Quarterly report pursuant to Section 13 or 15(d)

Summary of Significant Accounting Policies - Schedule of Potentially Dilutive Shares (Details)

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Summary of Significant Accounting Policies - Schedule of Potentially Dilutive Shares (Details) - shares
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Anti-dilutive securities (in shares) 18,486,864 29,722,758 29,868,864 29,759,417
Outstanding stock options        
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Anti-dilutive securities (in shares) 5,189,860 4,073,449 5,189,860 4,110,108
Outstanding restricted stock units        
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Anti-dilutive securities (in shares) 967,644 107,841 967,644 107,841
Warrants to purchase Series C-1 Preferred | Warrant liability        
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Anti-dilutive securities (in shares) 0 14,033 0 14,033
Warrants to purchase common stock associated with Solar loan agreement | Warrant liability        
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Anti-dilutive securities (in shares) 122,435 122,435 122,435 122,435
Warrants to purchase common stock associated with June 2016 Public Offering | Warrant liability        
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Anti-dilutive securities (in shares) 4,218,750 4,218,750 4,218,750 4,218,750
March 2018 Public Offering Series 1 [Member] | Warrant liability        
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Anti-dilutive securities (in shares) 0 13,198,075 0 13,198,075
March 2018 Public Offering Series 2 [Member] | Warrant liability        
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Anti-dilutive securities (in shares) 7,988,175 7,988,175 7,988,175 7,988,175
Common stock associated with 6% convertible senior notes        
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Anti-dilutive securities (in shares) 0 0 11,382,000 0