Capital Ratios (Details) $ in Thousands	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier One Capital	$ 183,348	$ 180,014
Banking Regulation, Common Equity Tier 1 Risk-Based Capital Ratio, Actual	0.1221	0.1254
CommonEquityTierOneRiskBasedCapitalRequiredForCapitalAdequacy	$ 105,136	$ 100,449
CommonEquityTierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets	7.00%	7.00%
Tier 1, Actual Amount	$ 183,348	$ 180,014
Tier 1, Actual Ratio	0.1221	0.1254
Capital Adequacy Minimum with Buffer	$ 127,665	$ 121,973
TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	0.0850	0.0850
Banking Regulation, Total Capital, Actual	$ 226,179	$ 222,772
Capital to Risk Weighted Assets	0.1506	0.1552
CapitalRequiredForCapitalAdequacyMinimumCapitalRequiredPlusCapitalConservationBuffer	$ 157,703	$ 150,673
CapitalRequiredForCapitalAdequacyToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	0.1050	0.1050
Tier One Leverage Capital	$ 183,348	$ 180,014
Tier One Leverage Capital to Average Assets	0.0886	0.0876
TierOneLeverageCapitalRequiredForCapitalAdequacyMinimumCapitalRequiredPlusCapitalConservationBuffer	$ 82,803	$ 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	$ 172,589	$ 178,316
Banking Regulation, Common Equity Tier 1 Risk-Based Capital Ratio, Actual	0.1153	0.1248
CommonEquityTierOneRiskBasedCapitalRequiredForCapitalAdequacy	$ 104,783	$ 100,043
CommonEquityTierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets	7.00%	7.00%
Banking Regulation, Common Equity Tier 1 Risk-Based Capital, Well Capitalized, Minimum	$ 97,299	$ 92,897
Tier 1 Capital to Risk Weighted Assets	0.0650	0.0650
Tier 1, Actual Amount	$ 172,589	$ 178,316
Tier 1, Actual Ratio	0.1153	0.1248
Capital Adequacy Minimum with Buffer	$ 127,237	$ 121,481
TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	0.0850	0.0850
Tier One Risk Based Capital Required to be Well Capitalized	$ 119,753	$ 114,335
Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio	0.0800	0.0800
Banking Regulation, Total Capital, Actual	$ 185,997	$ 191,739
Capital to Risk Weighted Assets	0.1243	0.1342
CapitalRequiredForCapitalAdequacyMinimumCapitalRequiredPlusCapitalConservationBuffer	$ 157,175	$ 150,065
CapitalRequiredForCapitalAdequacyToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	0.1050	0.1050
Capital Required to be Well Capitalized	$ 149,691	$ 142,919
Capital Required to be Well Capitalized to Risk Weighted Assets	0.1000	0.1000
Tier One Leverage Capital	$ 172,589	$ 178,316
Tier One Leverage Capital to Average Assets	0.0836	0.0871
TierOneLeverageCapitalRequiredForCapitalAdequacyMinimumCapitalRequiredPlusCapitalConservationBuffer	$ 82,602	$ 81,935
TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	0.0400	0.0400
Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions Amoutnnt	$ 103,252	$ 102,419
Tier One Leverage Capital Required to be Well Capitalized to Average Assets	0.0500	0.0500

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