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	Mar. 31, 2024	Sep. 30, 2023	Mar. 31, 2023	Dec. 31, 2023	Jun. 30, 2023
Disaggregation of Revenue [Line Items]
Contract with customer, liability				$ 366			$ 398
Option continuation - payment due in the third quarter 2024	$ 100
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
Premium from Third Stock Purchase Agreement Amendment	$ 87
Deferred revenues as of January 29, 2024	335
Allocation of transaction price	522
Total revenues				145		$ 25
Deferred revenue recognized				142		25
Capitalized contract cost, amortization				2
Cost sharing receivable				4			3
R&D Activities for Domvanalimab
Disaggregation of Revenue [Line Items]
Deferred revenue recognized				25
Access Rights and Option Continuation Periods
Disaggregation of Revenue [Line Items]
Total revenues				8		8
Taiho Collaboration Agreement
Disaggregation of Revenue [Line Items]
Contract with customer, liability				45			23
Total revenues				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
Initial Gilead Collaboration Agreement And Subsequent Amendments
Disaggregation of Revenue [Line Items]
Capitalized contract cost, gross				3
Gilead | Gilead Collaboration Agreement
Disaggregation of Revenue [Line Items]
Revenue recognized				250
Gilead | Etrumadenant License and Research and Development Services Agreement
Disaggregation of Revenue [Line Items]
Contract with customer, liability	129			189			133
Contract with customer, liability, cumulative catch-up adjustment to revenue, change in measure of progress	14
Total revenues				24		8
Gilead | Quemliclustat, License and R&D services
Disaggregation of Revenue [Line Items]
Contract with customer, liability	130			71			132
Contract with customer, liability, cumulative catch-up adjustment to revenue, change in measure of progress	88
Total revenues				100		8
Deferred revenue recognized				200
Gilead | R&D Activities for Domvanalimab
Disaggregation of Revenue [Line Items]
Contract with customer, liability				27			25
Total revenues				7		$ 1
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			$ 71			54
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
Gilead | R&D Activities for Inflammation Programs
Disaggregation of Revenue [Line Items]
Contract with customer, liability				29			$ 31
Total revenues				$ 2
Gilead | Revenue Benchmark | Customer Concentration Risk
Disaggregation of Revenue [Line Items]
Percentage of revenues				97.00%		100.00%
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				$ 16
Payment for option exercise		$ 26
STAR-121 Development Activities | Taiho Collaboration Agreement | Forecast
Disaggregation of Revenue [Line Items]
Revenue recognized			$ 10