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Equity Incentive Plan - 2019 Employee Stock Purchase Plan (Details) - USD ($)
3 Months Ended 9 Months Ended
Jun. 19, 2019
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Stock-based compensation expense   $ 1,878,000 $ 342,000 $ 4,013,000 $ 832,000
Class A common stock | 2019 Employee Stock Purchase Plan          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Shares reserved for issuance 283,333        
Stock-based compensation expense   $ 177,000   $ 177,000  
Expected dividend       0.00%  
Class A common stock | 2019 Employee Stock Purchase Plan | Maximum          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Percentage of additional number of shares authorized annually 1.00%        
Additional number of shares authorized annually 416,666