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Stock-Based Compensation - Fair Value of Employee Option Grant Estimated on Date of Grant (Details) - Equity Option - $ / shares
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Estimated weighted-average fair value (USD per share) $ 11.81 $ 16.30 $ 9.91 $ 10.22
Weighted Average        
Weighted-average assumptions        
Expected volatility 79.00% 78.00% 81.00% 77.00%
Expected term, in years 6 years 1 month 6 days 6 years 1 month 6 days 6 years 6 years
Risk-free interest rate 2.10% 2.77% 2.44% 2.66%
Expected dividend yield 0.00% 0.00% 0.00% 0.00%