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Computation of Earnings Per Common Share (Schedule of Earnings Per Share) (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Earnings Per Share [Abstract]    
Average number of common shares outstanding for basic calculation 7,493,334 7,556,585
Average number of common shares outstanding used to calculate diluted earnings per common share 7,507,739 7,634,417
NET INCOME $ 3,131 $ 5,321
Basic (in dollars per share) $ 0.42 $ 0.70
Diluted (in dollars per share) $ 0.42 $ 0.70
Isabella Bank Corporation Restricted Stock Plan    
Earnings Per Share [Abstract]    
Average potential effect of common shares in the Directors Plan 14,405 28,348
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]    
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements 14,405 28,348
Isabella Bank Corporation and Related Companies Deferred Compensation Plan for Directors    
Earnings Per Share [Abstract]    
Average potential effect of common shares in the Directors Plan [1] 0 49,484
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]    
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements [1] 0 49,484
[1] Exclusive of shares held in the Rabbi Trust