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Stockholders' Equity - Weighted-Average assumptions (Details) - $ / shares
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Weighted-average assumptions    
Expected term 6 years 3 months 6 years 3 months
Expected volatility 71.34% 73.89%
Risk-free interest rate 2.62% 2.42%
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 $ 7.00 $ 13.70