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Stock-Based Compensation - Employee Stock Purchase Plan (Details) - Shares available for issuance under the 2018 Employee Stock Purchase Plan
3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Expected term (in years) 6 months 6 months 6 months 6 months
Expected volatility 73.20% 60.20% 73.20% 60.20%
Risk-free interest rate 0.20% 2.50% 0.20% 2.50%
Expected dividend yield 0.00% 0.00% 0.00% 0.00%