Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Cash flow from operating activities
Net income (loss)	$ (307)	$ (267)	$ 53
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Stock-based compensation expense	73	65	55
Depreciation and amortization	8	6	4
Noncash lease expense	8	8	3
Amortization of premiums (discounts) on marketable securities	(19)	0	5
Other items, net	2	3	0
Changes in operating assets and liabilities:
Receivable from collaboration partners ($19, $704 and ($17) from a related party)	15	704	(17)
Other assets (($4), ($2) and $— from a related party)	(14)	2	(3)
Accounts payable	(1)	8	(5)
Deferred revenue (($77), ($107) and ($361) to a related party)	(68)	(112)	(368)
Other liabilities	(3)	21	17
Net cash provided by (used in) operating activities	(306)	438	(256)
Cash flow from investing activities
Purchases of marketable securities	(788)	(1,241)	(719)
Proceeds from maturities of marketable securities	982	694	690
Proceeds from sales of marketable securities	24	143	51
Purchases of property and equipment	(24)	(6)	(26)
Purchases of in-process research and development	0	(6)	0
Collaboration reimbursements of in-process research and development from a related party	0	3	0
Net cash provided by (used in) investing activities	194	(413)	(4)
Cash flow from financing activities
Proceeds from issuance of common stock ($20, $— and $220 from a related party)	25	0	220
Proceeds from issuance of common stock pursuant to equity award plans	8	23	12
Proceeds from sale of future royalties	0	10	5
Net cash provided by financing activities	33	33	237
Net increase (decrease) in cash, cash equivalents and restricted cash	(79)	58	(23)
Cash, cash equivalents and restricted cash at beginning of period	209	151	174
Cash, cash equivalents and restricted cash at end of period	130	209	151
Supplemental disclosure of cash flow information
Income taxes paid	5	3	0
Non-cash investing and financing activities:
Unpaid portion of property and equipment purchases included in Accounts payable and Other current liabilities	2	3	1
Unpaid portion of other assets included in accrued research and development	0	0	1
Vesting of early exercised stock options and restricted stock	$ 0	$ 0	$ 1