Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Cash flows from operating activities
Net loss	$ (122,055)	$ (95,764)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	18,553	21,976
Premium on marketable securities	(1,189)	(9,459)
Amortization of premium on marketable securities	3,533	1,325
Depreciation	275	2,582
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	2,635	(262)
Collaboration receivable - related party	(1,291)	(24,766)
Other long-term assets	(340)	(51)
Right-of-use operating asset	1,350	1,333
Operating lease liabilities, current	55	44
Operating lease liabilities, non-current	(1,568)	(1,498)
Accounts payable	(3,251)	(406)
Accrued expenses and other liabilities	(4,507)	(4,140)
Net cash used in operating activities	(107,800)	(109,086)
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	292,539	182,970
Purchases of marketable securities	(301,928)	(841,786)
Purchases of property and equipment	(169)
Net cash used in investing activities	(9,558)	(658,816)
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	1,799	5,623
Net cash provided by financing activities	1,799	5,623
Net decrease in cash, cash equivalents and restricted cash	(115,559)	(762,279)
Cash, cash equivalents and restricted cash at beginning of period	295,502	1,662,798
Cash, cash equivalents and restricted cash at end of period	$ 179,943	900,519
Supplemental disclosure of non-cash operating activities
Lease asset de-recognized upon lease cancellation		$ 3,733