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Summary of Significant Accounting Policies - Additional Information (Details) |  |  | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Mar. 31, 2025 USD ($) | Mar. 01, 2025 USD ($) shares | Sep. 30, 2025 USD ($) shares | Sep. 30, 2024 USD ($) | Sep. 30, 2025 USD ($) Segment shares | Sep. 30, 2024 USD ($) | Dec. 31, 2024 USD ($) | Apr. 30, 2021 shares
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Stock repurchase program, number of shares authorized to be repurchased (in shares) | shares |  |  |  |  |  |  |  | 375,388
Stock repurchase program, percent of stock outstanding |  |  |  |  |  |  |  | 5.00%
Securities held to maturity, Accrued interest receivable |  |  | $ 2,300,000 |  | $ 2,300,000 |  | $ 2,000,000 |
Shares issued for directors and employee stock plans |  |  | $ 245,000 | $ 165,000 | $ 678,000 | $ 280,000 |  |
Number of days individually evaluation |  |  |  |  | 60 days |  |  |
Stock repurchased during period, shares (in shares) | shares |  |  |  |  | 0 |  |  |
Number of reportable segments | Segment |  |  |  |  | 1 |  |  |
ASU 2023-07 |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Change in Accounting Principle, Accounting Standards Update, Adopted [true false] |  |  | true |  | true |  |  |
Change in Accounting Principle, Accounting Standards Update, Adoption Date |  |  | Dec. 31, 2024 |  | Dec. 31, 2024 |  |  |
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false] |  |  | true |  | true |  |  |
Fentura Financial, Inc. [Member] |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Shares issued for directors and employee stock plans (in shares) | shares |  | 6,070,836 |  |  |  |  |  |
Cash price | $ 1,800,000 |  |  |  |  |  |  |
Common stock purchased |  | $ 57,807 |  |  |  |  |  |
Shares issued for directors and employee stock plans |  | $ 192,800,000 |  |  |  |  |  |
Directors' Stock Purchase Plan [Member] |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Shares issued for directors and employee stock plans (in shares) | shares |  |  | 6,136 |  | 16,442 |  |  |
Shares issued for directors and employee stock plans |  |  | $ 176,000 |  | $ 496,000 |  |  |
Employee Stock Purchase Plan [Member] |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Shares issued for directors and employee stock plans (in shares) | shares |  |  | 2,802 |  | 7,413 |  |  |
Shares issued for directors and employee stock plans |  |  | $ 69,000 |  | $ 182,000 |  |  |
FETM Shares Retired [Member] |  |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |  |
Shares issued for directors and employee stock plans (in shares) | shares |  | 6,750 |  |  |  |  |  |
Shares issued for directors and employee stock plans | $ 215,000 |  |  |  |  |  |  |

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