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Benefit Plans (Narrative) (Details) - USD ($) $ in Thousands |  | 12 Months Ended
--- | --- | ---
Mar. 01, 2007 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Retirement Benefits [Abstract] |  |  |  |
Employees contribution |  | 100.00% |  |
Corporation matching contribution, percent |  | 100.00% |  |
Employee contribution, percent |  | 5.00% |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Performance bonus expense |  | $ 1,101 | $ 1,070 | $ 500
Defined Benefit Plan, Net Periodic Benefit Cost (Credit), Gain (Loss) Due to Settlement |  | (152) | (128) | (269)
Plan expenses |  | 813 | 764 | 743
Highest consecutive years of compensation | 5 years |  |  |
Years of service | 10 years |  |  |
Accumulated other comprehensive income includes net unrecognized pension cost before income taxes |  | 3,474 |  |
Corporation self-funded medical plan |  | 75 |  |
Medical expenses |  | 1,868 | 2,445 | 2,695
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Performance Bonus Expense - Executive Plan |  | 165 |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Compensation expense |  |  | 171 | 45
Other Postretirement Benefits Plan [Member] |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Plan expenses |  | 373 | $ 355 | $ 356
Restricted Stock | Isabella Bank Corporation Restricted Stock Plan |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Compensation expense |  | 14 |  |
Unrecognized compensation expense |  | $ 68 |  |
Unrecognized compensation expense, period for recognition |  | 3 years 2 months 1 day |  |
Minimum [Member] | Restricted Stock | Isabella Bank Corporation Restricted Stock Plan |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Maximum award as a percentage of salary |  | 25.00% |  |
Maximum [Member] | Restricted Stock | Isabella Bank Corporation Restricted Stock Plan |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |
Maximum award as a percentage of salary |  | 40.00% |  |
Safe Harbor 401(k) [Member] |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Vesting percentage |  | 100.00% |  |
Fixed Income Funds [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Investment allocation in securities |  | 50.00% |  |
Equity Collective Trust [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Investment allocation in securities |  | 50.00% |  |

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