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, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Stock-based compensation expense for stock options and restricted common stock granted        
Stock-based compensation expense $ 1,694 $ 1,261 $ 3,504 $ 2,385
Unrecognized compensation cost related to non-vested stock options and unvested restricted common stock granted to employees 9,300   $ 9,300  
Weighted average period over which future compensation cost is expected to be recognized for awards granted to employees     3 years 2 months 12 days  
Unrecognized compensation cost related to non-vested stock options granted to non-employees 900   $ 900  
Research and development        
Stock-based compensation expense for stock options and restricted common stock granted        
Stock-based compensation expense 1,238 1,076 2,727 2,073
General and administrative        
Stock-based compensation expense for stock options and restricted common stock granted        
Stock-based compensation expense $ 456 $ 185 $ 777 $ 312