Derivative and Hedging Instruments - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2022
Derivative [Line Items]
Earnings related to forward points and interest differentials from forward contracts and swap agreements	$ (94)	$ (25)	$ 151
Net unrealized gain (loss)	(38)	(126)	917
Net unrealized gain (loss)			190
Pre-tax gains (losses) in other comprehensive income (loss) related to net investment hedges	(321)	(445)	$ 845
Accumulated other comprehensive income (loss)	(308)	(1,317)
Accrued liabilities
Derivative [Line Items]
Collateral received with counterparties	62
Other Assets
Derivative [Line Items]
Posted collateral	48
Net Investment Hedging
Derivative [Line Items]
Accumulated other comprehensive income (loss)	182	433
Designated as Hedging Instruments:
Derivative [Line Items]
Notional amount	11,700	11,000
Not Designated as Hedging Instruments:
Derivative [Line Items]
Notional amount	$ 1,900	$ 800