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Stock-Based Compensation (Details) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended 3 Months Ended 6 Months Ended
May 31, 2017
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Share-based Compensation [Abstract]          
Withholding taxes paid on shares issued upon conversion of RSUs       $ 33 $ 29
Stock-based compensation expense       2,008 1,715
General and Administrative Expense [Member]          
Share-based Compensation [Abstract]          
Stock-based compensation expense   $ 467 $ 442 855 834
Research and Development Expense [Member]          
Share-based Compensation [Abstract]          
Stock-based compensation expense   $ 655 $ 440 $ 1,154 $ 881
Stock Options [Member]          
Share-based Compensation [Abstract]          
Options granted (in shares)   336,000 331,000 1,006,000 356,000
Options granted, weighted average grant date fair value (in dollars per share)   $ 2.31 $ 2.72 $ 2.52 $ 2.65
Dividend yield       0.00% 0.00%
Expected stock price volatility       85.00% 84.00%
Risk-free interest rate       2.65% 1.94%
Expected option term       6 years 6 years
Unrecognized stock-based compensation expense expected to be recognized related to non-vested stock options   $ 7,160   $ 7,160  
Weighted average amortization period       3 years 29 days  
Options exercised (in shares)       0  
Restricted Stock Units (RSUs) [Member]          
Share-based Compensation [Abstract]          
RSUs granted (in shares)   223,997 220,664 243,997 220,664
Weighted average grant date fair value of RSU's granted (in dollars per share)   $ 3.19 $ 3.83 $ 3.28 $ 3.83
Withholding taxes paid on shares issued upon conversion of RSUs   $ 33 $ 29 $ 33 $ 29
Unrecognized stock-based compensation expense expected to be recognized related to non-vested RSUs   $ 2,001   $ 2,001  
Weighted average amortization period       2 years 8 months 19 days  
New shares of common stock issued as a result of vesting RSUs (in shares)       168,219  
2013 Plan [Member]          
Share-based Compensation [Abstract]          
Additional shares authorized for issuance (in shares) 2,500,000        
Shares authorized for issuance (in shares)   16,624,469   16,624,469  
Shares available for grant (in shares)   1,780,315   1,780,315