Nature of the Business and Basis of Presentation (Details) - USD ($)
$ in Thousands
1 Months Ended 3 Months Ended 12 Months Ended
Jul. 20, 2018
Jul. 31, 2018
Dec. 31, 2020
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Sale of Equity                            
Net proceeds after deducting underwriting discounts and commissions and other offering costs                         $ 257,866  
Net Loss     $ (40,539) $ (40,852) $ (37,854) $ (48,486) $ (44,472) $ (47,015) $ (39,390) $ (32,581) $ (167,731) $ (163,458) (89,195)  
Accumulated deficit     (480,471)       $ (312,740)       (480,471) $ (312,740) $ (150,082) $ (60,979)
Current cash and investments     $ 176,300               $ 176,300      
Common stock                            
Sale of Equity                            
Shares issued (in shares)                         12,055,450  
Conversion of redeemable convertible preferred stock to common stock (in shares) 51,845,438 51,845,438                     51,845,438  
IPO                            
Sale of Equity                            
Shares issued (in shares) 12,055,450                          
Net proceeds after deducting underwriting discounts and commissions and other offering costs $ 254,300                          
Overallotment                            
Sale of Equity                            
Shares issued (in shares) 1,572,450