License and Collaborations - Additional Information (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2026	Dec. 31, 2025	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2020	Dec. 31, 2017
License and Collaboration Agreements [Line Items]
Total revenues			$ 33,000,000	$ 51,000,000	$ 37,000,000
Receivable from collaboration partners, current		$ 11,000,000	11,000,000	20,000,000
Other current liabilities		154,000,000	154,000,000	123,000,000
Other noncurrent liabilities		135,000,000	135,000,000	157,000,000
Taiho Collaboration Agreement
License and Collaboration Agreements [Line Items]
Milestone payments			19,000,000
Milestone payments, received			12,000,000
Receivable from collaboration partners, current		10,000,000	10,000,000	14,000,000
Other liabilities		44,000,000	44,000,000
Taiho Collaboration Agreement | Forecast
License and Collaboration Agreements [Line Items]
Milestone payments, received	$ 7,000,000
AstraZeneca Agreement
License and Collaboration Agreements [Line Items]
Milestone payments						$ 24,000,000
Development cost recorded within research and development expenses			20,000,000	14,000,000	6,000,000
Other current liabilities		12,000,000	12,000,000
Other noncurrent liabilities				26,000,000
Taiho Pharmaceutical Co., Ltd | Taiho Agreement
License and Collaboration Agreements [Line Items]
Option period							5 years
Non refundable and non creditable cash payments							$ 35,000,000
Payment for option exercise		15,000,000		15,000,000
Royalties payable term							10 years
Payment for option exercise		15,000,000
Total revenues			7,000,000	15,000,000	5,000,000
Reimbursement of research and development expense			34,000,000	28,000,000	8,000,000
Taiho Pharmaceutical Co., Ltd | Taiho Agreement | Minimum
License and Collaboration Agreements [Line Items]
Payment for option exercise							$ 3,000,000
Taiho Pharmaceutical Co., Ltd | Taiho Agreement | Maximum
License and Collaboration Agreements [Line Items]
Payment for option exercise							15,000,000
Additional clinical and regulatory milestone payments receivable		172,000,000					130,000,000
Contingent milestone payments receivable		$ 145,000,000	145,000,000				$ 145,000,000
STAR-221 Development Activities | Taiho Collaboration Agreement
License and Collaboration Agreements [Line Items]
Milestone payments					28,000,000
STAR-121 Development Activities | Taiho Collaboration Agreement
License and Collaboration Agreements [Line Items]
Milestone payments				26,000,000
Milestone payments, received			10,000,000	16,000,000
WuXi Biologics License Agreement | anti-PD-1
License and Collaboration Agreements [Line Items]
Milestone payments			0	0	0
Royalty expense			0	0	0
WuXi Biologics License Agreement | anti-CD39
License and Collaboration Agreements [Line Items]
Milestone payments			0	0
Royalty expense			0	0
WuXi Biologics License Agreement | anti-CD39 | Research and Development
License and Collaboration Agreements [Line Items]
Milestone payments					1,000,000
WuXi Biologics License Agreement | Maximum | anti-PD-1
License and Collaboration Agreements [Line Items]
Clinical regulatory milestone payments			50,000,000
Clinical, regulatory and commercialization milestone payments			375,000,000
WuXi Biologics License Agreement | Maximum | anti-CD39
License and Collaboration Agreements [Line Items]
Additional clinical, regulatory and commercialization milestone payments			14,000,000
Abmuno License Agreement
License and Collaboration Agreements [Line Items]
Milestone payments			0	0	0
Royalty expense			0	$ 0	$ 0
Additional clinical, regulatory and commercialization milestone payments			$ 88,000,000