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Related Party Transactions (Ending balances of, and contributions to the Isabella Bank Foundation) (Details) - USD ($)
$ in Thousands
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Related Party Transaction [Line Items]      
Ending assets $ 2,086,241 $ 2,058,968  
Affiliated Entity [Member] | Isabella Bank Foundation [Member]      
Related Party Transaction [Line Items]      
Ending assets 1,236 1,221 $ 1,385
Cash Donations $ 0 $ 0 $ 50