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Stockholders' Equity - Additional Information (Details)
 1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
May 21, 2020
USD ($)
$ / shares
shares
Sep. 06, 2018
$ / shares
shares
May 21, 2018
$ / shares
shares
Jan. 04, 2018
USD ($)
Dec. 03, 2015
USD ($)
$ / shares
shares
Jan. 31, 2018
shares
Nov. 30, 2017
shares
Jun. 30, 2020
USD ($)
shares
Mar. 31, 2019
USD ($)
shares
Sep. 30, 2020
USD ($)
vote
$ / shares
shares
Sep. 30, 2019
USD ($)
Dec. 31, 2019
$ / shares
shares
Stockholders Equity [Line Items]                        
Common stock number of votes per share | vote                   1    
Issuance of common stock in connection with a public offering, net of underwriting discounts, commissions, and offering costs (in shares) 9,433,963                      
Offering price (in dollars per share) | $ / shares $ 13.25                      
Proceeds from issuance of common stock | $ $ 117,100,000                 $ 117,110,000 $ 291,311,000  
Common stock, par value (in dollars per share) | $ / shares                   $ 0.0001   $ 0.0001
Issuance of common stock in connection with a public offering net of underwriting discounts, commissions and offering costs | $               $ 117,094,000 $ 291,344,000      
Shares of common stock, repurchase (in shares)                   3,434,613   4,648,526
Series A Convertible Preferred Stock | Stock Restriction Agreements                        
Stockholders Equity [Line Items]                        
Shares subject to forfeiture (in shares)             1,305,427          
Founder                        
Stockholders Equity [Line Items]                        
Issuance of common stock in connection with a public offering, net of underwriting discounts, commissions, and offering costs (in shares)   1,795,023 251,547                  
Percentage of fully diluted share capital       15.00%                
Increased in equity capital amount | $       $ 300,000,000.0                
Common stock issued price per share | $ / shares   $ 9.63 $ 2.61                  
Additional shares of restricted stock subject to vesting restrictions (in shares)   1,795,023 251,547                  
Common stock                        
Stockholders Equity [Line Items]                        
Conversion of convertible preferred stock into common stock (in shares)                 30,493,460      
Issuance of common stock in connection with a public offering, net of underwriting discounts, commissions, and offering costs (in shares)               9,433,963 19,837,500      
Issuance of common stock in connection with a public offering net of underwriting discounts, commissions and offering costs | $               $ 1,000 $ 2,000      
Common stock | Stock Restriction Agreements                        
Stockholders Equity [Line Items]                        
Unrestricted shares of common stock were subject to service vesting conditions (in shares)           4,580,444            
Founder Shares                        
Stockholders Equity [Line Items]                        
Issuance of common stock in connection with a public offering, net of underwriting discounts, commissions, and offering costs (in shares)         9,160,888              
Common stock, par value (in dollars per share) | $ / shares         $ 0.0001              
Issuance of common stock in connection with a public offering net of underwriting discounts, commissions and offering costs | $         $ 4,100              
Common Stock vesting percentage       50.00%                
Common stock vesting period       5 years