Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
Cash flows from operating activities
Net loss	$ (333,180)	$ (368,822)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	83,981	75,250
Premium on marketable securities	(19,792)	(1,066)
Amortization of premium on marketable securities	8,722	628
Depreciation	3,849	1,981
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(11,522)	(3,647)
Collaboration receivable - related party	(24,185)
Other long-term assets	(534)	366
Right-of-use operating asset	3,898	4,914
Operating lease liabilities, current	152	(6)
Operating lease liabilities, non-current	(4,427)	(5,194)
Accounts payable	1,042	(10,913)
Accrued expenses and other liabilities	11,315	(40,182)
Net cash used in operating activities	(280,681)	(346,691)
Cash flows from investing activities
Proceeds from sales and maturities of marketable securities	699,212	766,339
Purchases of marketable securities	(1,671,399)	(368,829)
Purchases of property and equipment		(345)
Net cash provided by (used in) investing activities	(972,187)	397,165
Cash flows from financing activities
Proceeds from stock option exercises and employee stock purchase plan issuances	14,300	7,091
Net cash provided by financing activities	13,504	7,091
Payment of employee tax obligations related to vesting of restricted stock units	(796)
Net increase (decrease) in cash, cash equivalents and restricted cash	(1,239,364)	57,565
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	423,434	186,637
Supplemental disclosure of non-cash operating and investing activities
Lease asset de-recognized upon lease cancellation	$ 3,733	$ 2,310