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Computation of Earnings Per Common Share (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]      
Average number of common shares outstanding for basic calculation (in shares) 7,372,434 7,465,343 7,511,591
Average number of common shares outstanding used to calculate diluted earnings per common share (in shares) 7,385,862 7,482,374 7,575,492
Net income $ 18,910 $ 13,889 $ 18,167
Basic (in dollars per share) $ 2.56 $ 1.86 $ 2.42
Diluted (in dollars per share) $ 2.56 $ 1.86 $ 2.40
Isabella Bank Corporation and Related Companies Deferred Compensation Plan for Directors      
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]      
Average potential effect of common shares (in shares) [1] 0 0 34,962
Isabella Bank Corporation Restricted Stock Plan      
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]      
Average potential effect of common shares (in shares) 13,428 17,031 28,939
[1] Exclusive of shares held in the Rabbi Trust