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Statements of Cash Flows - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Operating activities      
Net loss $ (146,227) $ (68,606) $ (46,447)
Adjustments to reconcile net loss to net cash (used in) provided by operating activities:      
Depreciation expense 6,769 6,093 4,301
Noncash lease expense 3,266    
Share-based compensation expense 24,761 13,124 11,149
Intangible assets amortization 1,703 1,698 1,699
Investment amortization 773 (4,463) (1,214)
Fair value adjustment of convertible preferred stock warrant   2,266 (6)
Other 71 359 (18)
Changes in operating assets and liabilities:      
Accounts receivable, net 2,613 (7,817) 775
Inventory (4,994) (1,231) (3,046)
Prepaid expenses and other current assets (1,362) (8,576) (318)
Accounts payable and accrued liabilities 7,495 6,149 2,185
Deferred rent   78 (488)
Operating lease liabilities (886)    
Deferred revenue (43,389) 266,927 (649)
Other (276) (597) (182)
Net cash (used in) provided by operating activities (149,683) 205,404 (32,259)
Investing activities      
Purchases of property and equipment (18,803) (11,200) (6,299)
Purchases of marketable securities (694,965) (884,217) (146,503)
Proceeds from sales and maturities of marketable securities 596,724 413,720 153,538
Net cash (used in) provided by investing activities (117,044) (481,697) 736
Financing activities      
Proceeds from exercise of stock options 21,748 4,055 1,268
Proceeds from public offering of common stock, net of underwriting discounts and commissions 272,160 320,850  
Payment of public offering costs, net (321) (4,986)  
Proceeds from issuance of common stock upon the exercise of a common stock warrant   9  
Other   (12) (20)
Net cash provided by financing activities 293,587 319,916 1,248
Net increase (decrease) in cash, cash equivalents and restricted cash 26,860 43,623 (30,275)
Cash, cash equivalents and restricted cash at beginning of year 98,714 55,091 85,366
Cash, cash equivalents and restricted cash at end of year 125,574 98,714 55,091
Noncash investing and financing activities      
Purchases of equipment included in accounts payable and accrued liabilities 4,679 773 832
Landlord-funded leasehold improvements     $ 2,419
Noncash additions to property through lease financing arrangements   36,607  
Derecognition of lease financing arrangements upon adoption of guidance on accounting for leases $ 36,607    
Conversion of convertible preferred stock to common stock upon closing of initial public offering   561,931  
Conversion of convertible preferred stock warrant to common stock warrant upon closing of initial public offering   $ 2,602