Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Cash flows from operating activities
Net income (loss)	$ (532,784)	$ (457,892)	$ 606,073
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Stock-based compensation expense	61,602	104,629	95,994
Premium on marketable securities	(1,500)	(23,641)	(1,736)
Amortization of premium (discount) on marketable securities	5,853	13,046	1,048
Depreciation expense	1,122	4,182	2,630
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(10,985)	(17,020)	3,879
Collaboration receivable - related party	4,846	(18,506)
Other long-term assets	(519)	(910)	452
Right-of-use operating asset	5,577	5,221	6,397
Operating lease liabilities, current	175	206	36
Operating lease liabilities, non-current	(6,473)	(5,944)	(6,825)
Accounts payable	8,433	6,689	(11,511)
Accrued expenses and other liabilities	4,617	11,758	(32,157)
Net cash provided by (used in) operating activities	(460,036)	(378,182)	664,280
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	1,207,407	988,075	901,749
Purchases of marketable securities	(881,037)	(1,990,151)	(458,720)
Purchases of property and equipment	(937)	(372)	(345)
Net cash provided by (used in) investing activities	325,433	(1,002,448)	442,684
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	3,113	14,719	9,262
Payment of employee tax obligations related to vesting of restricted stock units	(43)	(1,385)
Proceeds from the sale of common stock under the Stock Purchase Agreement - related party			417,500
Net cash provided by financing activities	3,070	13,334	426,762
Net increase (decrease) in cash, cash equivalents and restricted cash	(131,533)	(1,367,296)	1,533,726
Cash, cash equivalents and restricted cash at beginning of period	295,502	1,662,798	129,072
Cash, cash equivalents and restricted cash at end of period	163,969	295,502	1,662,798
Supplemental disclosure of non-cash operating and investing activities
Purchases of property and equipment included in accounts payable	$ 137	70
Lease asset de-recognized upon lease cancellation		$ 3,733	$ 2,310