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, 2018	Sep. 30, 2017	Sep. 30, 2016
Derivative Instruments Gain Loss [Line Items]
Gain/(Loss) Recognized in AOCI	[1]	$ (2)	$ (10)	$ 1
(Gain)/Loss Reclassified from AOCI into Interest Expense in the Consolidated Statements of Operations	[1]	(5)
(Gain)/Loss Recognized in Interest Expense in the Consolidated Statements of Operations (Amount Excluded from Effectiveness Testing)	[1]	$ 2

[1] As noted above, effective October 1, 2017, the Company changed the method it uses to assess effectiveness from the method based on changes in forward exchange rates, in which all gains/losses were recognized in AOCI, to the method based on changes in spot exchange rates