Summary of Significant Accounting Policies - Leases (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Leases
Finance lease under PPA	$ 509	$ 550
Operating lease under PPA	75	121
Operating cash flows for operating leases	72	$ 78	$ 77
Finance Lease, Liability, Payment, Due	$ 1,030