Minimum Regulatory Capital Requirements (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Common Equity Tier 1 Capital [Abstract]
Common Equity Tier One Capital	$ 154,705	$ 149,013
CommonEquityTierOneRiskBasedCapitalToRiskWeightedAssets	12.58%	12.23%
CommonEquityTierOneRiskBasedCapitalRequiredForCapitalAdequacy	$ 49,212	$ 48,744
CommonEquityTierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets	6.375%	5.75%
Tier 1 capital to risk weighted assets
Actual Amount	$ 154,705	$ 149,013
Actual Ratio	12.58%	12.23%
Minimum Capital Requirement Amount	$ 49,212	$ 48,744
Minimum Capital Requirement Ratio	7.875%	7.25%
Total capital to risk weighted assets
Actual Amount	$ 163,080	$ 156,713
Actual Ratio	13.26%	12.86%
Minimum Capital Requirement Amount	$ 98,423	$ 97,488
Minimum Capital Requirement Ratio	9.875%	9.25%
Tier 1 capital to average assets
Actual Amount	$ 154,705	$ 149,013
Actual Ratio	8.72%	8.54%
Minimum Capital Requirement Amount	$ 70,996	$ 69,827
Minimum Capital Requirement Ratio	4.00%	4.00%
Isabella Bank [Member]
Common Equity Tier 1 Capital [Abstract]
Common Equity Tier One Capital	$ 143,429	$ 139,897
CommonEquityTierOneRiskBasedCapitalToRiskWeightedAssets	11.75%	11.56%
CommonEquityTierOneRiskBasedCapitalRequiredForCapitalAdequacy	$ 48,832	$ 48,404
CommonEquityTierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets	6.375%	5.75%
CommonEquityTierOneRiskBasedCapitalRequiredToBeWellCapitalized	$ 73,248	$ 72,605
CommonEquityTierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets	6.50%	6.50%
Tier 1 capital to risk weighted assets
Actual Amount	$ 143,429	$ 139,897
Actual Ratio	11.75%	11.56%
Minimum Capital Requirement Amount	$ 48,832	$ 48,404
Minimum Capital Requirement Ratio	7.875%	7.25%
Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 73,248	$ 72,605
Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio	8.00%	8.00%
Total capital to risk weighted assets
Actual Amount	$ 151,804	$ 147,597
Actual Ratio	12.43%	12.20%
Minimum Capital Requirement Amount	$ 97,664	$ 96,807
Minimum Capital Requirement Ratio	9.875%	9.25%
Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 122,080	$ 121,009
Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio	10.00%	10.00%
Tier 1 capital to average assets
Actual Amount	$ 143,429	$ 139,897
Actual Ratio	8.07%	8.07%
Minimum Capital Requirement Amount	$ 71,085	$ 69,373
Minimum Capital Requirement Ratio	4.00%	4.00%
Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 88,856	$ 86,717
Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio	5.00%	5.00%