<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.21.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>240</ContextCount>
  <ElementCount>565</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>72</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>5</UnitCount>
  <MyReports>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>00090 - Document - Document And Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DocumentDocumentAndEntityInformation</Role>
      <ShortName>Document And Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>00100 - Statement - Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/StatementConsolidatedBalanceSheets</Role>
      <ShortName>Consolidated Balance Sheets</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>00105 - Statement - Consolidated Balance Sheets (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/StatementConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>Consolidated Balance Sheets (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>00200 - Statement - Condensed Consolidated Statements of Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/StatementCondensedConsolidatedStatementsOfIncome</Role>
      <ShortName>Condensed Consolidated Statements of Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>00205 - Statement - Condensed Consolidated Statements of Income (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/StatementCondensedConsolidatedStatementsOfIncomeParenthetical</Role>
      <ShortName>Condensed Consolidated Statements of Income (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>00300 - Statement - Condensed Consolidated Statements of Comprehensive Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/StatementCondensedConsolidatedStatementsOfComprehensiveIncome</Role>
      <ShortName>Condensed Consolidated Statements of Comprehensive Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>00400 - Statement - Consolidated Statements of Stockholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/StatementConsolidatedStatementsOfStockholdersEquity</Role>
      <ShortName>Consolidated Statements of Stockholders' Equity</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>00405 - Statement - Consolidated Statements of Stockholders' Equity (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/StatementConsolidatedStatementsOfStockholdersEquityParenthetical</Role>
      <ShortName>Consolidated Statements of Stockholders' Equity (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>00500 - Statement - Condensed Consolidated Statements of Cash Flows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/StatementCondensedConsolidatedStatementsOfCashFlows</Role>
      <ShortName>Condensed Consolidated Statements of Cash Flows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>10101 - Disclosure - Nature of Operations and Summary of Significant Account Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureNatureOfOperationsAndSummaryOfSignificantAccountPolicies</Role>
      <ShortName>Nature of Operations and Summary of Significant Account Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>10201 - Disclosure - Securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureSecurities</Role>
      <ShortName>Securities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>10301 - Disclosure - Loans and Allowance for Loan Losses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureLoansAndAllowanceForLoanLosses</Role>
      <ShortName>Loans and Allowance for Loan Losses</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>10401 - Disclosure - Premises and Equipment</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosurePremisesAndEquipment</Role>
      <ShortName>Premises and Equipment</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>10501 - Disclosure - Loan Servicing</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureLoanServicing</Role>
      <ShortName>Loan Servicing</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>10601 - Disclosure - Time Deposits</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureTimeDeposits</Role>
      <ShortName>Time Deposits</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>10701 - Disclosure - Federal Home Loan Bank Advances</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureFederalHomeLoanBankAdvances</Role>
      <ShortName>Federal Home Loan Bank Advances</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>10801 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureIncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>10901 - Disclosure - Accumulated Other Comprehensive Loss</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureAccumulatedOtherComprehensiveLoss</Role>
      <ShortName>Accumulated Other Comprehensive Loss</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>11001 - Disclosure - Regulatory Matters</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureRegulatoryMatters</Role>
      <ShortName>Regulatory Matters</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>11101 - Disclosure - Related Party Transactions</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureRelatedPartyTransactions</Role>
      <ShortName>Related Party Transactions</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>11201 - Disclosure - Employee and Director Benefits</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureEmployeeAndDirectorBenefits</Role>
      <ShortName>Employee and Director Benefits</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>11301 - Disclosure - Operating Lease Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureOperatingLeaseIncome</Role>
      <ShortName>Operating Lease Income</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>11401 - Disclosure - Disclosure About Fair Values of Assets and Liabilities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureDisclosureAboutFairValuesOfAssetsAndLiabilities</Role>
      <ShortName>Disclosure About Fair Values of Assets and Liabilities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>11501 - Disclosure - Derivative Financial Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureDerivativeFinancialInstruments</Role>
      <ShortName>Derivative Financial Instruments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>11601 - Disclosure - Commitments and Credit Risk</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureCommitmentsAndCreditRisk</Role>
      <ShortName>Commitments and Credit Risk</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>11701 - Disclosure - Earnings Per Common Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureEarningsPerCommonShare</Role>
      <ShortName>Earnings Per Common Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>11801 - Disclosure - Equity Incentive Plan</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureEquityIncentivePlan</Role>
      <ShortName>Equity Incentive Plan</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>11901 - Disclosure - Multiemployer Defined Benefit Plan</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureMultiemployerDefinedBenefitPlan</Role>
      <ShortName>Multiemployer Defined Benefit Plan</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>12001 - Disclosure - Recent Accounting Pronouncements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureRecentAccountingPronouncements</Role>
      <ShortName>Recent Accounting Pronouncements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>12101 - Disclosure - Condensed Financial Information (Parent Company Only)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureCondensedFinancialInformationParentCompanyOnly</Role>
      <ShortName>Condensed Financial Information (Parent Company Only)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>20102 - Disclosure - Nature of Operations and Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureNatureOfOperationsAndSummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Nature of Operations and Summary of Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.cincinnatibancorp.com/role/DisclosureRecentAccountingPronouncements</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>30103 - Disclosure - Nature of Operations and Summary of Significant Account Policies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureNatureOfOperationsAndSummaryOfSignificantAccountPoliciesTables</Role>
      <ShortName>Nature of Operations and Summary of Significant Account Policies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cincinnatibancorp.com/role/DisclosureNatureOfOperationsAndSummaryOfSignificantAccountPolicies</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>30203 - Disclosure - Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureSecuritiesTables</Role>
      <ShortName>Securities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cincinnatibancorp.com/role/DisclosureSecurities</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>30303 - Disclosure - Loans and Allowance for Loan Losses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureLoansAndAllowanceForLoanLossesTables</Role>
      <ShortName>Loans and Allowance for Loan Losses (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cincinnatibancorp.com/role/DisclosureLoansAndAllowanceForLoanLosses</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>30403 - Disclosure - Premises and Equipment (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosurePremisesAndEquipmentTables</Role>
      <ShortName>Premises and Equipment (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cincinnatibancorp.com/role/DisclosurePremisesAndEquipment</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>30503 - Disclosure - Loan Servicing (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureLoanServicingTables</Role>
      <ShortName>Loan Servicing (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cincinnatibancorp.com/role/DisclosureLoanServicing</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>30603 - Disclosure - Time Deposits (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureTimeDepositsTables</Role>
      <ShortName>Time Deposits (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cincinnatibancorp.com/role/DisclosureTimeDeposits</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>30703 - Disclosure - Federal Home Loan Bank Advances (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureFederalHomeLoanBankAdvancesTables</Role>
      <ShortName>Federal Home Loan Bank Advances (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cincinnatibancorp.com/role/DisclosureFederalHomeLoanBankAdvances</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>30803 - Disclosure - Income Taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureIncomeTaxesTables</Role>
      <ShortName>Income Taxes (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cincinnatibancorp.com/role/DisclosureIncomeTaxes</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>30903 - Disclosure - Accumulated Other Comprehensive Loss (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureAccumulatedOtherComprehensiveLossTables</Role>
      <ShortName>Accumulated Other Comprehensive Loss (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cincinnatibancorp.com/role/DisclosureAccumulatedOtherComprehensiveLoss</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>31003 - Disclosure - Regulatory Matters (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureRegulatoryMattersTables</Role>
      <ShortName>Regulatory Matters (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cincinnatibancorp.com/role/DisclosureRegulatoryMatters</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>31103 - Disclosure - Related Party Transactions (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureRelatedPartyTransactionsTables</Role>
      <ShortName>Related Party Transactions (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cincinnatibancorp.com/role/DisclosureRelatedPartyTransactions</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>31203 - Disclosure - Employee and Director Benefits (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureEmployeeAndDirectorBenefitsTables</Role>
      <ShortName>Employee and Director Benefits (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cincinnatibancorp.com/role/DisclosureEmployeeAndDirectorBenefits</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>31403 - Disclosure - Disclosure About Fair Values of Assets and Liabilities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureDisclosureAboutFairValuesOfAssetsAndLiabilitiesTables</Role>
      <ShortName>Disclosure About Fair Values of Assets and Liabilities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cincinnatibancorp.com/role/DisclosureDisclosureAboutFairValuesOfAssetsAndLiabilities</ParentRole>
      <Position>44</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>31503 - Disclosure - Derivative Financial Instruments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureDerivativeFinancialInstrumentsTables</Role>
      <ShortName>Derivative Financial Instruments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cincinnatibancorp.com/role/DisclosureDerivativeFinancialInstruments</ParentRole>
      <Position>45</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>31603 - Disclosure - Commitments and Credit Risk (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureCommitmentsAndCreditRiskTables</Role>
      <ShortName>Commitments and Credit Risk (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cincinnatibancorp.com/role/DisclosureCommitmentsAndCreditRisk</ParentRole>
      <Position>46</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>31703 - Disclosure - Earnings Per Common Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureEarningsPerCommonShareTables</Role>
      <ShortName>Earnings Per Common Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cincinnatibancorp.com/role/DisclosureEarningsPerCommonShare</ParentRole>
      <Position>47</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>31803 - Disclosure - Equity Incentive Plan (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureEquityIncentivePlanTables</Role>
      <ShortName>Equity Incentive Plan (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cincinnatibancorp.com/role/DisclosureEquityIncentivePlan</ParentRole>
      <Position>48</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>31903 - Disclosure - Multiemployer Defined Benefit Plan (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureMultiemployerDefinedBenefitPlanTables</Role>
      <ShortName>Multiemployer Defined Benefit Plan (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cincinnatibancorp.com/role/DisclosureMultiemployerDefinedBenefitPlan</ParentRole>
      <Position>49</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>32103 - Disclosure - Condensed Financial Information (Parent Company Only) (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureCondensedFinancialInformationParentCompanyOnlyTables</Role>
      <ShortName>Condensed Financial Information (Parent Company Only) (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.cincinnatibancorp.com/role/DisclosureCondensedFinancialInformationParentCompanyOnly</ParentRole>
      <Position>50</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>40101 - Disclosure - Nature of Operations and Summary of Significant Accounting Policies (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureNatureOfOperationsAndSummaryOfSignificantAccountingPoliciesDetails</Role>
      <ShortName>Nature of Operations and Summary of Significant Accounting Policies (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cincinnatibancorp.com/role/DisclosureNatureOfOperationsAndSummaryOfSignificantAccountPoliciesTables</ParentRole>
      <Position>51</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>40201 - Disclosure - Securities - Amortized cost and gross unrealized gains and losses (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureSecuritiesAmortizedCostAndGrossUnrealizedGainsAndLossesDetails</Role>
      <ShortName>Securities - Amortized cost and gross unrealized gains and losses (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>40202 - Disclosure - Securities - Continuous unrealized loss position (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureSecuritiesContinuousUnrealizedLossPositionDetails</Role>
      <ShortName>Securities - Continuous unrealized loss position (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>40203 - Disclosure - Securities - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureSecuritiesAdditionalInformationDetails</Role>
      <ShortName>Securities - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>40301 - Disclosure - Loans and Allowance for Loan Losses - Categories (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureLoansAndAllowanceForLoanLossesCategoriesDetails</Role>
      <ShortName>Loans and Allowance for Loan Losses - Categories (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>40302 - Disclosure - Loans and Allowance for Loan Losses - Allowance for loan losses activity and recorded investment in loan by segment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureLoansAndAllowanceForLoanLossesAllowanceForLoanLossesActivityAndRecordedInvestmentInLoanBySegmentDetails</Role>
      <ShortName>Loans and Allowance for Loan Losses - Allowance for loan losses activity and recorded investment in loan by segment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>40303 - Disclosure - Loans and Allowance for Loan Losses - Credit risk profile (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureLoansAndAllowanceForLoanLossesCreditRiskProfileDetails</Role>
      <ShortName>Loans and Allowance for Loan Losses - Credit risk profile (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>40304 - Disclosure - Loans and Allowance for Loan Losses - Aging analysis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureLoansAndAllowanceForLoanLossesAgingAnalysisDetails</Role>
      <ShortName>Loans and Allowance for Loan Losses - Aging analysis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>40305 - Disclosure - Loans and Allowance for Loan Losses - Impaired loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureLoansAndAllowanceForLoanLossesImpairedLoansDetails</Role>
      <ShortName>Loans and Allowance for Loan Losses - Impaired loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>40306 - Disclosure - Loans and Allowance for Loan Losses - Nonaccrual loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureLoansAndAllowanceForLoanLossesNonaccrualLoansDetails</Role>
      <ShortName>Loans and Allowance for Loan Losses - Nonaccrual loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>40307 - Disclosure - Loans and Allowance for Loan Losses - Newly classified TDRs (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureLoansAndAllowanceForLoanLossesNewlyClassifiedTdrsDetails</Role>
      <ShortName>Loans and Allowance for Loan Losses - Newly classified TDRs (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>40308 - Disclosure - Loans and Allowance for Loan Losses - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureLoansAndAllowanceForLoanLossesAdditionalInformationDetails</Role>
      <ShortName>Loans and Allowance for Loan Losses - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>40401 - Disclosure - Premises and Equipment - Classification of premises and equipment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosurePremisesAndEquipmentClassificationOfPremisesAndEquipmentDetails</Role>
      <ShortName>Premises and Equipment - Classification of premises and equipment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>40402 - Disclosure - Premises and Equipment - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosurePremisesAndEquipmentAdditionalInformationDetails</Role>
      <ShortName>Premises and Equipment - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>40501 - Disclosure - Loan Servicing - Fair vale method (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureLoanServicingFairValeMethodDetails</Role>
      <ShortName>Loan Servicing - Fair vale method (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>40502 - Disclosure - Loan Servicing - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureLoanServicingAdditionalInformationDetails</Role>
      <ShortName>Loan Servicing - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>40601 - Disclosure - Time Deposits - Scheduled maturities of time deposits (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureTimeDepositsScheduledMaturitiesOfTimeDepositsDetails</Role>
      <ShortName>Time Deposits - Scheduled maturities of time deposits (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>40602 - Disclosure - Time Deposits - Category of deposits (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureTimeDepositsCategoryOfDepositsDetails</Role>
      <ShortName>Time Deposits - Category of deposits (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>40603 - Disclosure - Time Deposits - Additional Information (Details</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureTimeDepositsAdditionalInformationDetails</Role>
      <ShortName>Time Deposits - Additional Information (Details</ShortName>
      <MenuCategory>Uncategorized</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>40701 - Disclosure - Federal Home Loan Bank Advances - Aggregate annual maturities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureFederalHomeLoanBankAdvancesAggregateAnnualMaturitiesDetails</Role>
      <ShortName>Federal Home Loan Bank Advances - Aggregate annual maturities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>40702 - Disclosure - Federal Home Loan Bank Advances - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureFederalHomeLoanBankAdvancesAdditionalInformationDetails</Role>
      <ShortName>Federal Home Loan Bank Advances - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>40801 - Disclosure - Income Taxes - Provision for income taxes (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureIncomeTaxesProvisionForIncomeTaxesDetails</Role>
      <ShortName>Income Taxes - Provision for income taxes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>40802 - Disclosure - Income Taxes - Reconciliation of actual income tax expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureIncomeTaxesReconciliationOfActualIncomeTaxExpenseDetails</Role>
      <ShortName>Income Taxes - Reconciliation of actual income tax expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>40803 - Disclosure - Income Taxes - Reconciliation between statutory income tax and company's effective tax rate (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureIncomeTaxesReconciliationBetweenStatutoryIncomeTaxAndCompanySEffectiveTaxRateDetails</Role>
      <ShortName>Income Taxes - Reconciliation between statutory income tax and company's effective tax rate (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>40804 - Disclosure - Income Taxes - Tax effects of temporary differences related to deferred taxes (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureIncomeTaxesTaxEffectsOfTemporaryDifferencesRelatedToDeferredTaxesDetails</Role>
      <ShortName>Income Taxes - Tax effects of temporary differences related to deferred taxes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>40805 - Disclosure - Income Taxes - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureIncomeTaxesAdditionalInformationDetails</Role>
      <ShortName>Income Taxes - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>40901 - Disclosure - Accumulated Other Comprehensive Loss (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureAccumulatedOtherComprehensiveLossDetails</Role>
      <ShortName>Accumulated Other Comprehensive Loss (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cincinnatibancorp.com/role/DisclosureAccumulatedOtherComprehensiveLossTables</ParentRole>
      <Position>77</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>41001 - Disclosure - Regulatory Matters - Capital amounts and ratios (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureRegulatoryMattersCapitalAmountsAndRatiosDetails</Role>
      <ShortName>Regulatory Matters - Capital amounts and ratios (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>41002 - Disclosure - Regulatory Matters - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureRegulatoryMattersAdditionalInformationDetails</Role>
      <ShortName>Regulatory Matters - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>41101 - Disclosure - Related Party Transactions - Annual activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureRelatedPartyTransactionsAnnualActivityDetails</Role>
      <ShortName>Related Party Transactions - Annual activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>41102 - Disclosure - Related Party Transactions - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureRelatedPartyTransactionsAdditionalInformationDetails</Role>
      <ShortName>Related Party Transactions - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>41201 - Disclosure - Employee and Director Benefits - Summary of ESOP (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureEmployeeAndDirectorBenefitsSummaryOfEsopDetails</Role>
      <ShortName>Employee and Director Benefits - Summary of ESOP (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>41202 - Disclosure - Employee and Director Benefits - Plan's funded status and pension cost (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureEmployeeAndDirectorBenefitsPlanSFundedStatusAndPensionCostDetails</Role>
      <ShortName>Employee and Director Benefits - Plan's funded status and pension cost (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>41203 - Disclosure - Employee and Director Benefits - Amounts recognized in accumulated other comprehensive income not yet recognized (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureEmployeeAndDirectorBenefitsAmountsRecognizedInAccumulatedOtherComprehensiveIncomeNotYetRecognizedDetails</Role>
      <ShortName>Employee and Director Benefits - Amounts recognized in accumulated other comprehensive income not yet recognized (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>84</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>41204 - Disclosure - Employee and Director Benefits - Components of net periodic benefit cost (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureEmployeeAndDirectorBenefitsComponentsOfNetPeriodicBenefitCostDetails</Role>
      <ShortName>Employee and Director Benefits - Components of net periodic benefit cost (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>85</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>41205 - Disclosure - Employee and Director Benefits - Retiree accrued liability expected to be reversed (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureEmployeeAndDirectorBenefitsRetireeAccruedLiabilityExpectedToBeReversedDetails</Role>
      <ShortName>Employee and Director Benefits - Retiree accrued liability expected to be reversed (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>86</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>41206 - Disclosure - Employee and Director Benefits - Significant assumptions for the benefit plan liability (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureEmployeeAndDirectorBenefitsSignificantAssumptionsForBenefitPlanLiabilityDetails</Role>
      <ShortName>Employee and Director Benefits - Significant assumptions for the benefit plan liability (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>87</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>41207 - Disclosure - Employee and Director Benefits - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureEmployeeAndDirectorBenefitsAdditionalInformationDetails</Role>
      <ShortName>Employee and Director Benefits - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>88</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>41301 - Disclosure - Operating Lease Income - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureOperatingLeaseIncomeAdditionalInformationDetails</Role>
      <ShortName>Operating Lease Income - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>89</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>41401 - Disclosure - Disclosure About Fair Values of Assets and Liabilities - Measure on recurring basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureDisclosureAboutFairValuesOfAssetsAndLiabilitiesMeasureOnRecurringBasisDetails</Role>
      <ShortName>Disclosure About Fair Values of Assets and Liabilities - Measure on recurring basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>90</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R91.htm</HtmlFileName>
      <LongName>41402 - Disclosure - Disclosure About Fair Values of Assets and Liabilities - Reconciliation of beginning and ending balances (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureDisclosureAboutFairValuesOfAssetsAndLiabilitiesReconciliationOfBeginningAndEndingBalancesDetails</Role>
      <ShortName>Disclosure About Fair Values of Assets and Liabilities - Reconciliation of beginning and ending balances (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>91</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R92.htm</HtmlFileName>
      <LongName>41404 - Disclosure - Disclosure About Fair Values of Assets and Liabilities - Level 3 (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureDisclosureAboutFairValuesOfAssetsAndLiabilitiesLevel3Details</Role>
      <ShortName>Disclosure About Fair Values of Assets and Liabilities - Level 3 (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cincinnatibancorp.com/role/DisclosureDisclosureAboutFairValuesOfAssetsAndLiabilitiesTables</ParentRole>
      <Position>92</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R93.htm</HtmlFileName>
      <LongName>41405 - Disclosure - Disclosure About Fair Value of Assets and Liabilities - Fair values of financial instruments not previously presented (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureDisclosureAboutFairValueOfAssetsAndLiabilitiesFairValuesOfFinancialInstrumentsNotPreviouslyPresentedDetails</Role>
      <ShortName>Disclosure About Fair Value of Assets and Liabilities - Fair values of financial instruments not previously presented (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>93</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R94.htm</HtmlFileName>
      <LongName>41501 - Disclosure - Derivative Financial Instruments - Information on the company's derivative financial instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureDerivativeFinancialInstrumentsInformationOnCompanySDerivativeFinancialInstrumentsDetails</Role>
      <ShortName>Derivative Financial Instruments - Information on the company's derivative financial instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>94</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R95.htm</HtmlFileName>
      <LongName>41502 - Disclosure - Derivative Financial Instruments - Income related to derivative financial instruments included in nominated income (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureDerivativeFinancialInstrumentsIncomeRelatedToDerivativeFinancialInstrumentsIncludedInNominatedIncomeDetails</Role>
      <ShortName>Derivative Financial Instruments - Income related to derivative financial instruments included in nominated income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>95</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R96.htm</HtmlFileName>
      <LongName>41601 - Disclosure - Commitments and Credit Risk - Commitments to fund fixed rate loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureCommitmentsAndCreditRiskCommitmentsToFundFixedRateLoansDetails</Role>
      <ShortName>Commitments and Credit Risk - Commitments to fund fixed rate loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>96</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R97.htm</HtmlFileName>
      <LongName>41602 - Disclosure - Commitments and Credit Risk- Lines of credit (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureCommitmentsAndCreditRiskLinesOfCreditDetails</Role>
      <ShortName>Commitments and Credit Risk- Lines of credit (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>97</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R98.htm</HtmlFileName>
      <LongName>41701 - Disclosure - Earnings Per Common Share - Stockholders' equity and are excluded from weighted-average (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureEarningsPerCommonShareStockholdersEquityAndAreExcludedFromWeightedAverageDetails</Role>
      <ShortName>Earnings Per Common Share - Stockholders' equity and are excluded from weighted-average (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>98</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R99.htm</HtmlFileName>
      <LongName>41801 - Disclosure - Equity Incentive plan - Stock Option Plan (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureEquityIncentivePlanStockOptionPlanDetails</Role>
      <ShortName>Equity Incentive plan - Stock Option Plan (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>99</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R100.htm</HtmlFileName>
      <LongName>41802 - Disclosure - Equity Incentive Plan - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureEquityIncentivePlanAdditionalInformationDetails</Role>
      <ShortName>Equity Incentive Plan - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>100</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R101.htm</HtmlFileName>
      <LongName>41901 - Disclosure - Multiemployer Defined Benefit Plan - Federal's contributions to the plan (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureMultiemployerDefinedBenefitPlanFederalSContributionsToPlanDetails</Role>
      <ShortName>Multiemployer Defined Benefit Plan - Federal's contributions to the plan (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>101</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R102.htm</HtmlFileName>
      <LongName>41902 - Disclosure - Multiemployer Defined Benefit Plan - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureMultiemployerDefinedBenefitPlanAdditionalInformationDetails</Role>
      <ShortName>Multiemployer Defined Benefit Plan - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>102</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R103.htm</HtmlFileName>
      <LongName>42001 - Disclosure - Recent Accounting Pronouncements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureRecentAccountingPronouncementsDetails</Role>
      <ShortName>Recent Accounting Pronouncements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>103</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R104.htm</HtmlFileName>
      <LongName>42101 - Disclosure - Condensed Financial Information (Parent Company Only) - Condensed Balance Sheet (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureCondensedFinancialInformationParentCompanyOnlyCondensedBalanceSheetDetails</Role>
      <ShortName>Condensed Financial Information (Parent Company Only) - Condensed Balance Sheet (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cincinnatibancorp.com/role/DisclosureCondensedFinancialInformationParentCompanyOnlyTables</ParentRole>
      <Position>104</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R105.htm</HtmlFileName>
      <LongName>42102 - Disclosure - Condensed Financial Information (Parent Company Only) - Condensed Statement of Income and Comprehensive Income (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureCondensedFinancialInformationParentCompanyOnlyCondensedStatementOfIncomeAndComprehensiveIncomeDetails</Role>
      <ShortName>Condensed Financial Information (Parent Company Only) - Condensed Statement of Income and Comprehensive Income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cincinnatibancorp.com/role/DisclosureCondensedFinancialInformationParentCompanyOnlyTables</ParentRole>
      <Position>105</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R106.htm</HtmlFileName>
      <LongName>42103 - Disclosure - Condensed Financial Information (Parent Company Only) - Condensed Statement of Cash Flows (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureCondensedFinancialInformationParentCompanyOnlyCondensedStatementOfCashFlowsDetails</Role>
      <ShortName>Condensed Financial Information (Parent Company Only) - Condensed Statement of Cash Flows (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cincinnatibancorp.com/role/DisclosureCondensedFinancialInformationParentCompanyOnlyTables</ParentRole>
      <Position>106</Position>
    </Report>
    <Report instance="cnnb-20201231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R107.htm</HtmlFileName>
      <LongName>42104 - Disclosure - Condensed Financial Information (Parent Company Only) - Impact of COVID-19 on Cincinnati Bancorp, Inc. (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.cincinnatibancorp.com/role/DisclosureCondensedFinancialInformationParentCompanyOnlyImpactOfCovid19OnCincinnatiBancorpIncDetails</Role>
      <ShortName>Condensed Financial Information (Parent Company Only) - Impact of COVID-19 on Cincinnati Bancorp, Inc. (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.cincinnatibancorp.com/role/DisclosureCondensedFinancialInformationParentCompanyOnlyTables</ParentRole>
      <Position>107</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File>cnnb-20201231.xml</File>
    <File>cnnb-20201231.xsd</File>
    <File>cnnb-20201231_cal.xml</File>
    <File>cnnb-20201231_def.xml</File>
    <File>cnnb-20201231_lab.xml</File>
    <File>cnnb-20201231_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy>http://fasb.org/us-gaap/2020-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/srt/2020-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2020-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
