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Stock-Based Compensation - Expenses (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Stock-based compensation expense        
Stock-based compensation expense $ 2,048 $ 3,450 $ 4,536 $ 6,989
Research and development        
Stock-based compensation expense        
Stock-based compensation expense 978 1,871 2,289 3,764
General and administrative        
Stock-based compensation expense        
Stock-based compensation expense $ 1,070 $ 1,579 $ 2,247 $ 3,225