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Consolidated Statements of Cash Flows - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Cash flow from operating activities      
Net loss $ (115,976) $ (133,746) $ (109,954)
Adjustments to reconcile net loss to net cash used in operating activities:      
Stock-based compensation expense 23,156 27,243 26,598
Depreciation 3,959 2,830 3,254
Realized gain on corporate equity securities (16,366)    
Non-cash investment in equity securities     (3,667)
Other non-cash items, net 104 (2,928) (3,268)
Changes in operating assets and liabilities:      
Accounts receivable (5,630) (388) 649
Prepaid expenses and other current assets (4,643) (495) (3,410)
Right-of-use assets 3,633 (9,300)  
Other non-current assets (719) (15) (92)
Accounts payable 855 274 1,780
Accrued expenses 1,707 9,485 4,042
Deferred revenue (91,794) 55,395 22,889
Operating lease liabilities (2,946) 9,324  
Other current and non-current liabilities (2,683) 1,652 1,030
Non-cash research and development expenses 27,500   14,442
Net cash used in operating activities (179,843) (40,669) (45,707)
Cash flow from investing activities      
Purchases of property and equipment (7,162) (6,167) (4,754)
Proceeds from the sale of equipment 12 102 37
Purchases of marketable securities (458,404) (342,183) (459,370)
Proceeds from maturities of marketable securities 305,000 360,500 411,000
Proceeds from sale of corporate equity securities 20,032    
Net cash (used in) provided by investing activities (140,522) 12,252 (53,087)
Cash flow from financing activities      
Proceeds from offering of common stock, net of issuance costs 203,726 116,341 76,789
Proceeds from exercise of stock options 19,501 14,863 10,328
Payments on construction financing lease obligation     (857)
Issuance of common stock under benefit plans 895 620 680
Net cash provided by financing activities 224,122 131,824 86,940
Net increase (decrease) in cash, cash equivalents, and restricted cash (96,243) 103,407 (11,854)
Cash, cash equivalents, and restricted cash, beginning of period 239,802 136,395 148,249
Cash, cash equivalents, and restricted cash, end of period 143,559 239,802 136,395
Supplemental disclosure of cash and non-cash activities:      
Fixed asset additions included in accounts payable and accrued expenses 656 728 659
Cash paid in connection with operating lease liabilities $ 9,760 5,970  
Offering costs included in accounts payable and accrued expenses   15 92
Right-of-use assets obtained in exchange of operating lease obligations   $ 19,461  
Reclassification of liability for common stock subject to repurchase     4
Issuance of common stock for repayment of notes payable     22,030
Issuance of common stock for asset acquisition     $ 1,942