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Derivative Financial Instruments (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Derivative [Line Items]
Reduction in earnings from excluded forward points and ineffectiveness	$ 9	$ 12	$ 9
Expected amount of accumulated other comprehensive income (loss) expected to be reclassified	40
Fair value hedge assets	248	89
Fair value hedge liabilities		2
Earnings related to forward points and interest differentials from forward contracts and swap agreements	150	95
Other Assets
Derivative [Line Items]
Posted collateral	26
Accrued Liabilities
Derivative [Line Items]
Collateral received with counterparties	64
Foreign Exchange Contract \| Accrued Liabilities
Derivative [Line Items]
Derivative liability	$ 5	$ 19
3.15% Senior Notes due December 2025 \| Senior Notes
Derivative [Line Items]
Stated interest rate (percent)	3.15%	3.15%
Designated as Hedging Instrument
Derivative [Line Items]
Notional amount	$ 10,700	$ 10,900
Not Designated as Hedging Instrument \| Foreign Exchange Contract
Derivative [Line Items]
Notional amount	1,600	800
Not Designated as Hedging Instrument \| Foreign Exchange Contract \| Other Assets
Derivative [Line Items]
Derivative asset	7	3
Cash Flow Hedging \| Designated as Hedging Instrument
Derivative [Line Items]
Derivative asset	71	47
Derivative liability	39	31
Net Investment Hedging
Derivative [Line Items]
Derivative asset	186	$ 298
Derivative liability	$ 137