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Stockholders' Equity - Weighted-Average assumptions (Details) - $ / shares
9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Weighted-average assumptions    
Expected term 6 years 3 months 6 years 3 months
Expected volatility 84.17% 84.40%
Risk-free interest rate 0.75% 1.41%
Expected dividend yield 0.00% 0.00%
2013 Equity Incentive and Inducement Plan    
Share-based Compensation Arrangement by Share-based Payment Award    
Weighted-average fair value of the options granted $ 1.87 $ 3.17