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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, 2021	Mar. 31, 2020
Earnings Per Share [Abstract]
Average number of common shares outstanding for basic calculation		7,969,462	7,892,421
Average number of common shares outstanding used to calculate diluted earnings per common share		8,088,524	8,055,607
Net income		$ 5,398	$ 3,064
Basic (in dollars per share)		$ 0.68	$ 0.39
Diluted (in dollars per share)		$ 0.67	$ 0.38
Isabella Bank Corporation Restricted Stock Plan
Earnings Per Share [Abstract]
Average potential effect of common shares in the Directors Plan		4,678	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		4,678	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]	114,384	163,186
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]	114,384	163,186
[1] Exclusive of shares held in the Rabbi Trust

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