Warrants | Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Warrant liabilities	$ 2,639	$ 1,189