Quarterly report pursuant to Section 13 or 15(d)

4. Stock-Based Compensation (Details)

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4. Stock-Based Compensation (Details)
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Stock-based Compensation        
Risk free interest rate 2.75% 1.87%   1.87%
Risk free interest rate, minimum     2.65%  
Risk free interest rate, maximum     2.92%  
Dividend yield 0.00% 0.00% 0.00% 0.00%
Expected term 5 years 5 years 5 years 5 years
Expected volatility 53.40% 44.60%   44.60%
Expected volatility, minimum     49.00%  
Expected volatility, maximum     53.40%