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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, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Expected stock price volatility 75.50% 82.50% 80.70% 83.10%
Risk free interest rate 2.70% 1.80% 2.40% 2.00%
Expected life of options (years) 5 years 7 months 15 days 5 years 10 months 20 days 5 years 7 months 15 days 6 years 2 months 16 days
Expected annual dividend per share (in dollars per share) $ 0 $ 0 $ 0 $ 0