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Stockholders' Equity - Weighted-Average assumptions (Details) - $ / shares
9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Weighted-average assumptions    
Expected term 6 years 3 months 6 years 3 months
Expected volatility 71.15% 73.83%
Risk-free interest rate 2.54% 2.53%
Expected dividend yield 0.00% 0.00%
2013 Equity Incentive Plan    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Weighted-average fair value of the options granted $ 7.17 $ 13.11