Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (298,665)	$ (174,683)	$ (115,884)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	4,400	3,294	1,618
Amortization of intangible assets	1,042	1,223	1,138
Impairment of intangible assets	832
Amortization of premiums (accretion of discounts) on investments, net	1,548	(179)	(328)
Noncash lease expense	3,371
Change in estimated fair value of contingent consideration	38,394	8,330	250
Payment of contingent consideration in excess of acquisition date fair value	(15,752)
Initial fair value of derivative liability		13,599
Change in estimated fair value of derivative liability	16,796	(1,150)
Change in estimated fair value of convertible preferred stock warrant liability		2,049	95
Preferred stock issued in connection with asset acquisition			1,750
Common stock issued in connection with license agreement		617
Change in deferred income taxes	(52)		(480)
Stock-based compensation	27,600	8,719	5,053
Loss on write-off and disposal of property and equipment		345	198
Other	23
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(4,475)	(4,619)	(4,172)
Other assets	(1,100)	(1,881)	151
Accounts payable	(790)	964	1,471
Accrued liabilities and other long-term liabilities	46,614	10,211	7,171
Operating lease liabilities	(3,684)
Deferred revenue	(7,043)	3,529	7,873
Net cash used in operating activities	(190,941)	(129,632)	(94,096)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of property and equipment	(6,549)	(8,940)	(8,192)
Purchases of investments	(403,841)	(643,898)	(123,105)
Maturities of investments	400,348	396,680	72,574
Proceeds from sale of property and equipment			25
Asset acquisitions			(1,743)
Proceeds from disposal of an asset held for sale	180
Net cash used in investing activities	(9,862)	(256,158)	(60,441)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock	323,214	126,411
Proceeds from issuance of convertible preferred stock, net of issuance costs		317,335	14,254
Proceeds received from financing lease obligation		1,202
Payment of contingent consideration	(4,248)
Payment of principal on financing lease obligations	(250)	(95)
Proceeds from repayment of promissory notes		3,265
Proceeds from exercise of stock options	4,059	1,129	584
Cash paid in lieu of fractional shares related to reverse stock split		(3)
Advanced proceeds from convertible preferred stock financing			10,140
Net cash provided by financing activities	529,474	449,244	24,978
Net increase (decrease) in cash, cash equivalents and restricted cash and cash equivalents	328,671	63,454	(129,559)
Cash, cash equivalents and restricted cash and cash equivalents at beginning of period	122,816	59,362	188,921
Cash, cash equivalents and restricted cash and cash equivalents at end of period	451,487	122,816	59,362
NONCASH INVESTING AND FINANCING ACTIVITIES:
Property and equipment purchases included in accounts payable and accrued liabilities	382	892	1,996
Issuance of preferred stock in connection with asset acquisition			593
Conversion of preferred stock into common stock upon completion of initial public offering		636,612
Operating lease liabilities obtained in exchange of right-of-use asset	48,495
Advanced proceeds applied to convertible preferred stock issuance		10,140
Reclassification of derivative liability to additional paid-in capital	29,245
Reclassification of preferred stock warrant liability to additional paid-in capital		3,073
RECONCILIATION OF CASH, CASH EQUIVALENTS AND RESTRICTED CASH TO THE CONSOLIDATED BALANCE SHEETS:
Cash and cash equivalents	436,575	109,335	47,598
Restricted cash and cash equivalents, current	7,993	6,181	10,761
Restricted cash and cash equivalents, noncurrent	6,919	7,300	1,003
Cash, cash equivalents and restricted cash and cash equivalents at end of period	451,487	$ 122,816	$ 59,362
Collaboration Agreement
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock	$ 206,699