Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Cash flows from operating activities
Net income (loss)	$ (457,892)	$ 606,073	$ (680,238)
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Stock-based compensation expense	104,629	95,994	153,231
Premium on marketable securities	(23,641)	(1,736)	(3,674)
Amortization of premium (discount) on marketable securities	13,046	1,048	(6,966)
Depreciation	4,182	2,630	2,283
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(17,020)	3,879	(4,781)
Collaboration receivable - related party	(18,506)
Other long-term assets	(910)	452	(3,793)
Right-of-use operating asset	5,221	6,397	8,168
Operating lease liabilities, current	206	36	2,804
Operating lease liabilities, non-current	(5,944)	(6,825)	(10,761)
Accounts payable	6,689	(11,511)	(18,783)
Accrued expenses and other liabilities	11,758	(32,157)	33,804
Net cash provided by (used in) operating activities	(378,182)	664,280	(528,706)
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	988,075	901,749	1,171,270
Purchases of marketable securities	(1,990,151)	(458,720)	(1,308,675)
Purchases of property and equipment	(372)	(345)	(5,751)
Net cash provided by (used in) investing activities	(1,002,448)	442,684	(143,156)
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	14,719	9,262	48,850
Payment of employee tax obligations related to vesting of restricted stock units	(1,385)		(2,175)
Payments of offering costs			(328)
Proceeds from the sale of common stock under the Stock Purchase Agreement - related party		417,500
Proceeds from public offerings of common stock, net of commissions and underwriting discounts			561,277
Net cash provided by financing activities	13,334	426,762	607,624
Net increase (decrease) in cash, cash equivalents and restricted cash	(1,367,296)	1,533,726	(64,238)
Cash, cash equivalents and restricted cash at beginning of period	1,662,798	129,072	193,310
Cash, cash equivalents and restricted cash at end of period	295,502	1,662,798	129,072
Supplemental disclosure of non-cash operating and investing activities
Purchases of property and equipment included in accounts payable	70		65
Right-of-use assets obtained in exchange for new operating lease liabilities			$ 872
Lease asset de-recognized upon lease cancellation	$ 3,733	$ 2,310