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License and Collaborations (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2026	Dec. 31, 2025	Jun. 30, 2025	Mar. 31, 2025	Jun. 30, 2026	Jun. 30, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2020	Dec. 31, 2017
License And Collaboration Agreements [Line Items]
Receivable from collaboration partners	$ 36	$ 11			$ 36
Other current liabilities	145	154			145
Other noncurrent liabilities	124	135			124
Taiho Collaboration Agreement
License And Collaboration Agreements [Line Items]
Milestone payments	30
Receivable from collaboration partners	36	10			36
Other liabilities	35	44			35
AstraZeneca Agreement
License And Collaboration Agreements [Line Items]
Milestone payments									$ 24
Development cost recorded within research and development expenses	4		$ 4		9	$ 8
Other current liabilities	22				22
Other noncurrent liabilities		12
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	6		8		12	17
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		172								130
Contingent milestone payments receivable		$ 145								$ 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
PRISM-1 Development Activities \| Taiho Collaboration Agreement
License And Collaboration Agreements [Line Items]
Milestone payments				$ 19
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