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Summary of Significant Accounting Policies - Additional Information (Details) |  |  | 3 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Mar. 01, 2025 USD ($) shares | Jul. 26, 2024 USD ($) $ / shares shares | Mar. 31, 2025 USD ($) Segment shares | Mar. 31, 2024 USD ($) | Dec. 31, 2024 USD ($) | Dec. 31, 2023 USD ($) | Apr. 30, 2021 shares
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Shares issued for directors and employee stock plans (in shares) | shares |  | 1,380,000 |  |  |  |  |
Sales price per share (in dollars per share) | $ / shares |  | $ 25 |  |  |  |  |
Issuance of common stock |  | $ 34,500,000 | $ 172,000 | $ 56,000 |  |  |
Provision for credit losses on loans |  |  | 13,163,000 | 0 |  |  |
Allowance for credit loss |  |  | 34,567,000 | 16,037,000 | $ 16,552,000 | $ 15,685,000 |
Estimated accretable interest income |  |  | 59,800,000 |  |  |  |
Merger-related expenses |  |  | 64,700,000 |  |  |  |
Securities held to maturity, Accrued interest receivable |  |  | 2,300,000 |  | $ 2,000,000 |  |
Shares issued for directors and employee stock plans |  |  | $ 172,000 | $ 90,000 |  |  |
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 |  |  |  |
Merger PCD loans |  |  | $ 1,300,000,000 |  |  |  |
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 |  |  |  |
Change in Accounting Principle, Accounting Standards Update, Adoption Date |  |  | Dec. 31, 2024 |  |  |  |
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false] |  |  | true |  |  |  |
Fentura Financial, Inc. [Member] |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Shares issued for directors and employee stock plans (in shares) | shares | 6,070,836 |  |  |  |  |  |
Shares issued for directors and employee stock plans | $ 193,000,000 |  |  |  |  |  |
Non-PCD Loans [Member] |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Provision for credit losses on loans | 12,000,000 |  |  |  |  |  |
Loans purchased |  |  | $ 51,600,000 |  |  |  |
Merger PCD loans | $ 1,300,000,000 |  |  |  |  |  |
PCD Loans [Member] |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Allowance for credit loss |  |  | 4,900,000 |  |  |  |
Loans purchased |  |  | $ 13,100,000 |  |  |  |
Directors' Stock Purchase Plan [Member] |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Shares issued for directors and employee stock plans (in shares) | shares |  |  | 3,301 |  |  |  |
Shares issued for directors and employee stock plans |  |  | $ 118,000 |  |  |  |
Employee Stock Purchase Plan [Member] |  |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Shares issued for directors and employee stock plans (in shares) | shares |  |  | 2,214 |  |  |  |
Shares issued for directors and employee stock plans |  |  | $ 54,000 |  |  |  |

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