License and Collaborations (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended						9 Months Ended		12 Months Ended
	Oct. 31, 2025	Mar. 31, 2026	Dec. 31, 2025	Sep. 30, 2025	Mar. 31, 2025	Sep. 30, 2024	Mar. 31, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2023	Dec. 31, 2020	Dec. 31, 2017	Dec. 31, 2024
License And Collaboration Agreements [Line Items]
Total revenues				$ 6		$ 7		$ 22	$ 28
Receivable from collaboration partners, current				11				11					$ 20
Other noncurrent assets				65				65					73
Other current liabilities				141				141					123
Other noncurrent liabilities				152				152					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				42				42					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				7		4		16	10
Other current liabilities				28				28
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				7		3		24	10
Total revenues						18			25
Taiho Pharmaceutical Co., Ltd | Taiho Agreement | Forecast
License And Collaboration Agreements [Line Items]
Payment for option exercise			$ 15
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
Taiho Pharmaceutical Co., Ltd | Taiho Agreement | Maximum | Subsequent Event
License And Collaboration Agreements [Line Items]
Additional clinical and regulatory milestone payments receivable	$ 172
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