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License and Collaborations (Details) - USD ($) $ in Millions	3 Months Ended						6 Months Ended		12 Months Ended
	Mar. 31, 2026	Jun. 30, 2025	Mar. 31, 2025	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Jun. 30, 2025	Jun. 30, 2024	Dec. 31, 2023	Dec. 31, 2020	Dec. 31, 2017	Dec. 31, 2024
License And Collaboration Agreements [Line Items]
Total revenues		$ 8			$ 11		$ 16	$ 21
Receivable from collaboration partners, current		15					15					$ 20
Other noncurrent assets		69					69					73
Other current liabilities		110					110					123
Other noncurrent liabilities		159					159					157
Taiho Collaboration Agreement
License And Collaboration Agreements [Line Items]
Milestone payments				$ 19
Milestone payments, received			$ 12
Receivable from collaboration partners, current		11					11					14
Other liabilities		45					45					36
Taiho Collaboration Agreement \| Forecast
License And Collaboration Agreements [Line Items]
Milestone payments, received	$ 7
AstraZeneca Agreement
License And Collaboration Agreements [Line Items]
Milestone payments										$ 24
Development cost recorded within research and development expenses		4			4		8	6
Other current liabilities		21					21
Other noncurrent liabilities												$ 26
Gilead Collaboration Agreement
License And Collaboration Agreements [Line Items]
Other noncurrent assets									$ 6
Development cost recorded within research and development expenses								6
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
Royalties payable term											10 years
Reimbursement of research and development expense		8			4		17	7
Total revenues					3			7
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]
Milestone payments									$ 28
STAR-121 Development Activities \| Taiho Collaboration Agreement
License And Collaboration Agreements [Line Items]
Milestone payments						$ 26
Milestone payments, received			$ 10			$ 16
WuXi Biologics License Agreement \| anti-PD-1
License And Collaboration Agreements [Line Items]
Development milestone expense		0			0		0	0
WuXi Biologics License Agreement \| anti-CD39
License And Collaboration Agreements [Line Items]
Development milestone expense		0			0		0	0
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]
Development milestone expense		$ 0			$ 0		0	$ 0
Additional clinical, regulatory and commercialization milestone payments							$ 88