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Related Party Transactions - Ending Balances of, and Contributions to the Isabella Bank Foundation (Details) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Related Party Transaction [Line Items]
Total assets	$ 2,209,448	$ 2,086,241
Affiliated Entity | Isabella Bank Foundation
Related Party Transaction [Line Items]
Total assets	1,521	1,236	$ 1,221
Donations	$ 0	$ 0	$ 0

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