Revenues - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended		3 Months Ended			9 Months Ended
	Jan. 29, 2024	Jul. 31, 2024	Jan. 31, 2024	Mar. 31, 2025	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023	Jun. 30, 2023
Disaggregation of Revenue [Line Items]
Contract with customer, liability					$ 394		$ 394		$ 398
Contract with customer, liability, cumulative catch-up adjustment to revenue, change in measure of progress and change in estimate of transaction price	$ 107
Contract with customer, liability, cumulative catch-up adjustment to revenue, earnings per share, basic (in dollars per share)							$ 1.19
Contract with customer, liability, cumulative catch-up adjustment to revenue, earnings per share, diluted (in dollars per share)							$ 1.19
Option continuation payment received in the third quarter of 2024	100
Premium from Third Stock Purchase Agreement Amendment	87
Deferred revenue as of January 29, 2024	335
Allocation of transaction price	522
Total revenues					48	$ 32	$ 232	$ 86
Revenue recognized from amounts in deferred revenue at the beginning of the period					33	28	212	79
Capitalized contract cost, amortization							3	1
Cost sharing receivable					3		3		3
R&D Activities for Domvanalimab
Disaggregation of Revenue [Line Items]
Revenue recognized from amounts in deferred revenue at the beginning of the period							25
Access Rights and Option Continuation Periods
Disaggregation of Revenue [Line Items]
Total revenues					5	8	22	25
Taiho Collaboration Agreement
Disaggregation of Revenue [Line Items]
Contract with customer, liability					39		39		23
Total revenues					3	4	10	4
Third Gilead Collaboration Agreement Amendment, Third Stock Purchase Agreement Amendment and the Second Investor Rights Agreement Amendment
Disaggregation of Revenue [Line Items]
Capitalized contract cost, gross	8
Third Stock Purchase Agreement Amendment
Disaggregation of Revenue [Line Items]
Capitalized contract cost, gross					5		5
Initial Gilead Collaboration Agreement and Subsequent Amendments
Disaggregation of Revenue [Line Items]
Capitalized contract cost, gross					3		3
Gilead | Etrumadenant License and Research and Development Services Agreement
Disaggregation of Revenue [Line Items]
Contract with customer, liability	129				176		176		133
Contract with customer liability revenue recognized excluding opening balance							250
Contract with customer, liability, cumulative catch-up adjustment to revenue, change in measure of progress	14
Total revenues					9	21	36	39
Gilead | Quemliclustat, License and R&D services
Disaggregation of Revenue [Line Items]
Contract with customer, liability	130				39		39		132
Contract with customer liability revenue recognized excluding opening balance							200
Contract with customer, liability, cumulative catch-up adjustment to revenue, change in measure of progress	88
Total revenues					13	(4)	132	11
Gilead | R&D Activities for Domvanalimab
Disaggregation of Revenue [Line Items]
Contract with customer, liability					23		23		25
Total revenues					1	1	11	3
Contract with customer, liability, cumulative catch-up adjustment to revenue, change in estimate of transaction price	5
Gilead | Access Rights and Option Continuation Periods
Disaggregation of Revenue [Line Items]
Contract with customer, liability	51				58		$ 58		54
Current and future programs exclusive access period							10 years
Contingent milestone payments receivable	200				300		$ 300
Option continuation payment receivable upon sixth anniversary of agreement	$ 100
Gilead | Rights to Certain Studies
Disaggregation of Revenue [Line Items]
Contract with customer, liability					34		34
Gilead | R&D Activities for Inflammation Programs
Disaggregation of Revenue [Line Items]
Contract with customer, liability					25		25		$ 31
Total revenues					$ 2	$ 2	$ 6	$ 3
Gilead | Revenue Benchmark | Customer Concentration Risk
Disaggregation of Revenue [Line Items]
Percentage of revenues					63.00%	88.00%	89.00%	95.00%
STAR-221 Development Activities | Taiho Collaboration Agreement
Disaggregation of Revenue [Line Items]
Contract with customer, liability										$ 35
Payment for option exercise							$ 28
STAR-121 Development Activities | Taiho Collaboration Agreement
Disaggregation of Revenue [Line Items]
Revenue recognized from amounts in deferred revenue at the beginning of the period							16
Payment for option exercise			$ 26
STAR-121 Development Activities | Taiho Collaboration Agreement | Forecast
Disaggregation of Revenue [Line Items]
Revenue recognized from amounts in deferred revenue at the beginning of the period				$ 10
Taiho Pharmaceutical Co., Ltd | Taiho Agreement
Disaggregation of Revenue [Line Items]
Total revenues					$ 18	$ 4	$ 25	$ 4
Payment for option exercise		$ 15			$ 15