Derivatives - Summary Impact of Cross-currency Swaps to AOCI and Consolidated Statements of Operations (Detail) - Cross Currency Swaps [Member] - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Derivative Instruments Gain Loss [Line Items]
Gain/(Loss) Recognized in AOCI	[1]	$ (8)	$ (1)	$ (13)	$ (8)
(Gain)/Loss Reclassified from AOCI into Interest Expense in the Consolidated Statements of Operations	[1]	(1)		(2)
(Gain)/Loss Recognized in Interest Expense in the Consolidated Statements of Operations (Amount Excluded from Effectiveness Testing)	[1]	$ 1		$ 1

[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.