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Stock-based Compensation - Expense (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2017
Stock-based compensation expense    
Stock-based compensation expense $ 676 $ 1,278
Research and development    
Stock-based compensation expense    
Stock-based compensation expense 168 305
General and administrative    
Stock-based compensation expense    
Stock-based compensation expense 508 973
Stock options    
Stock-based compensation expense    
Stock-based compensation expense 527 981
Restricted stock    
Stock-based compensation expense    
Stock-based compensation expense $ 149 $ 297