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Stockholders' Equity - Key assumptions of stock options granted (Details)
9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Stockholders' Equity    
Expected term 5 years 8 months 1 day 5 years 6 months
Risk-free interest rate 1.82% 2.72%
Expected dividend yield 0.00% 0.00%
Expected volatility 80.38% 77.29%