Quarterly report pursuant to Section 13 or 15(d)

Stock-Based Payments - Stock-Based Compensation Expense (Details)

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Stock-Based Payments - Stock-Based Compensation Expense (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 for stock options and restricted common stock granted        
Stock-based compensation expense $ 1,744 $ 1,156 $ 3,440 $ 2,040
Employee Stock options        
Stock-based compensation expense for stock options and restricted common stock granted        
Unrecognized compensation cost related to non-vested stock options granted to employees 17,500   $ 17,500  
Weighted average period over which future compensation cost is expected to be recognized for awards granted to employees     3 years 2 months 12 days  
Research and development        
Stock-based compensation expense for stock options and restricted common stock granted        
Stock-based compensation expense 670 448 $ 1,235 778
General and administrative        
Stock-based compensation expense for stock options and restricted common stock granted        
Stock-based compensation expense $ 1,074 $ 708 $ 2,205 $ 1,262