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Stock-based Compensation - Fair Value of Options Granted Using the Black-Scholes Pricing Model (Details) - Stock options
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Expected volatility rate 47.80%      
Volatility, minimum   42.40% 41.70% 38.60%
Volatility, maximum   44.10% 44.20% 47.80%
Risk free interest rate 2.80%      
Risk-free interest rate, minimum   1.90% 1.90% 2.60%
Risk-free interest rate, maximum   2.10% 2.20% 2.90%
Expected term (in years) 6 years 1 month 6 days 6 years 3 months    
Dividend rate 0.00% 0.00% 0.00% 0.00%
Minimum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Expected term (in years)   5 years 6 months 5 years 6 months 5 years 6 months
Maximum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Expected term (in years)     6 years 3 months 18 days 6 years 3 months 18 days