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Fair Value Measurements - Additional Information (Details) - USD ($) | 6 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2020 | Mar. 31, 2019 | Jan. 01, 2020 | Sep. 30, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Debt Securities, Held-to-maturity | $ 920,255,000 |  |  | $ 1,443,480,000
Available-for-sale, at amortized cost | 1,667,467,000 |  |  | $ 1,458,071,000
Available-for-sale securities purchased | 159,884,000 | $ 290,574,000 |  |
Proceeds from sales of available-for-sale securities | 204,351,000 | 491,000 |  |
Held-to-maturity securities purchased | 0 | 0 |  |
Proceeds from sale of held-to-maturity securities | $ 0 | $ 0 |  |
Term of contractual due dates of substantially all mortgage-backed securities (in years) | 10 years |  |  |
Accounting Standards Update 2019-04 | Mortgage-backed securities |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |
Debt Securities, Held-to-maturity |  |  | $ (374,680,000) |
Available-for-sale, at amortized cost |  |  | $ 374,680,000 |

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