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Stockholders' Equity - Weighted-Average assumptions (Details) - $ / shares
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Stockholders' Equity      
Weighted-average fair value of the options granted $ 1.85 $ 3.17 $ 7.17
Weighted-average assumptions      
Expected term 6 years 3 months 6 years 3 months 6 years 3 months
Expected volatility 84.19% 84.40% 71.15%
Risk-free interest rate 0.78% 1.41% 2.54%
Expected dividend yield 0.00% 0.00% 0.00%