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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, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Expected stock price volatility 73.60% 75.50% 74.10% 80.70%
Risk free interest rate 1.60% 2.70% 2.40% 2.40%
Expected life of options (years) 5 years 8 months 4 days 5 years 7 months 13 days 5 years 8 months 4 days 5 years 7 months 13 days
Expected annual dividend per share (in dollars per share) $ 0 $ 0 $ 0 $ 0