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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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