Accumulated Other Comprehensive Income (Loss) - Amounts Reclassified Out of AOCI (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
(Gains) Losses Reclassified to Interest Expense [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	$ (1)	$ (1)
Amortization of Actuarial Losses [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	$ 1	1
Settlement and Curtailment Gain [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax		(6)
Accumulated Other Comprehensive Income (Loss) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax		(6)
Tax impact		(3)
Total after tax		$ (9)