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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, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Expected stock price volatility 74.20% 81.10% 74.20% 81.20%
Risk free interest rate 2.00% 2.80% 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