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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
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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

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