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Share-Based Compensation - Fair Value Assumption for Stock Options (Details) - Options to purchase Common Stock - $ / shares
9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected term, in years 6 years 4 months 6 years 5 months 9 days
Expected volatility 84.90% 83.80%
Risk-free interest rate 2.60% 2.10%
Expected dividend yield 0.00% 0.00%
Weighted-average exercise price (in usd per share) $ 7.48 $ 11.48