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New Accounting Pronouncements (Details) - USD ($) | Mar. 31, 2020 | Jan. 01, 2020 | Dec. 31, 2019 | Oct. 01, 2019 | Sep. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2018 | Sep. 30, 2018
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New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Held-to-maturity securities, at amortized cost | $ 920,255,000 |  |  |  | $ 1,443,480,000 |  |  |
Available-for-sale, at amortized cost | 1,667,467,000 |  |  |  | 1,458,071,000 |  |  |
Mortgage-backed securities available-for-sale, fair value |  | $ 390,669,000 |  |  |  |  |  |
Allowance for loan losses | 139,501,000 |  | $ 132,513,000 |  | 131,534,000 | $ 133,086,000 | $ 131,165,000 | $ 129,257,000
Increase in other asssets | 303,467,000 |  |  |  | $ 210,989,000 |  |  |
Accounting Standards Update 2019-04 |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Mortgage-backed securities held-to-maturity, fair value |  | (390,669,000) |  |  |  |  |  |
Accounting Standards Update 2016-13 | Pro Forma |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Allowance for loan losses | $ 176,000,000 |  |  |  |  |  |  |
Allowance for credit losses, percentage of gross loans outstanding | 1.31% |  |  |  |  |  |  |
Accounting Standards Update 2016-02 |  |  |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |  |  |
Increase in other asssets |  |  |  | $ 29,013,000 |  |  |  |
Increase in other liabilities |  |  |  | 29,013,000 |  |  |  |
Cumulative effect adjustment to retained earnings |  |  |  | $ 0 |  |  |  |
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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