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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 6 Months Ended
Jun. 30, 2024
Jun. 30, 2023
Jun. 30, 2024
Jun. 30, 2023
Earnings Per Share [Abstract]        
Average number of common shares outstanding for basic calculation 7,477,310 7,498,584 7,485,743 7,528,251
Average number of common shares outstanding used to calculate diluted earnings per common share 7,494,828 7,567,527 7,501,437 7,601,613
Net income $ 3,481 $ 4,630 $ 6,612 $ 9,951
Basic (in dollars per share) $ 0.47 $ 0.62 $ 0.88 $ 1.32
Diluted (in dollars per share) $ 0.46 $ 0.61 $ 0.88 $ 1.31
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 38,166 0 43,793
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 38,166 0 43,793
Isabella Bank Corporation Restricted Stock Plan        
Earnings Per Share [Abstract]        
Average potential effect of common shares in the Directors Plan 17,518 30,777 15,694 29,569
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 17,518 30,777 15,694 29,569
[1] Exclusive of shares held in the Rabbi Trust