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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, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Earnings Per Share [Abstract]        
Average number of common shares outstanding for basic calculation 7,944,455 7,924,318 7,956,889 7,927,298
Average number of common shares outstanding used to calculate diluted earnings per common share 8,063,164 8,068,748 8,075,763 8,081,475
NET INCOME $ 4,621 $ 4,187 $ 10,019 $ 7,251
Basic (in dollars per share) $ 0.58 $ 0.53 $ 1.26 $ 0.91
Diluted (in dollars per share) $ 0.57 $ 0.52 $ 1.24 $ 0.90
Isabella Bank Corporation Restricted Stock Plan        
Earnings Per Share [Abstract]        
Average potential effect of common shares in the Directors Plan 16,257 0 10,489 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 16,257 0 10,489 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] 102,452 144,430 108,385 154,177
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] 102,452 144,430 108,385 154,177
[1] Exclusive of shares held in the Rabbi Trust