Derivative Instruments and Hedging Activities - Narrative (Details) - USD ($)	3 Months Ended	6 Months Ended
Mar. 31, 2024	Mar. 31, 2023	Mar. 31, 2024	Mar. 31, 2023	Sep. 30, 2023
Derivative [Line Items]
Pre-tax gains (losses) associated with net investment hedges	$ 17,000,000	$ (9,000,000)	$ (15,000,000)	$ (73,000,000)
Maximum amount of loss	173,000,000		173,000,000
Cash collateral received	6,000,000	55,000,000	6,000,000	55,000,000	$ 40,000,000
IRPAs
Derivative [Line Items]
Notional amount	0	$ 0	0	$ 0	$ 0
Pre-tax net gains associated with interest rate hedges	$ 19,000,000		$ 19,000,000
Forward foreign currency exchange contracts
Derivative [Line Items]
Derivative notional amount percent of required need coverage	90.00%		90.00%

Derivative Notional Amount Percent Of Required Need Coverage

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- Definition

Maximum potential amount of future undiscounted payments that could be required under the credit derivative, before reduction for potential recoveries under recourse or collateralization provisions.

+ References

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- Definition

Amount of obligation to return cash collateral under master netting arrangements that have not been offset against derivative assets.

+ References

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

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- Definition

The estimated net amount of unrealized gains or losses on interest rate cash flow hedges as of the balance sheet date expected to be reclassified to earnings within the next twelve months.

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- Definition

Amount, before tax, of reclassification from accumulated other comprehensive income (AOCI) for gain (loss) from derivative designated and qualifying as net investment hedge.

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