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Investment Securities - Narrative (Details) - USD ($) $ in Thousands | 12 Months Ended
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Sep. 30, 2022 | Sep. 30, 2021 | Sep. 30, 2020
Investments, Debt and Equity Securities [Abstract] |  |  |
Available-for-sale securities purchased | $ 587,942 | $ 530,227 | $ 1,064,815
Payments to acquire held-to-maturity securities | 195,357 | 0 | 0
Proceeds from sales of available-for-sale investment securities | 5,020 | 1,499 | $ 204,351
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 | $ 5,694 | $ 3,459 |
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 | $ 1,108 | $ 944 |
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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