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Share-Based Compensation - Weighted-average Assumptions (Details) - Options to purchase common stock - $ / shares
3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Expected stock price volatility 65.50% 74.90% 66.40% 75.20%
Risk free interest rate 0.70% 0.40% 0.50% 1.60%
Expected life of options (years) 5 years 4 months 24 days 5 years 8 months 12 days 5 years 4 months 24 days 5 years 8 months 12 days
Expected annual dividend per share (in dollars per share) $ 0 $ 0 $ 0 $ 0