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Share-Based Compensation - Weighted-average Assumptions (Details) - Options to purchase common stock - $ / shares
3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Expected stock price volatility 74.20% 73.60% 75.20% 74.10%
Risk free interest rate 0.30% 1.60% 1.60% 2.40%
Expected life of options 5 years 8 months 1 day 5 years 8 months 4 days 5 years 8 months 1 day 5 years 8 months 4 days
Expected annual dividend per share (in dollars per share) $ 0 $ 0 $ 0 $ 0