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Stockholders' Equity - Stock Option Plan - Fair Value Assumptions (Details)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Employee Stock Option      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Remaining contractual term (in years) 6 years 6 years 6 years
Expected volatility 60.20% 67.70% 61.90%
Risk-free interest rate 2.70% 2.10% 1.40%
Expected dividend rate 0.00% 0.00% 0.00%
Non-employee Stock Options      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Remaining contractual term (in years) 5 years 6 months 8 years 10 months 24 days 7 years 3 months 18 days
Expected volatility 59.40% 67.90% 68.90%
Risk-free interest rate 2.80% 2.30% 1.70%
Expected dividend rate 0.00% 0.00% 0.00%
Employee Stock Purchase Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Remaining contractual term (in years) 6 months 6 months 6 months
Expected volatility 50.90% 59.20% 72.00%
Risk-free interest rate 1.90% 0.90% 0.40%
Expected dividend rate 0.00% 0.00% 0.00%