Revenues - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2025	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2020	Jun. 30, 2025	Jan. 31, 2024
Disaggregation of Revenue [Line Items]
Total transaction price		$ 78	$ 78	$ 319				$ 522
Contract with customer, liability, cumulative catch-up adjustment to revenue, change in measure of progress and change in estimate of transaction price			$ 143	$ 107
Contract with customer, liability, cumulative catch-up adjustment to revenue, earnings per share, diluted (in dollars per share)			$ 1.33	$ 1.19
Contract with customer, liability, cumulative catch-up adjustment to revenue, earnings per share, basic (in dollars per share)			$ 1.33	$ 1.19
Option continuation payment received in the third quarter of 2024				$ 100				100
Premium from Third Stock Purchase Agreement Amendment				87				87
Deferred revenue as of January 2024				335				$ 335
Total transaction price		256	$ 256	522			$ 256
Deferred revenue, non-refundable payment	$ 725
Total revenues			247	258	$ 117
Total revenues			33	51	37
Access rights and option continuation periods
Disaggregation of Revenue [Line Items]
Total revenues			16	28	33
Gilead
Disaggregation of Revenue [Line Items]
Total transaction price				522
Gilead | R&D services
Disaggregation of Revenue [Line Items]
Total revenues			214	207	75
Gilead | Access rights and option continuation periods
Disaggregation of Revenue [Line Items]
Current and future programs exclusive access period						10 years
Contingent milestone payments receivable						$ 300
Option continuation payment receivable upon sixth anniversary of agreement						$ 100
Gilead | Rights to certain studies
Disaggregation of Revenue [Line Items]
Total transaction price		17	17
Gilead | Research and Development Activities for Inflammation Programs
Disaggregation of Revenue [Line Items]
Total revenues			$ 10	$ 8	$ 4
Gilead | Revenue Benchmark | Customer Concentration Risk
Disaggregation of Revenue [Line Items]
Percentage of revenues			97.00%	94.00%	96.00%
STAR-221 Development Activities | Taiho Collaboration Agreement
Disaggregation of Revenue [Line Items]
Total transaction price					$ 35
Taiho Pharmaceutical Co., Ltd
Disaggregation of Revenue [Line Items]
Total transaction price		15	$ 15
Taiho Pharmaceutical Co., Ltd | Taiho Agreement
Disaggregation of Revenue [Line Items]
Total transaction price		7	7
Total revenues			$ 7	$ 15	$ 5
Payment for option exercise		$ 15		$ 15