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Investment Securities - Narrative (Details) - USD ($) | 12 Months Ended |
--- | --- | ---
Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018 | Jan. 01, 2020
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Held-to-maturity securities, at amortized cost | $ 705,838,000 | $ 1,443,480,000 |  |
Available-for-sale, at amortized cost | 2,210,099,000 | 1,458,071,000 |  |
Available-for-sale securities purchased | 1,064,815,000 | 358,709,000 | $ 272,780,000 |
Payments to acquire held-to-maturity securities | 0 | 0 | 170,836,000 |
Proceeds from sales of available-for-sale investment securities | 204,351,000 | 491,000 | $ 0 |
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 | $ 3,285,000 | 3,712,000 |  |
Accrued interest for held-to-maturity securities | $ 1,811,000 | $ 3,716,000 |  |
Mortgage-backed securities | Accounting Standards Update 2019-04 |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Held-to-maturity securities, at amortized cost |  |  |  | $ (374,680,000)
Available-for-sale, at amortized cost |  |  |  | $ 374,680,000

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