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Stock Incentive Plans - Schedule of Weighted Average Assumptions for Employee Stock Options (Details) - Employee stock option grants - $ / shares
6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Risk-free interest rate 2.82% 1.88%
Dividend yield 0.00% 0.00%
Expected term 6 years 15 days 6 years 18 days
Expected volatility 85.45% 84.28%
Weighted-average fair value of common stock (in dollars per share) $ 12.37 $ 16.89