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Share-Based Compensation - Fair Value Assumption for Stock Options (Details) - Options to purchase Common Stock - $ / shares
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected term, in years 6 years 4 months 6 days 5 years
Expected volatility 84.20% 84.00%
Risk-free interest rate 2.10% 1.20%
Expected dividend yield 0.00% 0.00%
Weighted average fair value of underlying common stock (in usd per share) $ 11.37 $ 1.05