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Stock-Based Compensation - Assumptions (Details)
12 Months Ended
Dec. 31, 2017
shares
Dec. 31, 2017
component
shares
Dec. 31, 2016
Dec. 31, 2015
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)   2,078,000    
Employee stock options        
Valuation of Stock Option Grants to Employees        
Expected volatility, minimum   40.75% 63.06% 69.65%
Expected volatility, maximum   62.93% 72.49% 78.82%
Expected dividend rate   0.00% 0.00% 0.00%
Risk free interest rate, minimum   1.67% 0.97% 1.56%
Risk free interest rate, maximum   2.16% 1.92% 2.32%
Employee stock options | Minimum        
Valuation of Stock Option Grants to Employees        
Expected term (years)   5 years 1 month 21 days 5 years 1 month 6 days 5 years 6 months 29 days
Employee stock options | Maximum        
Valuation of Stock Option Grants to Employees        
Expected term (years)   5 years 2 months 27 days 5 years 2 months 12 days 10 years
Non-employee stock options        
Stock Based Compensation        
Options granted (in shares) 135,746      
Options unvested (in shares) 3,401 3,401    
Valuation of Stock Option Grants to Employees        
Expected volatility, minimum   41.22% 50.90% 70.17%
Expected volatility, maximum   52.18% 72.61% 75.35%
Expected dividend rate   0.00% 0.00% 0.00%
Risk free interest rate, minimum   1.42% 0.68% 1.74%
Risk free interest rate, maximum   1.95% 1.38% 2.24%
Non-employee stock options | Minimum        
Valuation of Stock Option Grants to Employees        
Expected term (years)   2 years 8 months 1 day 2 years 7 months 24 days 4 years 11 months 9 days
Non-employee stock options | Maximum        
Valuation of Stock Option Grants to Employees        
Expected term (years)   2 years 8 months 16 days 3 years 1 month 28 days 9 years 9 months 11 days