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 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Stock-based compensation expense for stock options and restricted common stock granted        
Stock-based compensation expense $ 1,108 $ 1,694 $ 3,148 $ 3,504
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 11,500   $ 11,500  
Weighted average period over which future compensation cost is expected to be recognized for awards granted to employees     2 years 8 months 12 days  
Research and development        
Stock-based compensation expense for stock options and restricted common stock granted        
Stock-based compensation expense 443 1,238 $ 1,221 2,727
General and administrative        
Stock-based compensation expense for stock options and restricted common stock granted        
Stock-based compensation expense $ 665 $ 456 $ 1,927 $ 777