Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Cash flows from operating activities
Net loss	$ (541,489)	$ (532,784)	$ (457,892)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	73,367	61,602	104,629
Premium on marketable securities	(132)	(1,500)	(23,641)
Amortization of premium (discount) on marketable securities	(6,340)	5,853	13,046
Depreciation expense	1,393	1,122	4,182
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	19,001	(10,985)	(17,020)
Collaboration receivable - related party	(69,349)	4,846	(18,506)
Other long-term assets	(1,778)	(519)	(910)
Right-of-use operating asset	6,074	5,577	5,221
Operating lease liabilities, current	(2,478)	175	206
Operating lease liabilities, non-current	(4,491)	(6,473)	(5,944)
Accounts payable	(8,495)	8,433	6,689
Accrued expenses and other liabilities	(5,868)	4,617	11,758
Net cash used in operating activities	(540,585)	(460,036)	(378,182)
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	1,038,136	1,207,407	988,075
Purchases of marketable securities	(594,670)	(881,037)	(1,990,151)
Purchases of property and equipment	(553)	(937)	(372)
Net cash provided by (used in) investing activities	442,913	325,433	(1,002,448)
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	6,930	3,113	14,719
Payment of employee tax obligations related to vesting of restricted stock units	(903)	(43)	(1,385)
Net cash provided by financing activities	6,027	3,070	13,334
Net decrease in cash, cash equivalents and restricted cash	(91,645)	(131,533)	(1,367,296)
Cash, cash equivalents and restricted cash at beginning of period	163,969	295,502	1,662,798
Cash, cash equivalents and restricted cash at end of period	$ 72,324	163,969	295,502
Supplemental disclosure of non-cash operating activities
Purchases of property and equipment included in accounts payable		$ 137	70
Lease asset de-recognized upon lease cancellation			$ 3,733