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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, 2022
Mar. 31, 2021
Earnings Per Share [Abstract]    
Average number of common shares outstanding for basic calculation 7,533,711 7,969,462
Average number of common shares outstanding used to calculate diluted earnings per common share 7,639,688 8,088,524
NET INCOME $ 4,734 $ 5,398
Basic (in dollars per share) $ 0.63 $ 0.68
Diluted (in dollars per share) $ 0.62 $ 0.67
Isabella Bank Corporation Restricted Stock Plan    
Earnings Per Share [Abstract]    
Average potential effect of common shares in the Directors Plan 22,439 4,678
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 22,439 4,678
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] 83,538 114,384
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] 83,538 114,384
[1] Exclusive of shares held in the Rabbi Trust