BALANCE SHEET COMPONENTS - Accrued Expenses and Other Current Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Accrued payroll and payroll related expenses	$ 1,679	$ 1,385
Accrued stock issuance and transaction costs	285	4,695
Accrued outsourced engineering services	8,979	3,205
Accrued legal expenses	5,978	243
Other accrued liabilities	4,931	1,237
Current portion of lease financing liability	727	660
Total accrued expenses and other current liabilities	$ 22,579	$ 11,425