Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Cash flows from operating activities
Net loss	$ (203,009)	$ (263,087)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	49,639	55,142
Premium on marketable securities	(16,873)	(494)
Amortization of premium on marketable securities	4,688	304
Depreciation	3,475	1,331
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(6,025)	(2,162)
Collaboration receivable - related party	(48,438)
Other long-term assets	(258)	378
Right-of-use operating asset	2,602	3,461
Operating lease liabilities, current	97	(49)
Operating lease liabilities, non-current	(2,945)	(3,602)
Accounts payable	(774)	(12,815)
Accrued expenses and other liabilities	5,966	(34,082)
Net cash used in operating activities	(211,855)	(255,675)
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	360,939	595,769
Purchases of marketable securities	(1,308,143)	(211,771)
Purchases of property and equipment		(346)
Net cash provided by (used in) investing activities	(947,204)	383,652
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	12,484	3,546
Net cash provided by financing activities	11,688	3,546
Payment of employee tax obligations related to vesting of restricted stock units	(796)
Net increase (decrease) in cash, cash equivalents and restricted cash	(1,147,371)	131,523
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	515,427	260,595
Supplemental disclosure of non-cash operating and investing activities
Lease asset de-recognized upon lease cancellation	$ 3,733	$ 2,310