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Equity Plans - Stock-based Compensation Expense (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]      
Total stock-based compensation expense $ 7,899 $ 8,353 $ 6,919
Total unrecognized stock-based compensation costs related to stock options $ 16,000    
Period of unrecognized stock-based compensation costs to be recognized 2 years 8 months 12 days    
Research and Development      
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]      
Total stock-based compensation expense $ 4,121 4,350 3,424
General and Administrative      
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]      
Total stock-based compensation expense $ 3,778 $ 4,003 $ 3,495