Revenues - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended	3 Months Ended
	Jan. 29, 2024	Jan. 31, 2024	Mar. 31, 2025	Sep. 30, 2024	Mar. 31, 2024	Dec. 31, 2024	Dec. 31, 2023
Disaggregation of Revenue [Line Items]
Contract with customer, liability			$ 291			$ 319
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)					$ 1.24
Contract with customer, liability, cumulative catch-up adjustment to revenue, earnings per share, diluted (in dollars per share)					$ 1.24
Option continuation payment received in the third quarter of 2024		100
Premium from Third Stock Purchase Agreement Amendment		87
Deferred revenue as of January 2024		335
Allocation of transaction price		522
Total revenues			28		$ 145
Revenue recognized from amounts in deferred revenue at the beginning of the period			28		142
Capitalized contract cost, amortization					2
Cost sharing receivable			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			6		8
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
Initial Gilead Collaboration Agreement and Subsequent Amendments
Disaggregation of Revenue [Line Items]
Capitalized contract cost, gross			3
Gilead | Etrumadenant License and Research and Development Services Agreement
Disaggregation of Revenue [Line Items]
Contract with customer, liability		129	153			166
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			13		24
Gilead | Quemliclustat, License and R&D services
Disaggregation of Revenue [Line Items]
Contract with customer, liability		130	18			23
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			5		100
Gilead | R&D Activities for Domvanalimab
Disaggregation of Revenue [Line Items]
Contract with customer, liability			19			21
Total revenues			2		7
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	$ 46			52
Current and future programs exclusive access period			10 years
Contingent milestone payments receivable		200	$ 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			21			$ 23
Total revenues			$ 2		$ 2
Gilead | Revenue Benchmark | Customer Concentration Risk
Disaggregation of Revenue [Line Items]
Percentage of revenues			100.00%		97.00%
STAR-221 Development Activities | Taiho Collaboration Agreement
Disaggregation of Revenue [Line Items]
Contract with customer, liability							$ 35
Taiho Pharmaceutical Co., Ltd | Taiho Agreement
Disaggregation of Revenue [Line Items]
Payment for option exercise				$ 15