Quarterly report pursuant to Section 13 or 15(d)

Capitalization and Equity Structure - Additional Information (Details Textual)

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Capitalization and Equity Structure - Additional Information (Details Textual) - USD ($)
1 Months Ended 9 Months Ended
Aug. 21, 2018
Dec. 31, 2015
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Class of Stock [Line Items]          
Common stock, shares authorized (in shares)     141,429,000   141,429,000
Preferred Stock, shares authorized (in shares)     10,000,000   10,000,000
Common stock, shares outstanding (in shares)     62,617,000   59,943,000
Common stock, shares issued (in shares)     62,617,000   59,943,000
Preferred stock, shares outstanding (in shares)     0   0
Preferred stock, shares issued (in shares)     0   0
Proceeds from issuance of common stock and warrants, net     $ 3,961,000 $ 42,463,000  
Warrants issued (in shares)     0    
2015 Warrants          
Class of Stock [Line Items]          
Exercise price (in dollars per share)   $ 3.74 $ 3.74    
Warrants issued (in shares)   2,122,000 0    
Information Agent Warrants          
Class of Stock [Line Items]          
Warrants called (in shaes)       200,000  
Exercise price (in dollars per share)     $ 1.5 $ 1.5  
Warrants issued (in shares)     0    
Common Stock | At-The-Market Offering          
Class of Stock [Line Items]          
Aggregate value of offering $ 25,000,000        
Number of shares sold (in share)     1,747,000    
Price per share sold (in dollars per share)     $ 2.48    
Value of stock available for issuance     $ 20,664,000