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 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Anti-dilutive securities (in shares) 30,257,565 29,834,858 30,257,565 29,834,858
Outstanding stock options        
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Anti-dilutive securities (in shares) 5,578,978 4,076,415 5,578,978 4,076,415
Outstanding restricted stock units        
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]        
Anti-dilutive securities (in shares) 967,227 101,425 967,227 101,425
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 13,313,625 0 13,313,625
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) 11,382,000 0 11,382,000 0