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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
	Mar. 31, 2025	Mar. 31, 2024
Earnings Per Share [Abstract]
Average number of common shares outstanding for basic calculation	7,419,739	7,493,334
Average number of common shares outstanding used to calculate diluted earnings per common share	7,432,162	7,507,739
Net income	$ 3,949	$ 3,131
Basic (in dollars per share)	$ 0.53	$ 0.42
Diluted (in dollars per share)	$ 0.53	$ 0.42
Isabella Bank Corporation Restricted Stock Plan
Earnings Per Share [Abstract]
Average potential effect of common shares in the Directors Plan	12,423	14,405
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	12,423	14,405

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