Derivatives - Summary Impact of Cross-currency Swaps to AOCI and Consolidated Statements of Operations (Detail) - Cross Currency Swaps [Member] - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Derivative Instruments Gain Loss [Line Items]
Gain/(Loss) Recognized in AOCI	$ 1	$ 23	$ (2)
(Gain)/Loss Reclassified from AOCI into Interest Expense in the Consolidated Statements of Operations	(5)	(5)	(5)
(Gain)/Loss Recognized in Interest Expense in the Consolidated Statements of Operations (Amount Excluded from Effectiveness Testing)	$ 2	$ 1	$ 2