Quarterly report pursuant to Section 13 or 15(d)

Fair Value of Warrants - Summary of Derivative Warrant Activity (Details)

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Fair Value of Warrants - Summary of Derivative Warrant Activity (Details) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Apr. 02, 2018
Dec. 31, 2017
Class of Warrants Outstanding [Roll Forward]        
Fair value of warrants outstanding as of December 31, 2017 $ 4,403      
Change in fair value of warrants (2,858)      
Reclassification of warrants from liability to equity (423) $ 0    
Fair value of warrants outstanding as of September 30, 2018 $ 1,122      
Warrants, number of shares of common stock with right to purchase (in shares) 10,055,000     10,055,000
2017 Debt        
Class of Warrants Outstanding [Roll Forward]        
Fair value of warrants outstanding as of December 31, 2017 $ 501      
Change in fair value of warrants (78)      
Reclassification of warrants from liability to equity (423)      
Fair value of warrants outstanding as of September 30, 2018 0      
Warrants, number of shares of common stock with right to purchase (in shares)     443,262  
2016 Offerings        
Class of Warrants Outstanding [Roll Forward]        
Fair value of warrants outstanding as of December 31, 2017 1,929      
Change in fair value of warrants (1,134)      
Reclassification of warrants from liability to equity 0      
Fair value of warrants outstanding as of September 30, 2018 795      
2017 Offering        
Class of Warrants Outstanding [Roll Forward]        
Fair value of warrants outstanding as of December 31, 2017 1,973      
Change in fair value of warrants (1,646)      
Reclassification of warrants from liability to equity 0      
Fair value of warrants outstanding as of September 30, 2018 $ 327