CONDENSED CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS’ EQUITY (DEFICIT) - USD ($)
$ in Thousands
Common stock
Additional paid-in capital
Accumulated other comprehensive loss
Accumulated deficit
Convertible preferred stock
Total
Balances at Beginning of period (in shares) at Dec. 31, 2017 14,977,317       43,933,006  
Balances at Beginning of period at Dec. 31, 2017 $ 15 $ 17,277   $ (60,979) $ 139,790 $ (43,687)
Issuance of common stock upon exercise of stock options (in shares) 19,750          
Issuance of common stock upon exercise of stock options   4       4
Stock-based compensation expense   2,613       2,613
Unrealized gains (losses) on investments     $ (45)     (45)
Net loss       (14,411)   (14,411)
Balances at End of period (in shares) at Mar. 31, 2018 14,997,067       51,845,438  
Balances at End of period at Mar. 31, 2018 $ 15 19,802 (45) (75,298) $ 240,776 (55,526)
Convertible preferred stock            
Issuance of stock, net of issuance costs (in shares)         7,912,432  
Issuance of stock, net of issuance costs         $ 100,986  
Balances at Beginning of period (in shares) at Dec. 31, 2017 14,977,317       43,933,006  
Balances at Beginning of period at Dec. 31, 2017 $ 15 17,277   (60,979) $ 139,790 (43,687)
Net loss           (62,012)
Balances at End of period (in shares) at Sep. 30, 2018 79,042,603          
Balances at End of period at Sep. 30, 2018 $ 79 533,846 (19) (122,899)   411,007
Cumulative effect adjustment for adoption | ASU 2018-07   (92)   92    
Balances at Beginning of period (in shares) at Mar. 31, 2018 14,997,067       51,845,438  
Balances at Beginning of period at Mar. 31, 2018 $ 15 19,802 (45) (75,298) $ 240,776 (55,526)
Issuance of common stock upon exercise of stock options (in shares) 12,750          
Issuance of common stock upon exercise of stock options   14       14
Stock-based compensation expense   5,902       5,902
Vesting of restricted common stock   122       122
Unrealized gains (losses) on investments     18     18
Net loss       (21,239)   (21,239)
Balances at End of period (in shares) at Jun. 30, 2018 15,009,817       51,845,438  
Balances at End of period at Jun. 30, 2018 $ 15 25,840 (27) (96,537) $ 240,776 (70,709)
Shares issued 12,055,450          
Issuance of common stock, initial public offering, net of issuance cost $3,548 $ 12 254,306       254,318
Cashless exercise of warrants 131,273          
Issuance of common stock upon exercise of stock options (in shares) 625          
Stock-based compensation expense   9,887       9,887
Vesting of restricted common stock   36       36
Conversion of preferred stock warrant to common stock warrant upon closing of initial public offering   3,053       3,053
Conversion of redeemable convertible preferred stock to common stock (in shares) 51,845,438       (51,845,438)  
Conversion of redeemable convertible preferred stock to common stock $ 52 240,724     $ (240,776) 240,776
Unrealized gains (losses) on investments     8     8
Net loss       (26,362)   (26,362)
Balances at End of period (in shares) at Sep. 30, 2018 79,042,603          
Balances at End of period at Sep. 30, 2018 $ 79 533,846 (19) (122,899)   411,007
Balances at Beginning of period (in shares) at Dec. 31, 2018 79,234,853          
Balances at Beginning of period at Dec. 31, 2018 $ 79 543,040 (29) (150,082)   393,008
Issuance of common stock upon exercise of stock options (in shares) 619,342          
Issuance of common stock upon exercise of stock options $ 1 798       799
Stock-based compensation expense   9,478       9,478
Vesting of restricted common stock   22       22
Unrealized gains (losses) on investments     110     110
Net loss       (32,581)   (32,581)
Balances at End of period (in shares) at Mar. 31, 2019 79,854,195          
Balances at End of period at Mar. 31, 2019 $ 80 553,338 81 (181,863)   $ 371,636
Balances at Beginning of period (in shares) at Dec. 31, 2018           0
Balances at Beginning of period (in shares) at Dec. 31, 2018 79,234,853          
Balances at Beginning of period at Dec. 31, 2018 $ 79 543,040 (29) (150,082)   $ 393,008
Net loss           (118,986)
Balances at End of period (in shares) at Sep. 30, 2019 79,777,733          
Balances at End of period at Sep. 30, 2019 $ 80 575,365 166 (268,268)   $ 307,343
Balances at Beginning of period (in shares) at Dec. 31, 2018           0
Balances at End of period (in shares) at Sep. 30, 2019           0
Cumulative effect adjustment for adoption | ASU 2016-02       800   $ 800
Balances at Beginning of period (in shares) at Mar. 31, 2019 79,854,195          
Balances at Beginning of period at Mar. 31, 2019 $ 80 553,338 81 (181,863)   371,636
Issuance of common stock upon exercise of stock options (in shares) 223,484          
Issuance of common stock upon exercise of stock options   572       572
Stock-based compensation expense   10,604       10,604
Vesting of restricted common stock   13       13
Unrealized gains (losses) on investments     209     209
Net loss       (39,390)   (39,390)
Balances at End of period (in shares) at Jun. 30, 2019 80,077,679          
Balances at End of period at Jun. 30, 2019 $ 80 564,527 290 (221,253)   343,644
Issuance of common stock upon exercise of stock options (in shares) 367,971          
Issuance of common stock upon exercise of stock options $ 1 471       472
Stock-based compensation expense   10,358       10,358
Repurchase of unvested restricted common stock (in shares) (667,917)          
Repurchase of unvested restricted common stock $ (1)         (1)
Vesting of restricted common stock   9       9
Unrealized gains (losses) on investments     (124)     (124)
Net loss       (47,015)   (47,015)
Balances at End of period (in shares) at Sep. 30, 2019 79,777,733          
Balances at End of period at Sep. 30, 2019 $ 80 $ 575,365 $ 166 $ (268,268)   $ 307,343
Balances at End of period (in shares) at Sep. 30, 2019           0