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Stock Option Plan - Weighted Average Assumptions (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 term 6 years 14 days 6 years 11 days 5 years 11 months 23 days 5 years 18 days
Dividend yield 0.00% 0.00% 0.00% 0.00%
Minimum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Expected volatility 41.00% 42.00% 41.00% 41.00%
Risk-free interest rate 2.74% 1.73% 2.35% 1.73%
Maximum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Expected volatility 44.00% 46.00% 46.00% 55.00%
Risk-free interest rate 2.80% 2.15% 2.93% 2.75%