Income Taxes - Components of Provision (Benefit) for Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Current
Current income tax expense (benefit)	$ 131	$ 33	$ 124
Deferred
Deferred income tax expense (benefit)	(28)	(1)	(124)
Income tax provision	103	32	0
Foreign tax authority | Ireland
Current
Current foreign tax expense	1	0	0
Deferred
Deferred foreign tax expense	0	0	0
Foreign tax authority | Other Foreign
Current
Current foreign tax expense	116	28	120
Deferred
Deferred foreign tax expense	(28)	(1)	(125)
United States
Current
US - Federal and State	14	5	4
Deferred
US - Federal and State	$ 0	$ 0	$ 1