<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.24.0.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>801</ContextCount>
  <ElementCount>550</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>139</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>6</UnitCount>
  <MyReports>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0000001 - Document - Cover</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/Cover</Role>
      <ShortName>Cover</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>0000002 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/DocumentandEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>0000003 - Statement - Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/ConsolidatedBalanceSheets</Role>
      <ShortName>Consolidated Balance Sheets</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>0000004 - Statement - Consolidated Balance Sheets (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/ConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>Consolidated Balance Sheets (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>0000005 - Statement - Consolidated Statements of Changes in Shareholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/ConsolidatedStatementsofChangesinShareholdersEquity</Role>
      <ShortName>Consolidated Statements of Changes in Shareholders' Equity</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>0000006 - Statement - Consolidated Statements of Changes in Shareholders' Equity (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/ConsolidatedStatementsofChangesinShareholdersEquityParenthetical</Role>
      <ShortName>Consolidated Statements of Changes in Shareholders' Equity (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>0000007 - Statement - Consolidated Statements of Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/ConsolidatedStatementsofIncome</Role>
      <ShortName>Consolidated Statements of Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>0000008 - Statement - Consolidated Statements of Comprehensive Income (Loss)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/ConsolidatedStatementsofComprehensiveIncomeLoss</Role>
      <ShortName>Consolidated Statements of Comprehensive Income (Loss)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>0000009 - Statement - Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/ConsolidatedStatementsofCashFlows</Role>
      <ShortName>Consolidated Statements of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>0000010 - Statement - Pending Accounting Standards Updates</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/PendingAccountingStandardsUpdates</Role>
      <ShortName>Pending Accounting Standards Updates</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>0000011 - Disclosure - Basis of Presentation (Note)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/BasisofPresentationNote</Role>
      <ShortName>Basis of Presentation (Note)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>0000012 - Disclosure - AFS Securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/AFSSecurities</Role>
      <ShortName>AFS Securities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>0000013 - Disclosure - Loans and ACL</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/LoansandACL</Role>
      <ShortName>Loans and ACL</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>0000014 - Disclosure - Premises and Equipment</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/PremisesandEquipment</Role>
      <ShortName>Premises and Equipment</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>0000015 - Disclosure - Goodwill and Other Intangible Assets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/GoodwillandOtherIntangibleAssets</Role>
      <ShortName>Goodwill and Other Intangible Assets</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>0000016 - Disclosure - Deposits</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/Deposits</Role>
      <ShortName>Deposits</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>0000017 - Disclosure - Borrowed Funds</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/BorrowedFunds</Role>
      <ShortName>Borrowed Funds</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>0000018 - Disclosure - Off-Balance-Sheet Activities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/OffBalanceSheetActivities</Role>
      <ShortName>Off-Balance-Sheet Activities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>0000020 - Disclosure - Commitments and Other Matters</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/CommitmentsandOtherMatters</Role>
      <ShortName>Commitments and Other Matters</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>0000021 - Disclosure - Minimum Regulatory Capital Requirements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/MinimumRegulatoryCapitalRequirements</Role>
      <ShortName>Minimum Regulatory Capital Requirements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>0000022 - Disclosure - Computation of Earnings Per Common Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/ComputationofEarningsPerCommonShare</Role>
      <ShortName>Computation of Earnings Per Common Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>0000023 - Disclosure - Benefit Plans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/BenefitPlans</Role>
      <ShortName>Benefit Plans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>0000024 - Disclosure - Revenue (Notes)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.isabellabank.com/role/RevenueNotes</Role>
      <ShortName>Revenue (Notes)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>0000025 - Disclosure - Other Noninterest Expenses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/OtherNoninterestExpenses</Role>
      <ShortName>Other Noninterest Expenses</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>0000026 - Disclosure - Federal Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/FederalIncomeTaxes</Role>
      <ShortName>Federal Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>0000027 - Disclosure - Accumulated Other Comprehensive Income (Loss)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/AccumulatedOtherComprehensiveIncomeLoss</Role>
      <ShortName>Accumulated Other Comprehensive Income (Loss)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>0000028 - Disclosure - Fair Value</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/FairValue</Role>
      <ShortName>Fair Value</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>0000029 - Disclosure - Related Party Transactions</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/RelatedPartyTransactions</Role>
      <ShortName>Related Party Transactions</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>0000030 - Disclosure - Operating Segments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/OperatingSegments</Role>
      <ShortName>Operating Segments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>0000031 - Disclosure - Parent Company Only Financial Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/ParentCompanyOnlyFinancialInformation</Role>
      <ShortName>Parent Company Only Financial Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>0000032 - Disclosure - Subsequent Events</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/SubsequentEvents</Role>
      <ShortName>Subsequent Events</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>9954471 - Disclosure - Basis of Presentation (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/BasisofPresentationPolicies</Role>
      <ShortName>Basis of Presentation (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>9954472 - Disclosure - Receivables, Loans, Notes Receivable, and Others (Policies)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.isabellabank.com/role/ReceivablesLoansNotesReceivableandOthersPolicies</Role>
      <ShortName>Receivables, Loans, Notes Receivable, and Others (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>9954473 - Disclosure - Fair Value Fair Value Measurement, Policy (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/FairValueFairValueMeasurementPolicyPolicies</Role>
      <ShortName>Fair Value Fair Value Measurement, Policy (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>9954474 - Disclosure - Organization, Consolidation and Presentation of Financial Statements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/OrganizationConsolidationandPresentationofFinancialStatementsTables</Role>
      <ShortName>Organization, Consolidation and Presentation of Financial Statements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>9954475 - Disclosure - AFS Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/AFSSecuritiesTables</Role>
      <ShortName>AFS Securities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/AFSSecurities</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>9954476 - Disclosure - Loans and ACL (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/LoansandACLTables</Role>
      <ShortName>Loans and ACL (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/LoansandACL</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>9954477 - Disclosure - Premises and Equipment (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/PremisesandEquipmentTables</Role>
      <ShortName>Premises and Equipment (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/PremisesandEquipment</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>9954478 - Disclosure - Goodwill and Other Intangible Assets (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/GoodwillandOtherIntangibleAssetsTables</Role>
      <ShortName>Goodwill and Other Intangible Assets (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/GoodwillandOtherIntangibleAssets</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>9954479 - Disclosure - Deposits (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/DepositsTables</Role>
      <ShortName>Deposits (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/Deposits</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>9954480 - Disclosure - Borrowed Funds (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/BorrowedFundsTables</Role>
      <ShortName>Borrowed Funds (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/BorrowedFunds</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>9954481 - Disclosure - Off-Balance-Sheet Activities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/OffBalanceSheetActivitiesTables</Role>
      <ShortName>Off-Balance-Sheet Activities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/OffBalanceSheetActivities</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>9954482 - Disclosure - Minimum Regulatory Capital Requirements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/MinimumRegulatoryCapitalRequirementsTables</Role>
      <ShortName>Minimum Regulatory Capital Requirements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/MinimumRegulatoryCapitalRequirements</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>9954483 - Disclosure - Computation of Earnings Per Common Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/ComputationofEarningsPerCommonShareTables</Role>
      <ShortName>Computation of Earnings Per Common Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/ComputationofEarningsPerCommonShare</ParentRole>
      <Position>44</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>9954484 - Disclosure - Benefit Plans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/BenefitPlansTables</Role>
      <ShortName>Benefit Plans (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/BenefitPlans</ParentRole>
      <Position>45</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>9954485 - Disclosure - Revenue (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/RevenueTables</Role>
      <ShortName>Revenue (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/RevenueNotes</ParentRole>
      <Position>46</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>9954486 - Disclosure - Other Noninterest Expenses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/OtherNoninterestExpensesTables</Role>
      <ShortName>Other Noninterest Expenses (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/OtherNoninterestExpenses</ParentRole>
      <Position>47</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>9954487 - Disclosure - Federal Income Taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/FederalIncomeTaxesTables</Role>
      <ShortName>Federal Income Taxes (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/FederalIncomeTaxes</ParentRole>
      <Position>48</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>9954488 - Disclosure - Accumulated Other Comprehensive Income (Loss) (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/AccumulatedOtherComprehensiveIncomeLossTables</Role>
      <ShortName>Accumulated Other Comprehensive Income (Loss) (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/AccumulatedOtherComprehensiveIncomeLoss</ParentRole>
      <Position>49</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>9954489 - Disclosure - Fair Value (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/FairValueTables</Role>
      <ShortName>Fair Value (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/FairValue</ParentRole>
      <Position>50</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>9954490 - Disclosure - Related Party Transactions (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/RelatedPartyTransactionsTables</Role>
      <ShortName>Related Party Transactions (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/RelatedPartyTransactions</ParentRole>
      <Position>51</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>9954491 - Disclosure - Parent Company Only Financial Information (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/ParentCompanyOnlyFinancialInformationTables</Role>
      <ShortName>Parent Company Only Financial Information (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/ParentCompanyOnlyFinancialInformation</ParentRole>
      <Position>52</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>9954493 - Disclosure - Basis of Presentation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/BasisofPresentationDetails</Role>
      <ShortName>Basis of Presentation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/BasisofPresentationPolicies</ParentRole>
      <Position>53</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>9954494 - Disclosure - Nature of Operations and Summary of Significant Accounting Policies (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/NatureofOperationsandSummaryofSignificantAccountingPoliciesNarrativeDetails</Role>
      <ShortName>Nature of Operations and Summary of Significant Accounting Policies (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>9954495 - Disclosure - AFS Securities (Amortized cost and fair value of AFS securities, with gross unrealized gains and losses) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/AFSSecuritiesAmortizedcostandfairvalueofAFSsecuritieswithgrossunrealizedgainsandlossesDetails</Role>
      <ShortName>AFS Securities (Amortized cost and fair value of AFS securities, with gross unrealized gains and losses) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/AFSSecuritiesTables</ParentRole>
      <Position>55</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>9954496 - Disclosure - AFS Securities (Amortized cost and fair value of AFS securities by contractual maturity) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/AFSSecuritiesAmortizedcostandfairvalueofAFSsecuritiesbycontractualmaturityDetails</Role>
      <ShortName>AFS Securities (Amortized cost and fair value of AFS securities by contractual maturity) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/AFSSecuritiesTables</ParentRole>
      <Position>56</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>9954497 - Disclosure - AFS Securities (Activity related to sales of AFS securities) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/AFSSecuritiesActivityrelatedtosalesofAFSsecuritiesDetails</Role>
      <ShortName>AFS Securities (Activity related to sales of AFS securities) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/AFSSecuritiesTables</ParentRole>
      <Position>57</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>9954498 - Disclosure - AFS Securities (AFS securities with gross unrealized losses) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/AFSSecuritiesAFSsecuritieswithgrossunrealizedlossesDetails</Role>
      <ShortName>AFS Securities (AFS securities with gross unrealized losses) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/AFSSecuritiesTables</ParentRole>
      <Position>58</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>9954499 - Disclosure - Loans and ACL (Summary of Loan Composition) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/LoansandACLSummaryofLoanCompositionDetails</Role>
      <ShortName>Loans and ACL (Summary of Loan Composition) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/LoansandACLTables</ParentRole>
      <Position>59</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>9954500 - Disclosure - Loans and ACL (Summary of Nonaccrual Loans with No ACL) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/LoansandACLSummaryofNonaccrualLoanswithNoACLDetails</Role>
      <ShortName>Loans and ACL (Summary of Nonaccrual Loans with No ACL) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/LoansandACLTables</ParentRole>
      <Position>60</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>9954501 - Disclosure - Loans and ACL (Summary of Past due and current loans) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/LoansandACLSummaryofPastdueandcurrentloansDetails</Role>
      <ShortName>Loans and ACL (Summary of Past due and current loans) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/LoansandACLTables</ParentRole>
      <Position>61</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>9954502 - Disclosure - Loans and ACL (Summary of Credit Quality Indicator by Years of Origination) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/LoansandACLSummaryofCreditQualityIndicatorbyYearsofOriginationDetails</Role>
      <ShortName>Loans and ACL (Summary of Credit Quality Indicator by Years of Origination) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/LoansandACLTables</ParentRole>
      <Position>62</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>9954503 - Disclosure - Loans and ALLL (Credit Quality Indicators for Commercial and Agricultural Exposure) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/LoansandALLLCreditQualityIndicatorsforCommercialandAgriculturalExposureDetails</Role>
      <ShortName>Loans and ALLL (Credit Quality Indicators for Commercial and Agricultural Exposure) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>9954504 - Disclosure - Loans and ACL (Summary of Loan Modifications by Type) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/LoansandACLSummaryofLoanModificationsbyTypeDetails</Role>
      <ShortName>Loans and ACL (Summary of Loan Modifications by Type) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/LoansandACLTables</ParentRole>
      <Position>64</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>9954505 - Disclosure - Loans and ACL (Summary of Loan Modification Effects) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/LoansandACLSummaryofLoanModificationEffectsDetails</Role>
      <ShortName>Loans and ACL (Summary of Loan Modification Effects) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/LoansandACLTables</ParentRole>
      <Position>65</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>9954506 - Disclosure - Loans and ACL (Summary of Loan Modifications by Past Due Status) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/LoansandACLSummaryofLoanModificationsbyPastDueStatusDetails</Role>
      <ShortName>Loans and ACL (Summary of Loan Modifications by Past Due Status) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/LoansandACLTables</ParentRole>
      <Position>66</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>9954507 - Disclosure - Loans and ACL (Summary of Changes in ACL by segment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/LoansandACLSummaryofChangesinACLbysegmentDetails</Role>
      <ShortName>Loans and ACL (Summary of Changes in ACL by segment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/LoansandACLTables</ParentRole>
      <Position>67</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>9954508 - Disclosure - Loans and ACL (Summary of Specific Reserves by Segments) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/LoansandACLSummaryofSpecificReservesbySegmentsDetails</Role>
      <ShortName>Loans and ACL (Summary of Specific Reserves by Segments) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/LoansandACLTables</ParentRole>
      <Position>68</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>9954509 - Disclosure - Loans and ACL (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/LoansandACLNarrativeDetails</Role>
      <ShortName>Loans and ACL (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/LoansandACLTables</ParentRole>
      <Position>69</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>9954510 - Disclosure - Premises and Equipment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/PremisesandEquipmentDetails</Role>
      <ShortName>Premises and Equipment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/PremisesandEquipmentTables</ParentRole>
      <Position>70</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>9954511 - Disclosure - Premises and Equipment (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/PremisesandEquipmentNarrativeDetails</Role>
      <ShortName>Premises and Equipment (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/PremisesandEquipmentTables</ParentRole>
      <Position>71</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>9954512 - Disclosure - Goodwill and Other Intangible Assets (Identifiable intangible assets) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/GoodwillandOtherIntangibleAssetsIdentifiableintangibleassetsDetails</Role>
      <ShortName>Goodwill and Other Intangible Assets (Identifiable intangible assets) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/GoodwillandOtherIntangibleAssetsTables</ParentRole>
      <Position>72</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>9954513 - Disclosure - Goodwill and Other Intangible Assets (Estimated amortization expense) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/GoodwillandOtherIntangibleAssetsEstimatedamortizationexpenseDetails</Role>
      <ShortName>Goodwill and Other Intangible Assets (Estimated amortization expense) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/GoodwillandOtherIntangibleAssetsTables</ParentRole>
      <Position>73</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>9954514 - Disclosure - Goodwill and Other Intangible Assets (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/GoodwillandOtherIntangibleAssetsNarrativeDetails</Role>
      <ShortName>Goodwill and Other Intangible Assets (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/GoodwillandOtherIntangibleAssetsTables</ParentRole>
      <Position>74</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>9954515 - Disclosure - Deposits (Scheduled maturities of time deposits) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/DepositsScheduledmaturitiesoftimedepositsDetails</Role>
      <ShortName>Deposits (Scheduled maturities of time deposits) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/DepositsTables</ParentRole>
      <Position>75</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>9954516 - Disclosure - Deposits (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/DepositsNarrativeDetails</Role>
      <ShortName>Deposits (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/DepositsTables</ParentRole>
      <Position>76</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>9954517 - Disclosure - Borrowed Funds (Short-term borrowings) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/BorrowedFundsShorttermborrowingsDetails</Role>
      <ShortName>Borrowed Funds (Short-term borrowings) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/BorrowedFundsTables</ParentRole>
      <Position>77</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>9954518 - Disclosure - Borrowed Funds (Pledged financial instruments) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/BorrowedFundsPledgedfinancialinstrumentsDetails</Role>
      <ShortName>Borrowed Funds (Pledged financial instruments) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/BorrowedFundsTables</ParentRole>
      <Position>78</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>9954519 - Disclosure - Borrowed Funds (Maturity and weighted average interest rates of FHLB advances) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/BorrowedFundsMaturityandweightedaverageinterestratesofFHLBadvancesDetails</Role>
      <ShortName>Borrowed Funds (Maturity and weighted average interest rates of FHLB advances) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/BorrowedFundsTables</ParentRole>
      <Position>79</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>9954520 - Disclosure - Borrowed Funds (Subordinated Debt) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/BorrowedFundsSubordinatedDebtDetails</Role>
      <ShortName>Borrowed Funds (Subordinated Debt) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/BorrowedFundsTables</ParentRole>
      <Position>80</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>9954521 - Disclosure - Borrowed Funds (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/BorrowedFundsNarrativeDetails</Role>
      <ShortName>Borrowed Funds (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/BorrowedFundsTables</ParentRole>
      <Position>81</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>9954522 - Disclosure - Off-Balance-Sheet Activities (Contractual amount of credit related financial instruments) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/OffBalanceSheetActivitiesContractualamountofcreditrelatedfinancialinstrumentsDetails</Role>
      <ShortName>Off-Balance-Sheet Activities (Contractual amount of credit related financial instruments) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/OffBalanceSheetActivitiesTables</ParentRole>
      <Position>82</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>9954523 - Disclosure - On-Balance Sheet Activities (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/OnBalanceSheetActivitiesNarrativeDetails</Role>
      <ShortName>On-Balance Sheet Activities (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>9954524 - Disclosure - Commitments and Other Matters (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/CommitmentsandOtherMattersNarrativeDetails</Role>
      <ShortName>Commitments and Other Matters (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/CommitmentsandOtherMatters</ParentRole>
      <Position>84</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>9954525 - Disclosure - Minimum Regulatory Capital Requirements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/MinimumRegulatoryCapitalRequirementsDetails</Role>
      <ShortName>Minimum Regulatory Capital Requirements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/MinimumRegulatoryCapitalRequirementsTables</ParentRole>
      <Position>85</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>9954526 - Disclosure - Computation of Earnings Per Common Share (Schedule of Earnings Per Share) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/ComputationofEarningsPerCommonShareScheduleofEarningsPerShareDetails</Role>
      <ShortName>Computation of Earnings Per Common Share (Schedule of Earnings Per Share) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/ComputationofEarningsPerCommonShareTables</ParentRole>
      <Position>86</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>9954527 - Disclosure - Benefit Plans (Changes in the defined benefit pension plan) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/BenefitPlansChangesinthedefinedbenefitpensionplanDetails</Role>
      <ShortName>Benefit Plans (Changes in the defined benefit pension plan) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/BenefitPlansTables</ParentRole>
      <Position>87</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>9954528 - Disclosure - Benefit Plans (Components of net periodic benefit cost) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/BenefitPlansComponentsofnetperiodicbenefitcostDetails</Role>
      <ShortName>Benefit Plans (Components of net periodic benefit cost) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/BenefitPlansTables</ParentRole>
      <Position>88</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>9954529 - Disclosure - Benefit Plans (Actuarial assumptions used) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/BenefitPlansActuarialassumptionsusedDetails</Role>
      <ShortName>Benefit Plans (Actuarial assumptions used) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/BenefitPlansTables</ParentRole>
      <Position>89</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>9954530 - Disclosure - Benefit Plans (Fair values of our pension plan assets by asset category) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/BenefitPlansFairvaluesofourpensionplanassetsbyassetcategoryDetails</Role>
      <ShortName>Benefit Plans (Fair values of our pension plan assets by asset category) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/BenefitPlansTables</ParentRole>
      <Position>90</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R91.htm</HtmlFileName>
      <LongName>9954531 - Disclosure - Benefit Plans (Estimated future benefit payments) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/BenefitPlansEstimatedfuturebenefitpaymentsDetails</Role>
      <ShortName>Benefit Plans (Estimated future benefit payments) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/BenefitPlansTables</ParentRole>
      <Position>91</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R92.htm</HtmlFileName>
      <LongName>9954532 - Disclosure - Benefit Plans (Components of shares eligible to be issued under the Directors Plan) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/BenefitPlansComponentsofshareseligibletobeissuedundertheDirectorsPlanDetails</Role>
      <ShortName>Benefit Plans (Components of shares eligible to be issued under the Directors Plan) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/BenefitPlansTables</ParentRole>
      <Position>92</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R93.htm</HtmlFileName>
      <LongName>9954533 - Disclosure - Benefit Plans (Nonvested Restricted Stock Awards Activity) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/BenefitPlansNonvestedRestrictedStockAwardsActivityDetails</Role>
      <ShortName>Benefit Plans (Nonvested Restricted Stock Awards Activity) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/BenefitPlansTables</ParentRole>
      <Position>93</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R94.htm</HtmlFileName>
      <LongName>9954534 - Disclosure - Benefit Plans (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/BenefitPlansNarrativeDetails</Role>
      <ShortName>Benefit Plans (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/BenefitPlansTables</ParentRole>
      <Position>94</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R95.htm</HtmlFileName>
      <LongName>9954535 - Disclosure - Revenue (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/RevenueDetails</Role>
      <ShortName>Revenue (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/RevenueTables</ParentRole>
      <Position>95</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R96.htm</HtmlFileName>
      <LongName>9954536 - Disclosure - Other Noninterest Expenses (Expenses included in other noninterest expenses) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/OtherNoninterestExpensesExpensesincludedinothernoninterestexpensesDetails</Role>
      <ShortName>Other Noninterest Expenses (Expenses included in other noninterest expenses) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/OtherNoninterestExpensesTables</ParentRole>
      <Position>96</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R97.htm</HtmlFileName>
      <LongName>9954537 - Disclosure - Federal Income Taxes (Components of the consolidated provision for federal income taxes) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/FederalIncomeTaxesComponentsoftheconsolidatedprovisionforfederalincometaxesDetails</Role>
      <ShortName>Federal Income Taxes (Components of the consolidated provision for federal income taxes) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/FederalIncomeTaxesTables</ParentRole>
      <Position>97</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R98.htm</HtmlFileName>
      <LongName>9954538 - Disclosure - Federal Income Taxes (Reconciliation of the provision for federal income taxes) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/FederalIncomeTaxesReconciliationoftheprovisionforfederalincometaxesDetails</Role>
      <ShortName>Federal Income Taxes (Reconciliation of the provision for federal income taxes) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/FederalIncomeTaxesTables</ParentRole>
      <Position>98</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R99.htm</HtmlFileName>
      <LongName>9954539 - Disclosure - Federal Income Taxes (Significant components of our deferred tax assets and liabilities) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/FederalIncomeTaxesSignificantcomponentsofourdeferredtaxassetsandliabilitiesDetails</Role>
      <ShortName>Federal Income Taxes (Significant components of our deferred tax assets and liabilities) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/FederalIncomeTaxesTables</ParentRole>
      <Position>99</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R100.htm</HtmlFileName>
      <LongName>9954540 - Disclosure - Federal Income Taxes (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/FederalIncomeTaxesNarrativeDetails</Role>
      <ShortName>Federal Income Taxes (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/FederalIncomeTaxesTables</ParentRole>
      <Position>100</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R101.htm</HtmlFileName>
      <LongName>9954541 - Disclosure - Accumulated Other Comprehensive Income (Loss) (Changes in AOCI by component) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/AccumulatedOtherComprehensiveIncomeLossChangesinAOCIbycomponentDetails</Role>
      <ShortName>Accumulated Other Comprehensive Income (Loss) (Changes in AOCI by component) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/AccumulatedOtherComprehensiveIncomeLossTables</ParentRole>
      <Position>101</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R102.htm</HtmlFileName>
      <LongName>9954542 - Disclosure - Accumulated Other Comprehensive Income (Loss) (Components of unrealized holding gains on AFS securities) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/AccumulatedOtherComprehensiveIncomeLossComponentsofunrealizedholdinggainsonAFSsecuritiesDetails</Role>
      <ShortName>Accumulated Other Comprehensive Income (Loss) (Components of unrealized holding gains on AFS securities) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/AccumulatedOtherComprehensiveIncomeLossTables</ParentRole>
      <Position>102</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R103.htm</HtmlFileName>
      <LongName>9954543 - Disclosure - Accumulated Other Comprehensive Income (Loss) (Reclassification adjustments) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/AccumulatedOtherComprehensiveIncomeLossReclassificationadjustmentsDetails</Role>
      <ShortName>Accumulated Other Comprehensive Income (Loss) (Reclassification adjustments) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/AccumulatedOtherComprehensiveIncomeLossTables</ParentRole>
      <Position>103</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R104.htm</HtmlFileName>
      <LongName>9954544 - Disclosure - Fair Value (Quantitative information about impaired loans) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/FairValueQuantitativeinformationaboutimpairedloansDetails</Role>
      <ShortName>Fair Value (Quantitative information about impaired loans) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/FairValueTables</ParentRole>
      <Position>104</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R105.htm</HtmlFileName>
      <LongName>9954545 - Disclosure - Fair Value (Carrying amount and estimated fair value of financial instruments) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/FairValueCarryingamountandestimatedfairvalueoffinancialinstrumentsDetails</Role>
      <ShortName>Fair Value (Carrying amount and estimated fair value of financial instruments) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/FairValueTables</ParentRole>
      <Position>105</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R106.htm</HtmlFileName>
      <LongName>9954546 - Disclosure - Fair Value (Recorded amount of assets and liabilities measured at fair value) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/FairValueRecordedamountofassetsandliabilitiesmeasuredatfairvalueDetails</Role>
      <ShortName>Fair Value (Recorded amount of assets and liabilities measured at fair value) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/FairValueTables</ParentRole>
      <Position>106</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R107.htm</HtmlFileName>
      <LongName>9954547 - Disclosure - Fair Value (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/FairValueNarrativeDetails</Role>
      <ShortName>Fair Value (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/FairValueTables</ParentRole>
      <Position>107</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R108.htm</HtmlFileName>
      <LongName>9954548 - Disclosure - Related Party Transactions (Annual activity) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/RelatedPartyTransactionsAnnualactivityDetails</Role>
      <ShortName>Related Party Transactions (Annual activity) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/RelatedPartyTransactionsTables</ParentRole>
      <Position>108</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R109.htm</HtmlFileName>
      <LongName>9954549 - Disclosure - Related Party Transactions (Ending balances of, and contributions to the Isabella Bank Foundation) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/RelatedPartyTransactionsEndingbalancesofandcontributionstotheIsabellaBankFoundationDetails</Role>
      <ShortName>Related Party Transactions (Ending balances of, and contributions to the Isabella Bank Foundation) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/RelatedPartyTransactionsTables</ParentRole>
      <Position>109</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R110.htm</HtmlFileName>
      <LongName>9954550 - Disclosure - Related Party Transactions (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/RelatedPartyTransactionsNarrativeDetails</Role>
      <ShortName>Related Party Transactions (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/RelatedPartyTransactionsTables</ParentRole>
      <Position>110</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R111.htm</HtmlFileName>
      <LongName>9954551 - Disclosure - Operating Segments (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/OperatingSegmentsNarrativeDetails</Role>
      <ShortName>Operating Segments (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/OperatingSegments</ParentRole>
      <Position>111</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R112.htm</HtmlFileName>
      <LongName>9954552 - Disclosure - Parent Company Only Financial Information (Interim Condensed Balance Sheets) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/ParentCompanyOnlyFinancialInformationInterimCondensedBalanceSheetsDetails</Role>
      <ShortName>Parent Company Only Financial Information (Interim Condensed Balance Sheets) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/ParentCompanyOnlyFinancialInformationTables</ParentRole>
      <Position>112</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R113.htm</HtmlFileName>
      <LongName>9954553 - Disclosure - Parent Company Only Financial Information (Interim Condensed Statements of Income) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/ParentCompanyOnlyFinancialInformationInterimCondensedStatementsofIncomeDetails</Role>
      <ShortName>Parent Company Only Financial Information (Interim Condensed Statements of Income) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/ParentCompanyOnlyFinancialInformationTables</ParentRole>
      <Position>113</Position>
    </Report>
    <Report instance="isba-20231231.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R114.htm</HtmlFileName>
      <LongName>9954554 - Disclosure - Parent Company Only Financial Information (Interim Condensed Statements of Cash Flows) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.isabellabank.com/role/ParentCompanyOnlyFinancialInformationInterimCondensedStatementsofCashFlowsDetails</Role>
      <ShortName>Parent Company Only Financial Information (Interim Condensed Statements of Cash Flows) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.isabellabank.com/role/ParentCompanyOnlyFinancialInformationTables</ParentRole>
      <Position>114</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File doctype="10-K" isDefinitelyFs="true" isUsgaap="true" original="isba-20231231.htm">isba-20231231.htm</File>
    <File>isba-20231231.xsd</File>
    <File>isba-20231231_cal.xml</File>
    <File>isba-20231231_def.xml</File>
    <File>isba-20231231_lab.xml</File>
    <File>isba-20231231_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="3">http://fasb.org/srt/2023</BaseTaxonomy>
    <BaseTaxonomy items="2984">http://fasb.org/us-gaap/2023</BaseTaxonomy>
    <BaseTaxonomy items="35">http://xbrl.sec.gov/dei/2023</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
