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Investment Securities - Narrative (Details) - USD ($) | 12 Months Ended
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Sep. 30, 2023 | Sep. 30, 2022 | Sep. 30, 2021
Investments, Debt and Equity Securities [Abstract] |  |  |
Available-for-sale securities purchased | $ 376,481,000 | $ 587,942,000 | $ 530,227,000
Payments to acquire held-to-maturity securities | 0 | 195,357,000 | 0
Proceeds from sales of available-for-sale investment securities | 1,169,000 | 5,020,000 | $ 1,499,000
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 | $ 8,641,000 | $ 5,694,000 |
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 | $ 1,013,000 | $ 1,108,000 |
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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