Capital Ratios (Details) $ in Thousands	Mar. 31, 2025 USD ($)	Dec. 31, 2024 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier One Capital	$ 184,713	$ 183,348
Banking Regulation, Common Equity Tier 1 Risk-Based Capital Ratio, Actual	0.1258	0.1221
CommonEquityTierOneRiskRatedCapitalRequiredForCapitalAdequacyMinimumPlusCapitalConservationBuffer	$ 102,748	$ 105,136
CommonEquityTierOneRiskBasedCapitalToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	7.00%	7.00%
Tier 1, Actual Amount	$ 184,713	$ 183,348
Tier 1, Actual Ratio	0.1258	0.1221
Capital Adequacy Minimum with Buffer	$ 124,765	$ 127,665
TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	0.0850	0.0850
Banking Regulation, Total Capital, Actual	$ 227,512	$ 226,179
Capital to Risk Weighted Assets	0.1550	0.1506
CapitalRequiredForCapitalAdequacyMinimumCapitalRequiredPlusCapitalConservationBuffer	$ 154,121	$ 157,703
CapitalRequiredForCapitalAdequacyToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	0.1050	0.1050
Tier One Leverage Capital	$ 184,713	$ 183,348
Tier One Leverage Capital Ratio to Average Assets	0.0896	0.0886
TierOneLeverageCapitalRequiredForCapitalAdequacyMinimumCapitalRequiredPlusCapitalConservationBuffer	$ 82,465	$ 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	$ 176,720	$ 172,589
Banking Regulation, Common Equity Tier 1 Risk-Based Capital Ratio, Actual	0.1208	0.1153
CommonEquityTierOneRiskRatedCapitalRequiredForCapitalAdequacyMinimumPlusCapitalConservationBuffer	$ 102,391	$ 104,783
CommonEquityTierOneRiskBasedCapitalToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	7.00%	7.00%
Banking Regulation, Common Equity Tier 1 Risk-Based Capital, Well Capitalized, Minimum	$ 95,077	$ 97,299
Tier 1 Capital to Risk Weighted Asssets	0.0650	0.0650
Tier 1, Actual Amount	$ 176,720	$ 172,589
Tier 1, Actual Ratio	0.1208	0.1153
Capital Adequacy Minimum with Buffer	$ 124,331	$ 127,237
TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	0.0850	0.0850
Tier One Risk Based Capital Required to be Well Capitalized	$ 117,018	$ 119,753
Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions Ratio	0.0800	0.0800
Banking Regulation, Total Capital, Actual	$ 190,072	$ 185,997
Capital to Risk Weighted Assets	0.1299	0.1243
CapitalRequiredForCapitalAdequacyMinimumCapitalRequiredPlusCapitalConservationBuffer	$ 153,586	$ 157,175
CapitalRequiredForCapitalAdequacyToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	0.1050	0.1050
Capital Required to be Well Capitalized	$ 146,272	$ 149,691
Capital Required to be Well Capitalized to Risk Weighted Assets	0.1000	0.1000
Tier One Leverage Capital	$ 176,720	$ 172,589
Tier One Leverage Capital Ratio to Average Assets	0.0859	0.0836
TierOneLeverageCapitalRequiredForCapitalAdequacyMinimumCapitalRequiredPlusCapitalConservationBuffer	$ 82,267	$ 82,602
TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer	0.0400	0.0400
Minimum To Be Well Capitalized Under Prompt Corrective Action Provisions Amount	$ 102,834	$ 103,252
Tier One Leverage Capital Required to be Well Capitalized to Average Assets	0.0500	0.0500

CapitalRequiredForCapitalAdequacyMinimumCapitalRequiredPlusCapitalConservationBuffer

No definition available.

Name:	isba_CapitalRequiredForCapitalAdequacyMinimumCapitalRequiredPlusCapitalConservationBuffer
Namespace Prefix:	isba_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

CapitalRequiredForCapitalAdequacyToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer

+ References

No definition available.

+ Details

Name:	isba_CapitalRequiredForCapitalAdequacyToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer
Namespace Prefix:	isba_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

CommonEquityTierOneRiskBasedCapitalToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer

+ References

No definition available.

+ Details

Name:	isba_CommonEquityTierOneRiskBasedCapitalToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer
Namespace Prefix:	isba_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

CommonEquityTierOneRiskRatedCapitalRequiredForCapitalAdequacyMinimumPlusCapitalConservationBuffer

+ References

No definition available.

+ Details

Name:	isba_CommonEquityTierOneRiskRatedCapitalRequiredForCapitalAdequacyMinimumPlusCapitalConservationBuffer
Namespace Prefix:	isba_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

TierOneLeverageCapitalRequiredForCapitalAdequacyMinimumCapitalRequiredPlusCapitalConservationBuffer

+ References

No definition available.

+ Details

Name:	isba_TierOneLeverageCapitalRequiredForCapitalAdequacyMinimumCapitalRequiredPlusCapitalConservationBuffer
Namespace Prefix:	isba_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer

+ References

No definition available.

+ Details

Name:	isba_TierOneLeverageCapitalRequiredForCapitalAdequacyToAverageAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer
Namespace Prefix:	isba_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

TierOneRiskBasedCapitalRequiredForCapitalAdequacyMinimumCapitalRequiredPlusCapitalConservationBuffer

+ References

No definition available.

+ Details

Name:	isba_TierOneRiskBasedCapitalRequiredForCapitalAdequacyMinimumCapitalRequiredPlusCapitalConservationBuffer
Namespace Prefix:	isba_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer

+ References

No definition available.

+ Details

Name:	isba_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssetsMinimumCapitalRequiredPlusCapitalConservationBuffer
Namespace Prefix:	isba_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio of minimum Common Equity Tier 1 risk-based capital to risk-weighted assets categorized as well capitalized as defined by regulatory framework for prompt corrective action.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 505
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477270/942-505-50-1

+ Details

Name:	us-gaap_BankingRegulationCommonEquityTierOneRiskBasedCapitalRatioWellCapitalizedMinimum
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of total capital as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_Capital
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of minimum total risk-based capital categorized as well capitalized as defined by regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_CapitalRequiredToBeWellCapitalized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Ratio of minimum total risk-based capital to risk-weighted assets categorized as well capitalized as defined by regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_CapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio of total risk-based capital to risk-weighted assets as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_CapitalToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of Common Equity Tier 1 risk-based capital as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_CommonEquityTierOneCapital
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Ratio of Common Equity Tier 1 risk-based capital to risk-weighted assets as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_CommonEquityTierOneCapitalRatio
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of minimum Common Equity Tier 1 risk-based capital categorized as well capitalized as defined by regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_CommonEquityTierOneCapitalRequiredToBeWellCapitalized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 505
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477270/942-505-50-1B

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 942
-SubTopic 505
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477270/942-505-50-1

+ Details

Name:	us-gaap_ComplianceWithRegulatoryCapitalRequirementsUnderBankingRegulationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of Tier 1 leverage capital as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapital
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of minimum Tier 1 leverage capital categorized as well capitalized as defined by regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Ratio of minimum Tier 1 leverage capital to average assets categorized as well capitalized as defined by regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapitalRequiredToBeWellCapitalizedToAverageAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio of Tier 1 capital to average assets as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_TierOneLeverageCapitalToAverageAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of Tier 1 risk-based capital as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapital
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of minimum Tier 1 risk-based capital categorized as well capitalized as defined by regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Ratio of minimum Tier 1 risk-based capital to risk-weighted assets categorized as well capitalized as defined by regulatory framework for prompt corrective action.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Ratio of Tier 1 risk-based capital to risk-weighted assets as defined by regulatory framework.

+ References

+ Details

Name:	us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	srt_ConsolidatedEntitiesAxis=srt_SubsidiariesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: