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Allowance for Loan Losses - Narrative (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2021
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |
Provision (release) for credit losses | $ 41,500 | $ 3,000 | $ 500
Allowance for credit loss, charge-off (recovery) | 45,101 | (3,508) |
Allowance for loan losses | 177,207 | 172,808 | 171,300
Loans Subject to Quantitative Evaluation |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |
Allowance for loan losses | 107,049 |  |
Loans Subject to Management Qualitative Evaluation |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |
Allowance for loan losses | 70,158 |  |
Non-Performing Loans |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |
Financing receivable, gross | $ 57,924 | $ 44,554 |
Ratio of non-performing assets to total assets | 0.26% | 0.21% |
Financial Asset Originated |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |
Non-accrual | $ 50,422 | $ 34,534 |
Commercial loans |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |
Allowance for loan losses | 137,194 | 133,476 | 125,375
Commercial loans | Financial Asset Originated |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |
Non-accrual | 34,644 | 16,296 |
Commercial loans | Commercial & industrial |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |
Allowance for loan losses | 58,773 | 57,210 | $ 45,957
Commercial loans | Commercial & industrial | Financial Asset Originated |  |  |
Financing Receivable, Allowance for Credit Loss [Line Items] |  |  |
Non-accrual | $ 6,082 | $ 5,693 |

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- Definition Amount excluding accrued interest, of allowance for credit loss on 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 + Details
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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 Amount, excluding accrued interest, of credit loss expense (reversal of expense) on financing receivable. Excludes net investment in lease. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 13 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479319/326-20-50-13 + Details
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