Business Segment - Summary of Segment Reporting Information (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2025	Jun. 30, 2025	Mar. 31, 2025	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Sep. 30, 2025	Sep. 30, 2024
Segment Reporting, Asset Reconciling Item [Line Items]
Total revenues	$ 26			$ 48			$ 214	$ 232
Less:
Impairment of long-lived assets	0			0		$ 20	0	20
Income tax expense	0			0			0	0
Net loss	(135)	$ 0	$ (112)	(92)	$ (93)	$ (4)	(247)	(189)
Reportable segment
Segment Reporting, Asset Reconciling Item [Line Items]
Total revenues	26			48			214	232
Less:
Late-stage development programs	78			69			207	196
Early-stage development and preclinical programs	33			34			113	92
Compensation and personnel costs	59			62			187	190
Depreciation and amortization	2			2			7	7
Impairment of long-lived assets	0			0			0	20
Interest income, net	(7)			(13)			(25)	(38)
Income tax expense	0			0			0	0
Other segment items	24			23			71	68
Partnership reimbursements	(28)			(37)			(99)	(114)
Net loss	$ (135)			$ (92)			$ (247)	$ (189)