Capital Ratios (Details) $ in Thousands	Jun. 30, 2025 USD ($)	Dec. 31, 2024 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier One Capital	$ 186,605	$ 183,348
Banking Regulation, Common Equity Tier 1 Risk-Based Capital Ratio, Actual	0.1246	0.1221
CommonEquityTierOneRiskRatedCapitalRequiredForCapitalAdequacyMinimumPlusCapitalConservationBuffer	$ 104,870	$ 105,136
CommonEquityTierOneRiskBasedCapitalToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	7.00%	7.00%
Tier 1, Actual Amount	$ 186,605	$ 183,348
Tier 1, Actual Ratio	0.1246	0.1221
Capital Adequacy Minimum with Buffer	$ 127,342	$ 127,665
TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	0.0850	0.0850
Banking Regulation, Total Capital, Actual	$ 229,759	$ 226,179
Capital to Risk Weighted Assets	0.1534	0.1506
CapitalRequiredForCapitalAdequacyMinimumCapitalRequiredPlusCapitalConservationBuffer	$ 157,305	$ 157,703
CapitalRequiredForCapitalAdequacyToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	0.1050	0.1050
Tier One Leverage Capital	$ 186,605	$ 183,348
Tier One Leverage Capital Ratio to Average Assets	0.0904	0.0886
TierOneLeverageCapitalRequiredForCapitalAdequacyMinimumCapitalRequiredPlusCapitalConservationBuffer	$ 82,580	$ 82,803
TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	0.0400	0.0400
Isabella Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier One Capital	$ 181,950	$ 172,589
Banking Regulation, Common Equity Tier 1 Risk-Based Capital Ratio, Actual	0.1218	0.1153
CommonEquityTierOneRiskRatedCapitalRequiredForCapitalAdequacyMinimumPlusCapitalConservationBuffer	$ 104,553	$ 104,783
CommonEquityTierOneRiskBasedCapitalToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	7.00%	7.00%
Banking Regulation, Common Equity Tier 1 Risk-Based Capital, Well Capitalized, Minimum	$ 97,085	$ 97,299
Tier 1 Capital to Risk Weighted Asssets	0.0650	0.0650
Tier 1, Actual Amount	$ 181,950	$ 172,589
Tier 1, Actual Ratio	0.1218	0.1153
Capital Adequacy Minimum with Buffer	$ 126,958	$ 127,237
TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	0.0850	0.0850
Tier One Risk Based Capital Required to be Well Capitalized	$ 119,490	$ 119,753
Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio	0.0800	0.0800
Banking Regulation, Total Capital, Actual	$ 195,636	$ 185,997
Capital to Risk Weighted Assets	0.1310	0.1243
CapitalRequiredForCapitalAdequacyMinimumCapitalRequiredPlusCapitalConservationBuffer	$ 156,830	$ 157,175
CapitalRequiredForCapitalAdequacyToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	0.1050	0.1050
Capital Required to be Well Capitalized	$ 149,362	$ 149,691
Capital Required to be Well Capitalized to Risk Weighted Assets	0.1000	0.1000
Tier One Leverage Capital	$ 181,950	$ 172,589
Tier One Leverage Capital Ratio to Average Assets	0.0883	0.0836
TierOneLeverageCapitalRequiredForCapitalAdequacyMinimumCapitalRequiredPlusCapitalConservationBuffer	$ 82,377	$ 82,602
TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	0.0400	0.0400
Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 102,972	$ 103,252
Tier One Leverage Capital Required to be Well Capitalized to Average Assets	0.0500	0.0500

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