Loans Receivable - Narrative (Details)	9 Months Ended
Jun. 30, 2024 USD ($) portfolioSegment	Sep. 30, 2023 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of portfolio segments for loans held for investment \| portfolioSegment	2
Loans held for sale	$ 468,527,000	$ 0
Accrued interest receivables for loans	92,457,000	77,349,000
Gross loans	22,466,841,000	19,648,504,000
Nonaccrual interest income recognized	706,000
Impaired, interest income on accrual accrual and performed loans	2,121,000
Loan modified, subsequent default	0
Commercial loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	13,235,910,000	12,094,294,000
Commercial loans \| Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	2,394,978,000	$ 2,321,717,000
Commercial loans \| Commercial & industrial \| 60 days past due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loan modified, accumulated amortized cost	0
Asset Pledged as Collateral \| Federal Reserve Bank Advances
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans	$ 17,407,607,000

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.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 310
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-Name Accounting Standards Codification
-Section 40
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481628/310-20-40-7

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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-Publisher FASB
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Amount, before allowance for credit loss, of accrued interest on financing receivable.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Amortized cost, before allowance for credit loss, fee, and loan in process, of financing receivable. Excludes financing receivable covered under loss sharing agreement and net investment in lease.

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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Accumulated amortized cost excluding accrued interest, of financing receivable modified for debtor experiencing financial difficulty.

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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Amount of interest income on financing receivable on nonaccrual status.

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Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

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Amount, after valuation allowance, of financing receivable held for sale and not part of disposal group. Excludes loan covered under loss sharing agreement and loan classified as investment in debt security.

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