Accumulated Other Comprehensive Income (Loss) - Amounts Reclassified Out of AOCI (Detail) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
(Gains) Losses Reclassified to Interest Expense [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax	$ (2)		$ (1)	$ (3)	$ (2)
(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	1		$ 1	1	1
Amortization of Actuarial Losses and Prior Service Cost (Credit) [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Total before tax				1	$ 1
Settlement Charge [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	(1)			5
Tax impact				(3)
Total after tax	$ (1)	$ 3		$ 2