Derivative Instruments and Hedging Activities - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020	Sep. 30, 2020
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Reclassification of terminated interest rate swaps to interest expense within the next 12 months	$ (2)		$ (2)
Cash flow hedges	83	$ (109)	111	$ (70)
Debt | Net Investment Hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, Notional Amount	1,500		1,500		$ 1,500
Foreign Exchange Contract
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, Notional Amount	1,600		1,600		2,500
Currency Swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, Notional Amount	3,000		3,000		3,000
Fixed to Floating | Fair Value Hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, Notional Amount					375
Interest Rate Swap | Cash Flow Hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative, Notional Amount	1,000		1,000		$ 1,500
Cash flow hedges	78	$ (111)	105	$ (74)
Derivative, Notional Amount, Terminated	$ 500		$ 500