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Computation of Earnings Per Common Share (Schedule of Earnings Per Share) (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Earnings Per Share [Abstract]      
Average number of common shares outstanding for basic calculation 7,853,398 7,959,705 7,909,794
Average number of common shares outstanding used to calculate diluted earnings per common share 7,965,961 8,106,091 8,095,042
Net Income (Loss) Attributable to Parent $ 19,499 $ 10,885 $ 13,024
Basic (in dollars per share) $ 2.48 $ 1.37 $ 1.65
Diluted (in dollars per share) $ 2.45 $ 1.34 $ 1.61
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] 99,813 143,878 185,248
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]      
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements [1] 99,813 143,878 185,248
Isabella Bank Corporation Restricted Stock Plan      
Earnings Per Share [Abstract]      
Average potential effect of common shares in the Directors Plan 12,750 2,508 0
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]      
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements 12,750 2,508 0
[1] Exclusive of shares held in the Rabbi Trust