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Stock-based Compensation - Summary of Stock-Based Compensation Included in Statement of Operations (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Stock-based compensation        
Total stock-based compensation expense $ 4,162 $ 3,526 $ 8,425 $ 6,254
Research and development expenses        
Stock-based compensation        
Total stock-based compensation expense 514 380 1,081 703
Selling, general and administrative expenses        
Stock-based compensation        
Total stock-based compensation expense $ 3,648 $ 3,146 $ 7,344 $ 5,551