CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Cash flows from operating activities:        
Net loss   $ (118,986) $ (62,012)  
Adjustments to reconcile net loss to net cash used in operating activities:        
Stock-based compensation expense   30,440 18,402  
Depreciation and amortization expense   2,294 879  
Change in fair value of preferred stock warrant liability $ 426   2,187  
Accretion of discount on investments   (1,939) (198)  
Loss on disposal of property and equipment   119    
Non-cash interest expense   201 26  
Changes in operating assets and liabilities:        
Prepaid expenses and other current assets   (2,129) (2,792)  
Operating lease, right-of-use-asset   3,336    
Other assets   (293)    
Accounts payable   3,184 2,402  
Accrued expenses and other current liabilities   3,725 2,472  
Deferred rent     (14)  
Operating lease liabilities   (1,411)    
Net cash used in operating activities   (81,459) (38,648)  
Cash flows from investing activities:        
Purchases of property, plant and equipment   (27,381) (11,671)  
Purchases of investments   (259,231) (87,369)  
Sales and maturities of investments   143,275 33,460  
Net cash used in investing activities   (143,337) (65,580)  
Cash flows from financing activities:        
Proceeds from issuance of convertible preferred stock, net of issuance costs     100,986  
Proceeds from initial public offering of common stock, net of commissions and underwriting discounts     257,866  
Payments of initial public offering costs     (3,449)  
Proceeds from repayment of promissory note     245  
Repurchase of unvested restricted common stock   (122)    
Proceeds from borrowings under loan and security agreement   25,000    
Proceeds from issuance of common stock upon exercise of stock options   1,843 18  
Net cash provided by financing activities   26,721 355,666  
Net increase (decrease) in cash, cash equivalents and restricted cash   (198,075) 251,438  
Cash, cash equivalents and restricted cash at beginning of period   309,421 104,572 $ 104,572
Cash, cash equivalents and restricted cash at end of period $ 356,010 111,346 356,010 $ 309,421
Supplemental cash flow information:        
Cash paid for interest, net of interest capitalized   1,706    
Cash paid for leases   3,194    
Supplemental disclosure of non-cash investing and financing information:        
Lease assets obtained in exchange for new operating lease liabilities   49,496    
Purchases of property, plant and equipment included in accounts payable or accrued expenses   6,073 1,353  
Amounts capitalized under build-to-suit lease transaction     24,657  
Deferred offering costs and issuance costs included in accounts payable or accrued expenses   $ 11    
Offering costs and issuance costs included in accounts payable and accrued expenses     99  
Reclassification of warrant liability to additional paid-in capital     3,053  
Conversion of preferred stock to common stock upon closing of the initial public offering     $ 240,776