Accumulated Other Comprehensive Income (Loss) - Amounts Reclassified Out of AOCI (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total after tax	$ (3)	$ 2
(Gains) Losses Reclassified to Interest Expense [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	(5)
(Gains) Losses Excluded from Effectiveness Testing and Amortized to Interest Expense [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	2
Amortization of Actuarial Losses (Gains) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	2	5	$ 3
Amortization of Prior Service (Credit) Cost [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	(3)	(3)	$ (3)
Pension and Other Postretirement Benefit Liability Adjustment [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	(4)	2
Tax impact	1
Total after tax	$ (3)	$ 2