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Income Taxes - Schedule of Components of Income Tax Expense (Benefit) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Income Tax Disclosure [Abstract]      
Tax at statutory federal rate $ (47,728) $ (42,997) $ (37,557)
State tax, net of federal benefit (8,218) (9,823) (6,527)
Research and development credits (4,327) (4,855) (4,775)
Stock-based compensation expense 4,675 2,906 (2,059)
Non-deductible compensation 1,455 4,720 901
Change in valuation allowance 53,621 49,479 50,834
Other 522 570 (817)
Provision for income taxes $ 0 $ 0 $ 0