License and Collaborations - Additional Information (Details) - USD ($) $ in Millions	1 Months Ended			3 Months Ended			9 Months Ended		12 Months Ended
	Oct. 31, 2024	Jul. 31, 2024	Jan. 31, 2024	Mar. 31, 2025	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2020	Dec. 31, 2017	Dec. 31, 2023
License And Collaboration Agreements [Line Items]
Revenue recognized from amounts in deferred revenue at the beginning of the period					$ 33	$ 28	$ 212	$ 79
Total revenues					48	32	232	86
Receivable from collaboration partners, current					16		16				$ 38
Other noncurrent assets					75		75				106
Other current liabilities					97		97				76
Other noncurrent liabilities					136		136				142
Research and development expense					123	82	347	247
Taiho Collaboration Agreement
License And Collaboration Agreements [Line Items]
Total revenues					3	4	10	4
Receivable from collaboration partners, current					14		14				18
Taiho Collaboration Agreement | Subsequent Event
License And Collaboration Agreements [Line Items]
Milestone payments	$ 19
AstraZeneca Agreement
License And Collaboration Agreements [Line Items]
Milestone payments									$ 24
Development cost recorded within research and development expenses					4	1	10	6
Other liabilities					22		22
Other current liabilities					10		10
Other noncurrent liabilities					12		12				11
Gilead Collaboration Agreement
License And Collaboration Agreements [Line Items]
Other noncurrent assets											$ 6
Development cost recorded within research and development expenses							6
Exelixis Collaboration
License And Collaboration Agreements [Line Items]
Research and development expense					2	1	5	1
Taiho Pharmaceutical Co., Ltd | Taiho Agreement
License And Collaboration Agreements [Line Items]
Option period										5 years
Non refundable and non creditable cash payments										$ 35
Payment for option exercise		$ 15			15
Royalties payable term										10 years
Total revenues					18	4	25	4
Reimbursement of research and development expense					3	2	10	4
Taiho Pharmaceutical Co., Ltd | Taiho Agreement | Minimum
License And Collaboration Agreements [Line Items]
Payment for option exercise										$ 3
Taiho Pharmaceutical Co., Ltd | Taiho Agreement | Maximum
License And Collaboration Agreements [Line Items]
Payment for option exercise										15
Additional clinical and regulatory milestone payments receivable										130
Contingent milestone payments receivable										$ 145
STAR-221 Development Activities | Taiho Collaboration Agreement
License And Collaboration Agreements [Line Items]
Payment for option exercise							28
STAR-121 Development Activities | Taiho Collaboration Agreement
License And Collaboration Agreements [Line Items]
Payment for option exercise			$ 26
Revenue recognized from amounts in deferred revenue at the beginning of the period							16
STAR-121 Development Activities | Taiho Collaboration Agreement | Forecast
License And Collaboration Agreements [Line Items]
Revenue recognized from amounts in deferred revenue at the beginning of the period				$ 10
WuXi Biologics License Agreement | anti-CD39 | Research and Development
License And Collaboration Agreements [Line Items]
Development milestone expense					$ 0	$ 0	0	$ 1
WuXi Biologics License Agreement | Maximum | anti-PD-1
License And Collaboration Agreements [Line Items]
Clinical regulatory milestone payments							50
Clinical, regulatory and commercialization milestone payments							375
WuXi Biologics License Agreement | Maximum | anti-CD39
License And Collaboration Agreements [Line Items]
Additional clinical, regulatory and commercialization milestone payments							14
Abmuno License Agreement
License And Collaboration Agreements [Line Items]
Additional clinical, regulatory and commercialization milestone payments							$ 88