Computation of Earnings Per Common Share (Schedule of Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Average number of common shares outstanding for basic calculation		7,959,705	7,909,794	7,872,077
Average number of common shares outstanding used to calculate diluted earnings per common share		8,106,091	8,095,042	8,072,848
Net Income (Loss) Attributable to Parent		$ 10,885	$ 13,024	$ 14,021
Basic (in dollars per share)		$ 1.37	$ 1.65	$ 1.78
Diluted (in dollars per share)		$ 1.34	$ 1.61	$ 1.74
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]	143,878	185,248	200,771
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]	143,878	185,248	200,771
Isabella Bank Corporation Restricted Stock Plan
Earnings Per Share [Abstract]
Average potential effect of common shares in the Directors Plan		2,508	0	0
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		2,508	0	0

[1] Exclusive of shares held in the Rabbi Trust