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License and Collaborations (Details) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Mar. 31, 2026	Dec. 31, 2025	Mar. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2020	Dec. 31, 2017
License And Collaboration Agreements [Line Items]
Total revenues	$ 7		$ 8
Receivable from collaboration partners	3	$ 11
Other current liabilities	136	154
Other noncurrent liabilities	119	135
Taiho Collaboration Agreement
License And Collaboration Agreements [Line Items]
Milestone payments			19
Receivable from collaboration partners	3	10
Other liabilities	23	44
AstraZeneca Agreement
License And Collaboration Agreements [Line Items]
Milestone payments						$ 24
Development cost recorded within research and development expenses	5		4
Other current liabilities	17
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
Total revenues	2
Reimbursement of research and development expense	5		9
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
Abmuno License Agreement
License And Collaboration Agreements [Line Items]
Additional clinical, regulatory and commercialization milestone payments	88
Development milestone expense	0		0
WuXi Biologics License Agreement \| anti-PD-1
License And Collaboration Agreements [Line Items]
Development milestone expense	0		0
WuXi Biologics License Agreement \| anti-CD39
License And Collaboration Agreements [Line Items]
Development milestone expense	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