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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 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Earnings Per Share [Abstract]        
Average number of common shares outstanding for basic calculation 7,932,227 7,966,811 7,948,578 7,945,762
Average number of common shares outstanding used to calculate diluted earnings per common share 8,044,572 8,111,283 8,065,252 8,096,802
NET INCOME $ 4,686 $ 4,357 $ 14,705 $ 11,608
Basic (in dollars per share) $ 0.59 $ 0.55 $ 1.85 $ 1.46
Diluted (in dollars per share) $ 0.58 $ 0.54 $ 1.82 $ 1.43
Isabella Bank Corporation Restricted Stock Plan        
Earnings Per Share [Abstract]        
Average potential effect of common shares in the Directors Plan 17,517 0 12,858 0
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 17,517 0 12,858 0
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] 94,828 144,472 103,816 151,040
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] 94,828 144,472 103,816 151,040
[1] Exclusive of shares held in the Rabbi Trust