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Share based Compensation - Weighted-average Assumptions (Details) - Options to purchase common stock - $ / shares
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected stock price volatility 74.20% 81.20%
Risk free interest rate 2.50% 2.30%
Expected life of options (years) 5 years 248 days 5 years 244 days
Expected annual dividend per share (in dollars per share) $ 0 $ 0