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Investment Securities - Narrative (Details) - USD ($) | 12 Months Ended
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Sep. 30, 2024 | Sep. 30, 2023 | Sep. 30, 2022
Investments, Debt and Equity Securities [Abstract] |  |  |
Available-for-sale securities purchased | $ 549,159,000 | $ 376,481,000 | $ 587,942,000
Payments to acquire held-to-maturity securities | 47,092,000 | 0 | 195,357,000
Proceeds from sales of available-for-sale investment securities | 182,682,000 | 1,169,000 | $ 5,020,000
Proceeds from sale of held-to-maturity securities | $ 0 |  |
Term of contractual due dates (in years) | 25 years |  |
Accrued interest for available-for-sale securities | $ 9,311,000 | $ 8,641,000 |
Debt Securities, Available-for-Sale, Accrued Interest, after Allowance for Credit Loss, Statement of Financial Position [Extensible Enumeration] | Interest receivable | Interest receivable |
Accrued interest for held-to-maturity securities | $ 1,154,000 | $ 1,013,000 |
Debt Securities, Held-to-Maturity, Accrued Interest, after Allowance for Credit Loss, Statement of Financial Position [Extensible Enumeration] | Interest receivable | Interest receivable |

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- Definition Amount, after allowance for credit loss, of accrued interest on investment in debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 5C -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-5C Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 45 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479344/326-20-45-5 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 326 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 3A -Publisher FASB -URI https://asc.fasb.org/1943274/2147479319/326-20-50-3A + Details
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