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Allowance for Losses on Loans - Narrative (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Mar. 31, 2025 | Mar. 31, 2024 | Mar. 31, 2025 | Mar. 31, 2024 | Sep. 30, 2024
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Provision for credit losses | $ 2,750 | $ 16,000 | $ 2,750 | $ 16,000 |
Allowance for credit loss, charge-off (recovery) | 5,063 | $ 146 | 5,294 | $ 33 |
Gross loans | $ 21,122,710 |  | $ 21,122,710 |  | $ 21,120,107
Percent past due | 0.27% |  | 0.27% |  | 0.25%
Financial Asset Originated |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Non-accrual | $ 59,886 |  | $ 59,886 |  | $ 69,541
Non-Performing Loans |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Gross loans | $ 70,884 |  | $ 70,884 |  | $ 77,418
Ratio of non-performing assets to total assets | 0.26% |  | 0.26% |  | 0.28%
Commercial loans |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Gross loans | $ 12,238,959 |  | $ 12,238,959 |  | $ 12,205,420
Percent past due | 0.16% |  | 0.16% |  | 0.16%
Commercial loans | Financial Asset Originated |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Non-accrual | $ 39,797 |  | $ 39,797 |  | $ 46,299
Commercial loans | Commercial & industrial |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Gross loans | $ 2,384,745 |  | $ 2,384,745 |  | $ 2,332,732
Percent past due | 0.00% |  | 0.00% |  | 0.09%
Commercial loans | Commercial & industrial | Financial Asset Originated |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |
Non-accrual | $ 0 |  | $ 0 |  | $ 0

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- Definition Amount, excluding accrued interest, of decrease (increase) in allowance for credit loss on financing receivable from writeoff (recovery). Excludes net investment in lease. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 13 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-13 + Details
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- Definition Amortized cost excluding accrued interest, before allowance for credit loss, of financing receivable. Excludes net investment in lease. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479344/326-20-45-1 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 3B -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-3B Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-5 + Details
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