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Stock Based Compensation (Details) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation expense $ 887 $ 833
General and Administrative Expense [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation expense 388 494
Research and Development Expense [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock-based compensation expense $ 498 $ 339
Stock Options [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Options granted (in shares) 670,000  
Options granted, weighted average grant date fair value (in dollars per share) $ 2.61  
Unrecognized stock-based compensation expense expected to be recognized related to non-vested stock options $ 7,179  
Weighted average amortization period 3 years 1 month 28 days  
Restricted Stock Units (RSUs) [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
RSUs granted (in shares) 20,000  
Weighted average grant date fair value of RSU's granted (in dollars per share) $ 4.05  
Unrecognized stock-based compensation expense expected to be recognized related to non-vested RSUs $ 1,614  
Weighted average amortization period 2 years 2 months 5 days  
2013 Plan [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Shares authorized for issuance (in shares) 16,624,469  
Shares available for grant (in shares) 2,238,562