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Stock Option Plan - Fair Value Assumptions (Details)
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]        
Expected volatility 59.00% 70.00% 59.00% 70.00%
Risk-free interest rate 2.00% 2.60%    
Risk-free interest rate, minimum     2.00% 1.40%
Risk-free interest rate, maximum     2.60% 2.60%
Dividend yield 0.00% 0.00% 0.00% 0.00%
Expected term (in years)   6 years 29 days   6 years 29 days
Minimum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Expected term (in years) 5 years 9 months   5 years 9 months  
Maximum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Expected term (in years) 6 years 29 days   6 years 29 days