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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions | Jan. 31, 2025 | Sep. 30, 2025 | Sep. 30, 2024
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Summary Of Significant Accounting Policies [Line Items] |  |  |
Interest bearing amounts held at other financial institutions |  | $ 124.0 | $ 103.2
Heartland Financial USA Inc [Member] |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |
Description of acquired entity | On January 31, 2025 (Acquisition Date), the Company acquired Heartland Financial USA, Inc. (HTLF) pursuant to an Agreement and Plan of Merger, dated as of April 28, 2024. |  |
Acquisition date | Jan. 31, 2025 |  |
Name of acquired entity | Heartland Financial USA, Inc. (HTLF) |  |
Date of merger | Apr. 28, 2024 |  |

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