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Share based Compensation - Weighted-average Assumptions (Details) - Options to purchase common stock - $ / shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected stock price volatility 78.60% 82.80% 81.30%
Risk free interest rate 2.40% 2.00% 1.50%
Expected life of options (years) 5 years 7 months 12 days 6 years 2 months 5 days 6 years 3 months
Expected annual dividend per share $ 0 $ 0 $ 0