Balance Sheet Components - Schedule of Accrued and Other Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Balance Sheet Related Disclosures [Abstract]
Research and development expenses	$ 23,227	$ 12,530
Payroll and related expenses	9,607	9,410
Excess funds payable under grant agreements	3,893	94
Operating lease liabilities, current	3,459
Restricted stock liability	119	1,434
Current portion of finance lease obligation	253	237
Other professional and consulting expenses	2,894	1,634
Other accrued expenses	887	1,156
Total accrued and other liabilities	$ 44,339	$ 26,495