Business Segment - Summary of Segment Reporting Information (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2026	Jun. 30, 2026	Mar. 31, 2026	Sep. 30, 2025	Mar. 31, 2025	Sep. 30, 2026	Sep. 30, 2025
Segment Reporting [Line Items]
Total revenues	$ 9			$ 26		$ 67	$ 214
Adjusted for expense (income):
Non-operating income, net	(3)			(7)		(14)	(25)
Net loss	(128)	$ (91)	$ (128)	(135)	$ (112)	(347)	(247)
Reportable segment
Segment Reporting [Line Items]
Total revenues	9			26		67	214
Adjusted for expense (income):
Late-stage development programs	54			78		165	207
Early-stage development and preclinical programs	17			33		56	113
Compensation and personnel costs	59			59		187	187
Depreciation and amortization	2			2		7	7
Partnership reimbursements	(12)			(28)		(48)	(99)
Other segment items	20			24		61	71
Non-operating income, net	(3)			(7)		(14)	(25)
Net loss	$ (128)			$ (135)		$ (347)	$ (247)

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