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, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Derivative Instruments Gain Loss [Line Items]
Gain/(Loss) Recognized in AOCI	$ 5	$ (8)	$ 13	$ (13)
(Gain)/Loss Reclassified from AOCI into Interest Expense in the Consolidated Statements of Operations	(2)	(1)	(3)	(2)
(Gain)/Loss Recognized in Interest Expense in the Consolidated Statements of Operations (Amount Excluded from Effectiveness Testing)	$ 1	$ 1	$ 1	$ 1