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Investment Securities - Narrative (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Sep. 30, 2021 | Sep. 30, 2020 | Sep. 30, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Available-for-sale securities purchased | $ 530,227 | $ 1,064,815 | $ 358,709
Payments to acquire held-to-maturity securities | 0 |  |
Proceeds from sales of available-for-sale investment securities | 1,499 | 204,351 | $ 491
Proceeds from sale of held-to-maturity securities | 0 |  |
Accrued interest for available-for-sale securities | 3,459 | 3,285 |
Accrued interest for held-to-maturity securities | $ 944 | 1,811 |
Term of contractual due dates (in years) | 25 years |  |
Accounting Standards Update 2019-04 |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Held-to-maturity, at amortized costs |  | (374,680) |
Available-for-sale, at amortized cost |  | $ 374,680 |

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