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Defined Benefit Plan, Curtailment, Years of Service

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Performance Bonus Expense

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Self Funded Medical Plan, Maximum Amount Paid Per Family

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Share-based Compensation Arrangement by Share-based Payment Award, Percentage of Employee Annual Salary

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Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

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Amount, before tax, of accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit.

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Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan.

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Amount of gain (loss) recognized in net periodic benefit (cost) credit from irrevocable action relieving primary responsibility for benefit obligation and eliminating risk related to obligation and assets used to effect settlement.

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Amount of cost for defined contribution plan.

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Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

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Percentage employer matches of the employee's percentage contribution matched.

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Percentage of employer's matching contributions to a defined contribution plan that vests in a given year.

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Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan.

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Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Amount of cost to be recognized for nonvested award under share-based payment arrangement. Excludes share and unit options.

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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%