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Share based Compensation - Weighted-average Assumptions (Details) - Options to purchase common stock - $ / shares
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected stock price volatility 74.10% 78.60% 82.80%
Risk free interest rate 2.40% 2.40% 2.00%
Expected life of options (years) 5 years 8 months 4 days 5 years 7 months 13 days 6 years 2 months 4 days
Expected annual dividend per share $ 0.00 $ 0.00 $ 0.00