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Stock-Based Compensation - Additional Information (Details) - USD ($)
1 Months Ended 3 Months Ended 9 Months Ended
Jul. 31, 2018
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Tax benefits for stock-based compensation expense recognized   $ 0 $ 0 $ 0 $ 0
Stock options granted       542,000  
Total unrecognized stock-based compensation expense relating to unvested stock options   41,000,000.0   $ 41,000,000.0  
Total stock-based compensation expense   $ 3,960,000 $ 979,000 $ 9,899,000 $ 2,031,000
Performance-Based Vesting          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Stock options granted   0 0 0 0
Market-Based Vesting          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Stock options granted   0 0 0 0
Service-Based Vesting | Non-Employees          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Stock options granted   0 0 0 0
Stock Options          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Weighted-average recognition period       3 years  
2018 Employee Stock Purchase Plan          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Common stock shares reserved for issuance 500,000        
Increase in number of common stock shares reserved for issuance per year 1,000,000        
Percentage increase in shares reserved for issuance per year 1.00%        
Price as a percentage of fair market value of common stock 85.00%        
Consecutive overlapping offering period 24 months        
First offering period commence date       Jul. 18, 2018  
First offering period end date       Aug. 15, 2020  
Second offering period commence date       Feb. 15, 2019  
Third offering period commence date       Aug. 16, 2019  
Total stock-based compensation expense   $ 200,000   $ 500,000  
2018 Equity Incentive Plan          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Common stock shares reserved for issuance 4,000,000        
Increase in number of common stock shares reserved for issuance per year 5,000,000        
Percentage increase in shares reserved for issuance per year 5.00%        
Maximum number of additional shares authorized to be added to the Plan       5,279,548  
2012 Equity Incentive Plan          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Award vesting period 4 years        
2012 Equity Incentive Plan | Maximum          
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]          
Option expiration period 10 years