License and Collaborations - Additional Information (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended				6 Months Ended	12 Months Ended
	Jan. 31, 2024	Mar. 31, 2025	Mar. 31, 2024	Sep. 30, 2023	Mar. 31, 2023	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2023	Dec. 31, 2018
License And Collaboration Agreements [Line Items]
Total revenues			$ 145		$ 25
Receivable from collaboration partners ($21 and $20 from a related party)			35					$ 38
Other current liabilities			64					76
Other noncurrent liabilities			140					142
Other noncurrent assets ($— and $6 from a related party)			78					106
Research and development (Net of recoveries of $16 and $33 from a related party)			109		81
Taiho Collaboration Agreement
License And Collaboration Agreements [Line Items]
Total revenues			4
Receivable from collaboration partners ($21 and $20 from a related party)			10					14
AstraZeneca Agreement
License And Collaboration Agreements [Line Items]
Milestone payments			24
Development cost recorded within research and development expenses			3		2
Other current liabilities			2
Other noncurrent liabilities			12					11
Gilead Collaboration Agreement
License And Collaboration Agreements [Line Items]
Development cost recorded within research and development expenses			6
Other noncurrent assets ($— and $6 from a related party)								$ 6
Exelixis Collaboration
License And Collaboration Agreements [Line Items]
Research and development (Net of recoveries of $16 and $33 from a related party)			1
Taiho Pharmaceutical Co., Ltd | Taiho Agreement
License And Collaboration Agreements [Line Items]
Option period						5 years
Non refundable and non creditable cash payments						$ 35
Royalties payable term							10 years
Total revenues			4
Reimbursement of research and development expense			3
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]
Payment for option exercise				$ 28
STAR-121 Development Activities | Taiho Collaboration Agreement
License And Collaboration Agreements [Line Items]
Payment for option exercise	$ 26
Revenue recognized			16
STAR-121 Development Activities | Taiho Collaboration Agreement | Forecast
License And Collaboration Agreements [Line Items]
Revenue recognized		$ 10
WuXi Biologics License Agreement | anti-CD39 | Research and Development
License And Collaboration Agreements [Line Items]
Development milestone expense					$ 1
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]
Additional clinical, regulatory and commercialization milestone payments			$ 88