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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, 2023
Dec. 31, 2022
Dec. 31, 2021
Earnings Per Share [Abstract]      
Average number of common shares outstanding for basic calculation 7,511,591 7,549,878 7,853,398
Average number of common shares outstanding used to calculate diluted earnings per common share 7,575,492 7,647,612 7,965,961
Net Income (Loss) Attributable to Parent $ 18,167 $ 22,238 $ 19,499
Basic (in dollars per share) $ 2.42 $ 2.95 $ 2.48
Diluted (in dollars per share) $ 2.40 $ 2.91 $ 2.45
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] 34,962 70,329 99,813
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] 34,962 70,329 99,813
Isabella Bank Corporation Restricted Stock Plan      
Earnings Per Share [Abstract]      
Average potential effect of common shares in the Directors Plan 28,939 27,405 12,750
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 28,939 27,405 12,750
[1] Exclusive of shares held in the Rabbi Trust