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Summary of Significant Accounting Policies (Details) $ / shares in Units, $ in Thousands |  | 6 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Feb. 08, 2021 $ / shares shares | Mar. 31, 2021 USD ($) branch | Mar. 31, 2020 USD ($) | Dec. 31, 2020 USD ($) | Sep. 30, 2020 USD ($) | Dec. 31, 2019 USD ($) | Sep. 30, 2019 USD ($)
Accounting Policies [Line Items] |  |  |  |  |  |  |
Number of bank branches | branch |  | 232 |  |  |  |  |
Increase (decrease) to retained earnings for the cumulative effect of adopting ASC 326 |  | $ 2,332,953 | $ 1,964,612 | $ 2,061,767 | $ 2,014,133 | $ 2,031,127 | $ 2,032,995
Cash collateral |  | 1,500 |  |  | 97,600 |  |
Loans in process |  | 1,982,225 |  |  | 1,456,072 |  |
Proceeds from issuance of preferred stock, net |  | 293,325 | 0 |  |  |  |
Dividend rate | 4.875% |  |  |  |  |  |
Public Offering |  |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |  |
Number of depositary shares | shares | 12,000,000 |  |  |  |  |  |
Percentage of preferred stock | 2.50% |  |  |  |  |  |
Public offering price (in usd per share) | $ / shares | $ 25.00 |  |  |  |  |  |
Public Offering | Series A Preferred Stock |  |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |  |
Number of depositary shares | shares | 300,000 |  |  |  |  |  |
Financing Receivable |  |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |  |
Loans in process |  | 3,348,654 |  |  | 2,738,095 |  |
Retained Earnings |  |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |  |
Increase (decrease) to retained earnings for the cumulative effect of adopting ASC 326 |  | $ 1,468,093 | $ 1,384,833 | $ 1,443,280 | $ 1,420,906 | $ 1,365,397 | 1,335,909
Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |  |
Increase (decrease) to retained earnings for the cumulative effect of adopting ASC 326 |  |  |  |  |  |  | (21,945)
Cumulative Effect, Period of Adoption, Adjustment | Retained Earnings |  |  |  |  |  |  |
Accounting Policies [Line Items] |  |  |  |  |  |  |
Increase (decrease) to retained earnings for the cumulative effect of adopting ASC 326 |  |  |  |  |  |  | $ (21,945)

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