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Stock-Based Compensation - Assumptions (Details)
12 Months Ended
Dec. 31, 2018
component
shares
Dec. 31, 2017
Dec. 31, 2016
Valuation of Stock Option Grants to Employees      
Number of components averaged to estimate expected term | component 2    
Employee and non-employee stock options      
Stock Based Compensation      
Options granted (in shares) 2,319,000    
Employee stock options      
Valuation of Stock Option Grants to Employees      
Expected volatility, minimum 40.28% 40.75% 63.06%
Expected volatility, maximum 42.53% 62.93% 72.49%
Expected dividend rate 0.00% 0.00% 0.00%
Risk free interest rate, minimum 2.37% 1.67% 0.97%
Risk free interest rate, maximum 3.06% 2.16% 1.92%
Employee stock options | Minimum      
Valuation of Stock Option Grants to Employees      
Expected term (years) 5 years 2 months 27 days 5 years 1 month 21 days 5 years 1 month 6 days
Employee stock options | Maximum      
Valuation of Stock Option Grants to Employees      
Expected term (years) 5 years 7 months 13 days 5 years 2 months 27 days 5 years 2 months 12 days
Non-employee stock options      
Stock Based Compensation      
Options granted (in shares) 131,865    
Options unvested (in shares) 36,344    
Valuation of Stock Option Grants to Employees      
Expected volatility, minimum 41.36% 41.22% 50.90%
Expected volatility, maximum 43.84% 52.18% 72.61%
Expected dividend rate 0.00% 0.00% 0.00%
Risk free interest rate, minimum 2.36% 1.42% 0.68%
Risk free interest rate, maximum 2.86% 1.95% 1.38%
Non-employee stock options | Minimum      
Valuation of Stock Option Grants to Employees      
Expected term (years) 2 years 8 months 19 days 2 years 8 months 1 day 2 years 7 months 24 days
Non-employee stock options | Maximum      
Valuation of Stock Option Grants to Employees      
Expected term (years) 2 years 9 months 4 days 2 years 8 months 16 days 3 years 1 month 28 days