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)
$ in Thousands
 3 Months Ended
Mar. 31, 2018
USD ($)
Class of Warrants Outstanding [Roll Forward]  
Fair value of warrants outstanding as of December 31, 2017 $ 4,403
Change in fair value of warrants (692)
Fair value of warrants outstanding as of March 31, 2018 3,711
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)
Fair value of warrants outstanding as of March 31, 2018 423
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 27
Fair value of warrants outstanding as of March 31, 2018 1,956
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 (641)
Fair value of warrants outstanding as of March 31, 2018 $ 1,332