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Benefit Plans - Narrative (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2025 | Dec. 31, 2024 | Dec. 31, 2023
Defined Contribution Plan Disclosure [Line Items] |  |  |
Employees contribution | 100.00% |  |
Corporation matching contribution, percent | 100.00% |  |
Employee contribution, percent | 5.00% |  |
Plan expenses | $ 954 | $ 965 | $ 885
Performance bonus expense - executive plan | 249 | 103 | 53
Performance bonus expense | 1,574 | 1,485 | 796
Corporation self-funded medical plan | 150 |  |
Medical expenses | 3,540 | 2,895 | 2,281
Other Postretirement Benefits Plan |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Plan expenses | 467 | 529 | 345
Isabella Bank Corporation Restricted Stock Plan | Restricted Stock |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Compensation expense | 64 | $ 95 | $ 253
Unrecognized compensation expense | $ 153 |  |
Unrecognized compensation expense, period for recognition (in years) | 2 years 9 months |  |
Minimum |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Executive Cash Incentive Plan, potential payout as percentage of employee's annual salary | 22.80% |  |
Minimum | Isabella Bank Corporation Restricted Stock Plan | Restricted Stock |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Award as a percentage of salary | 25.00% |  |
Maximum |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Executive Cash Incentive Plan, potential payout as percentage of employee's annual salary | 35.00% |  |
Maximum | Isabella Bank Corporation Restricted Stock Plan | Restricted Stock |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Award as a percentage of salary | 40.00% |  |
Equity Collective Trust |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Investment allocation in securities | 50.00% |  |
Plan assets, target long-term rate of return (percent) | 6.00% |  |
Fixed Income Funds |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Investment allocation in securities | 50.00% |  |
Safe Harbor 401(k) |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Vesting percentage | 100.00% |  |

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