Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Cash flows from operating activities
Net loss	$ (95,764)	$ (126,740)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	21,976	31,110
Premium on marketable securities	(9,459)	(240)
Amortization of premium (discount) on marketable securities	1,325	(18)
Depreciation	2,582	669
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(262)	1,221
Collaboration receivable - related party	(24,766)
Other long-term assets	(51)	99
Right-of-use operating asset	1,333	1,778
Operating lease liabilities, current	44	25
Operating lease liabilities, non-current	(1,498)	(1,897)
Accounts payable	(406)	(7,307)
Accrued expenses and other liabilities	(4,140)	(35,391)
Net cash used in operating activities	(109,086)	(136,691)
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	182,970	275,197
Purchases of marketable securities	(841,786)	(68,651)
Purchases of property and equipment		(276)
Net cash provided by (used in) investing activities	(658,816)	206,270
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	5,623	3,160
Net cash provided by financing activities	5,623	3,160
Net increase (decrease) in cash, cash equivalents and restricted cash	(762,279)	72,739
Cash, cash equivalents and restricted cash at beginning of period	1,662,798	129,072
Cash, cash equivalents and restricted cash at end of period	900,519	201,811
Supplemental disclosure of non-cash operating and investing activities
Purchases of property and equipment included in accounts payable		$ 15
Lease asset de-recognized upon lease cancellation	$ 3,733