Computation of Earnings Per Common Share (Schedule of Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Earnings Per Share [Abstract]
Average number of common shares outstanding for basic calculation		7,495,168	7,555,333	7,517,680	7,544,909
Average number of common shares outstanding used to calculate diluted earnings per common share		7,570,374	7,650,950	7,591,664	7,647,117
NET INCOME		$ 4,413	$ 5,887	$ 14,364	$ 15,916
Basic (in dollars per share)		$ 0.59	$ 0.78	$ 1.91	$ 2.11
Diluted (in dollars per share)		$ 0.58	$ 0.77	$ 1.89	$ 2.08
Isabella Bank Corporation Restricted Stock Plan
Earnings Per Share [Abstract]
Average potential effect of common shares in the Directors Plan		29,569	29,111	29,569	27,277
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		29,569	29,111	29,569	27,277
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]	45,637	66,506	44,415	74,931
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]	45,637	66,506	44,415	74,931

[1] Exclusive of shares held in the Rabbi Trust