Derivatives and Hedging Activities - Summary of Fair Value, Notional Amount and Balance Sheet Classification (Details) - USD ($) $ in Thousands	Mar. 31, 2025	Sep. 30, 2024
Other assets
Derivatives, Fair Value [Line Items]
Notional amount	$ 3,532,069	$ 1,981,554
Derivative assets, fair value	176,788	165,624
Other liabilities
Derivatives, Fair Value [Line Items]
Notional amount	1,427,728	3,614,512
Derivative liabilities, fair value	56,498	48,055
Client swap program hedges \| Other assets
Derivatives, Fair Value [Line Items]
Notional amount	1,027,728	1,044,512
Derivative assets, fair value	47,257	46,758
Client swap program hedges \| Other liabilities
Derivatives, Fair Value [Line Items]
Notional amount	1,027,728	1,044,512
Derivative liabilities, fair value	47,816	47,388
Commercial loan fair value hedges \| Other assets
Derivatives, Fair Value [Line Items]
Notional amount	34,341	37,042
Derivative assets, fair value	1,989	1,595
Commercial loan fair value hedges \| Other liabilities
Derivatives, Fair Value [Line Items]
Notional amount	0	0
Derivative liabilities, fair value	0	0
Mortgage loan fair value hedges \| Other assets
Derivatives, Fair Value [Line Items]
Notional amount	1,570,000	0
Derivative assets, fair value	6,876	0
Mortgage loan fair value hedges \| Other liabilities
Derivatives, Fair Value [Line Items]
Notional amount		2,570,000
Derivative liabilities, fair value	0	667
Mortgage backed securities fair value hedges \| Other assets
Derivatives, Fair Value [Line Items]
Notional amount	0
Derivative assets, fair value	0
Mortgage backed securities fair value hedges \| Other liabilities
Derivatives, Fair Value [Line Items]
Notional amount	400,000
Derivative liabilities, fair value	8,682
Borrowings cash flow hedges \| Other assets
Derivatives, Fair Value [Line Items]
Notional amount	900,000	900,000
Derivative assets, fair value	120,666	117,271
Borrowings cash flow hedges \| Other liabilities
Derivatives, Fair Value [Line Items]
Notional amount	0	0
Derivative liabilities, fair value	$ 0	$ 0

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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- Definition

Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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- Definition

Nominal or face amount used to calculate payment on derivative.

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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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