Capital Ratios (Details) $ in Thousands	Sep. 30, 2024 USD ($)	Dec. 31, 2023 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier One Capital	$ 181,981	$ 180,014
Banking Regulation, Common Equity Tier 1 Risk-Based Capital Ratio, Actual	0.1208	0.1254
CommonEquityTierOneRiskRatedCapitalRequiredForCapitalAdequacyMinimumPlusCapitalConservationBuffer	$ 105,507	$ 100,449
CommonEquityTierOneRiskBasedCapitalToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	7.00%	7.00%
Actual Amount	$ 181,981	$ 180,014
Actual Ratio	0.1208	0.1254
Capital Adequacy Minimum with Buffer	$ 128,116	$ 121,973
TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	0.0850	0.0850
Banking Regulation, Total Capital, Actual	$ 224,516	$ 222,772
Banking Regulation, Total Risk-Based Capital Ratio, Actual	0.1490	0.1552
CapitalRequiredForCapitalAdequacyMinimumCapitalRequiredPlusCapitalConservationBuffer	$ 158,261	$ 150,673
CapitalRequiredForCapitalAdequacyToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	0.1050	0.1050
Banking Regulation, Tier 1 Leverage Capital, Actual	$ 181,981	$ 180,014
Banking Regulation, Tier 1 Leverage Capital Ratio, Actual	0.0877	0.0876
TierOneLeverageCapitalRequiredForCapitalAdequacyMinimumCapitalRequiredPlusCapitalConservationBuffer	$ 83,044	$ 82,154
TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	0.0400	0.0400
Isabella Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier One Capital	$ 173,019	$ 178,316
Banking Regulation, Common Equity Tier 1 Risk-Based Capital Ratio, Actual	0.1152	0.1248
CommonEquityTierOneRiskRatedCapitalRequiredForCapitalAdequacyMinimumPlusCapitalConservationBuffer	$ 105,136	$ 100,043
CommonEquityTierOneRiskBasedCapitalToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	7.00%	7.00%
Banking Regulation, Common Equity Tier 1 Risk-Based Capital, Well Capitalized, Minimum	$ 97,627	$ 92,897
Tier 1 Capital to Risk Weighted Asssets	0.0650	0.0650
Actual Amount	$ 173,019	$ 178,316
Actual Ratio	0.1152	0.1248
Capital Adequacy Minimum with Buffer	$ 127,665	$ 121,481
TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	0.0850	0.0850
Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 120,156	$ 114,335
Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio	0.0800	0.0800
Banking Regulation, Total Capital, Actual	$ 186,151	$ 191,739
Banking Regulation, Total Risk-Based Capital Ratio, Actual	0.1239	0.1342
CapitalRequiredForCapitalAdequacyMinimumCapitalRequiredPlusCapitalConservationBuffer	$ 157,704	$ 150,065
CapitalRequiredForCapitalAdequacyToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	0.1050	0.1050
Banking Regulation, Total Risk-Based Capital, Well Capitalized, Minimum	$ 150,195	$ 142,919
Banking Regulation, Total Risk-Based Capital Ratio, Well Capitalized, Minimum	0.1000	0.1000
Banking Regulation, Tier 1 Leverage Capital, Actual	$ 173,019	$ 178,316
Banking Regulation, Tier 1 Leverage Capital Ratio, Actual	0.0835	0.0871
TierOneLeverageCapitalRequiredForCapitalAdequacyMinimumCapitalRequiredPlusCapitalConservationBuffer	$ 82,851	$ 81,935
TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	0.0400	0.0400
Banking Regulation, Tier 1 Leverage Capital, Well Capitalized, Minimum	$ 103,564	$ 102,419
Banking Regulation, Tier 1 Leverage Capital Ratio, Well Capitalized, Minimum	0.0500	0.0500