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Summary of Significant Accounting Policies - Additional Information (Details) |  | 3 Months Ended | 9 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Jul. 26, 2024 USD ($) $ / shares shares | Sep. 30, 2024 USD ($) shares | Sep. 30, 2023 USD ($) | Sep. 30, 2024 USD ($) shares | Sep. 30, 2023 USD ($) | Jul. 25, 2024 USD ($) Office shares | Dec. 31, 2023 USD ($) | Apr. 30, 2021 shares
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Stock issued during period, shares, new issues (in shares) | shares | 1,380,000 |  |  |  |  |  |  |
Sales price per share (in dollars per share) | $ / shares | $ 25 |  |  |  |  |  |  |
Issuance of common stock | $ 34,500,000 |  |  | $ 500,000 | $ 168,000 |  |  |
Number of shares each share of Fentura common stock outstanding will be converted into (in shares) | shares |  |  |  |  |  | 1.35 |  |
Consolidated total assets |  | $ 2,726,003,000 |  | 2,726,003,000 |  |  | $ 2,576,706,000 |
Merger-related expenses |  | 645,000 |  | 645,000 |  |  |  |
Stock issued during period, value, new issues |  | $ 165,000 | $ 131,000 | $ 280,000 | $ 428,000 |  |  |
Stock repurchased during period, shares (in shares) | shares |  |  |  | 0 |  |  |  |
Stock repurchase program, number of shares authorized to be repurchased (in shares) | shares |  |  |  |  |  |  |  | 375,388
Stock repurchase program, percent of stock outstanding |  |  |  |  |  |  |  | 5.00%
Number of days individually evaluation |  |  |  | 60 days |  |  |  |
Plan [Member] |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Consolidated total assets |  |  |  |  |  | $ 4,300,000,000 |  |
Number of offices after combination is completed | Office |  |  |  |  |  | 56 |  |
Directors' Stock Purchase Plan [Member] |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Stock issued during period, shares, new issues (in shares) | shares |  | 3,877 |  | 11,582 |  |  |  |
Stock issued during period, value, new issues |  | $ 111,000 |  | $ 330,000 |  |  |  |
Employee Stock Purchase Plan [Member] |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Stock issued during period, shares, new issues (in shares) | shares |  | 2,037 |  | 6,943 |  |  |  |
Stock issued during period, value, new issues |  | $ 53,000 |  | $ 170,000 |  |  |  |

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