Derivatives - Summary Impact of Cross-currency Swaps Net of Tax to AOCI and Consolidated Statements of Operations (Detail) - Cross Currency Swaps [Member] - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Derivative Instruments Gain Loss [Line Items]
Gain/(loss) recognized in AOCI	$ (8)	$ (6)	$ (13)
(Gain)/loss reclassified from AOCI into Interest expense	(4)	(4)	(6)
(Gain)/loss recognized in Interest expense (amount excluded from effectiveness testing)	$ 2	$ 2	$ 2
Derivative Instrument Gain Loss Reclassified from Aoci into Income Expense Effective Portion Statement of Income or Comprehensive Income [Extensible Enumeration]	Interest Expense, Operating and Nonoperating	Interest Expense, Operating and Nonoperating	Interest Expense, Operating and Nonoperating