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Summary of Significant Accounting Policies - Antidilutive Securities (Details) - shares
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Net Loss Per Share    
Antidilutive securities excluded from the calculation of basic and diluted loss per share (in shares) 9,025,380 11,496,978
Options to purchase Common Stock    
Net Loss Per Share    
Antidilutive securities excluded from the calculation of basic and diluted loss per share (in shares) 1,849,737 975,233
Warrants to purchase Common Stock    
Net Loss Per Share    
Antidilutive securities excluded from the calculation of basic and diluted loss per share (in shares) 4,490,159 10,516,377
Restricted Stock Units    
Net Loss Per Share    
Antidilutive securities excluded from the calculation of basic and diluted loss per share (in shares) 0 4,491
Series A Convertible Preferred Stock    
Net Loss Per Share    
Antidilutive securities excluded from the calculation of basic and diluted loss per share (in shares) 877 877
Series E Convertible Preferred Stock    
Net Loss Per Share    
Antidilutive securities excluded from the calculation of basic and diluted loss per share (in shares) 2,684,607 0