Derivative and Hedging Instruments - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Derivative [Line Items]
Increase (decrease) related to forward points and interest differentials from forward contracts and swap agreements	$ (45)	$ (94)	$ (25)
Net unrealized gain (loss)	(172)	(38)	(126)
Pre-tax gains (losses) to be reclassified from AOCI into the statement of operations	(157)
Pre-tax gains (losses) in other comprehensive income (loss) related to net investment hedges	(459)	(321)	$ (445)
Accumulated other comprehensive income (loss)	248	(308)
Accrued liabilities
Derivative [Line Items]
Collateral received with counterparties	45
Other Assets
Derivative [Line Items]
Posted collateral	192
Net Investment Hedging
Derivative [Line Items]
Accumulated other comprehensive income (loss)	$ (176)	$ 182

Amount, after tax, of accumulated increase (decrease) in equity from transaction and other event and circumstance from nonowner source.

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