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Stock-Based Compensation (Details) - USD ($)
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation $ 3,057,312 $ 2,411,455
Total unrecognized compensation cost related to unvested stock options $ 8,300,000  
Period over which total unrecognized compensation cost related to unvested stock options will be recognized 2 years 5 months  
Weighted average grant date fair value ($ per share) $ 4.29 $ 4.57
Restricted Stock Units (RSUs) [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Total unrecognized compensation cost related to unvested stock options $ 4,800,000  
Period over which total unrecognized compensation cost related to unvested stock options will be recognized 2 years 8 months  
Weighted average grant date fair value ($ per share) for restricted stock units $ 7.02 $ 7.56
Employees and Directors [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation $ 3,000,000 $ 2,200,000
Employees and Directors [Member] | Stock Options [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Risk-free interest rate 0.91% 0.99%
Expected volatility 76.00% 74.00%
Expected life in years 5 years 5 years
Dividend yield 0.00% 0.00%
Forfeiture rate 7.00% 8.00%
Non Employee [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation $ 197,000 $ 168,000
Research and Development Expense [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Allocated share-based compensation expense 1,700,000 1,400,000
General and Administrative Expense [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Allocated share-based compensation expense $ 1,300,000 $ 888,000