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Stock Based Compensation - Stock Based Compensation Expense (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]        
Stock based compensation expense $ 980 $ 994 $ 2,060 $ 1,934
Research and development        
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]        
Stock based compensation expense 173 378 473 801
General and administrative        
Share-based Payment Arrangement, Expensed and Capitalized, Amount [Line Items]        
Stock based compensation expense $ 807 $ 616 $ 1,587 $ 1,133