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Summary of Significant Accounting Policies - Antidilutive Securities (Details) - shares
6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Net Loss Per Share    
Antidilutive securities excluded from the calculation of basic and diluted loss per share (in shares) 10,142,486 17,805,051
Options to purchase Common Stock    
Net Loss Per Share    
Antidilutive securities excluded from the calculation of basic and diluted loss per share (in shares) 2,966,843 1,924,039
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 12,329,435
Restricted Stock Units    
Net Loss Per Share    
Antidilutive securities excluded from the calculation of basic and diluted loss per share (in shares) 0 2,241
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 3,548,459