<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.26.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>533</ContextCount>
  <ElementCount>467</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>90</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>11</UnitCount>
  <MyReports>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>995200090 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DocumentDocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>995200100 - Statement - CONDENSED CONSOLIDATED BALANCE SHEETS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/StatementCondensedConsolidatedBalanceSheets</Role>
      <ShortName>CONDENSED CONSOLIDATED BALANCE SHEETS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>995200105 - Statement - CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/StatementCondensedConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>995200200 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF INCOME</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/StatementCondensedConsolidatedStatementsOfIncome</Role>
      <ShortName>CONDENSED CONSOLIDATED STATEMENTS OF INCOME</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>995200300 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/StatementCondensedConsolidatedStatementsOfComprehensiveIncome</Role>
      <ShortName>CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>995200400 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/StatementCondensedConsolidatedStatementsOfStockholdersEquity</Role>
      <ShortName>CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>995200405 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/StatementCondensedConsolidatedStatementsOfStockholdersEquityParenthetical</Role>
      <ShortName>CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>995200500 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/StatementCondensedConsolidatedStatementsOfCashFlows</Role>
      <ShortName>CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>995210101 - Disclosure - Basis of Presentation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureBasisOfPresentation</Role>
      <ShortName>Basis of Presentation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>995210201 - Disclosure - Organization and Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureOrganizationAndSummaryOfSignificantAccountingPolicies</Role>
      <ShortName>Organization and Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>995210301 - Disclosure - Available for Sale Securities</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureAvailableForSaleSecurities</Role>
      <ShortName>Available for Sale Securities</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>995210401 - Disclosure - Loans and Allowance for Credit Losses</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureLoansAndAllowanceForCreditLosses</Role>
      <ShortName>Loans and Allowance for Credit Losses</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>995210501 - Disclosure - Premises and Equipment</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosurePremisesAndEquipment</Role>
      <ShortName>Premises and Equipment</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>995210601 - Disclosure - Deposits</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureDeposits</Role>
      <ShortName>Deposits</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>995210701 - Disclosure - Repurchase Agreements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureRepurchaseAgreements</Role>
      <ShortName>Repurchase Agreements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>995210801 - Disclosure - Earnings Per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureEarningsPerShare</Role>
      <ShortName>Earnings Per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>995210901 - Disclosure - Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureIncomeTaxes</Role>
      <ShortName>Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>995211001 - Disclosure - 401(k) Retirement Plan</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/Disclosure401KRetirementPlan</Role>
      <ShortName>401(k) Retirement Plan</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>995211101 - Disclosure - Subordinated Debt</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureSubordinatedDebt</Role>
      <ShortName>Subordinated Debt</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>995211201 - Disclosure - Fair Value Measurements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureFairValueMeasurements</Role>
      <ShortName>Fair Value Measurements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>995211301 - Disclosure - Derivative Financial Instruments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureDerivativeFinancialInstruments</Role>
      <ShortName>Derivative Financial Instruments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>995214101 - Disclosure - Segment Reporting</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureSegmentReporting</Role>
      <ShortName>Segment Reporting</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>995410 - Disclosure - Pay vs Performance Disclosure</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/PvpDisclosure</Role>
      <ShortName>Pay vs Performance Disclosure</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>995445 - Disclosure - Insider Trading Arrangements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://xbrl.sec.gov/ecd/role/InsiderTradingArrangements</Role>
      <ShortName>Insider Trading Arrangements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>99920202 - Disclosure - Organization and Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureOrganizationAndSummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Organization and Summary of Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.bankwithsouthern.com/role/DisclosureOrganizationAndSummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>99930303 - Disclosure - Available for Sale Securities (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureAvailableForSaleSecuritiesTables</Role>
      <ShortName>Available for Sale Securities (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankwithsouthern.com/role/DisclosureAvailableForSaleSecurities</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>99930403 - Disclosure - Loans and Allowance for Credit Losses (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureLoansAndAllowanceForCreditLossesTables</Role>
      <ShortName>Loans and Allowance for Credit Losses (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankwithsouthern.com/role/DisclosureLoansAndAllowanceForCreditLosses</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>99930503 - Disclosure - Premises and Equipment (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosurePremisesAndEquipmentTables</Role>
      <ShortName>Premises and Equipment (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankwithsouthern.com/role/DisclosurePremisesAndEquipment</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>99930603 - Disclosure - Deposits (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureDepositsTables</Role>
      <ShortName>Deposits (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankwithsouthern.com/role/DisclosureDeposits</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>99930703 - Disclosure - Repurchase Agreements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureRepurchaseAgreementsTables</Role>
      <ShortName>Repurchase Agreements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankwithsouthern.com/role/DisclosureRepurchaseAgreements</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>99930803 - Disclosure - Earnings Per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureEarningsPerShareTables</Role>
      <ShortName>Earnings Per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankwithsouthern.com/role/DisclosureEarningsPerShare</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>99930903 - Disclosure - Income Taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureIncomeTaxesTables</Role>
      <ShortName>Income Taxes (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankwithsouthern.com/role/DisclosureIncomeTaxes</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>99931203 - Disclosure - Fair Value Measurements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureFairValueMeasurementsTables</Role>
      <ShortName>Fair Value Measurements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankwithsouthern.com/role/DisclosureFairValueMeasurements</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>99931303 - Disclosure - Derivative Financial Instruments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureDerivativeFinancialInstrumentsTables</Role>
      <ShortName>Derivative Financial Instruments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.bankwithsouthern.com/role/DisclosureDerivativeFinancialInstruments</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>99940201 - Disclosure - Organization and Summary of Significant Accounting Policies - Organization (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureOrganizationAndSummaryOfSignificantAccountingPoliciesOrganizationDetails</Role>
      <ShortName>Organization and Summary of Significant Accounting Policies - Organization (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>99940202 - Disclosure - Organization and Summary of Significant Accounting Policies - Cash and Cash Equivalents (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureOrganizationAndSummaryOfSignificantAccountingPoliciesCashAndCashEquivalentsDetails</Role>
      <ShortName>Organization and Summary of Significant Accounting Policies - Cash and Cash Equivalents (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>99940203 - Disclosure - Organization and Summary of Significant Accounting Policies - Interest-Bearing Time Deposits (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureOrganizationAndSummaryOfSignificantAccountingPoliciesInterestBearingTimeDepositsDetails</Role>
      <ShortName>Organization and Summary of Significant Accounting Policies - Interest-Bearing Time Deposits (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>99940204 - Disclosure - Organization and Summary of Significant Accounting Policies - Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureOrganizationAndSummaryOfSignificantAccountingPoliciesLoansDetails</Role>
      <ShortName>Organization and Summary of Significant Accounting Policies - Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bankwithsouthern.com/role/DisclosureOrganizationAndSummaryOfSignificantAccountingPoliciesPolicies</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>99940205 - Disclosure - Organization and Summary of Significant Accounting Policies - Premises and Equipment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureOrganizationAndSummaryOfSignificantAccountingPoliciesPremisesAndEquipmentDetails</Role>
      <ShortName>Organization and Summary of Significant Accounting Policies - Premises and Equipment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>99940206 - Disclosure - Organization and Summary of Significant Accounting Policies - Goodwill and Intangible Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureOrganizationAndSummaryOfSignificantAccountingPoliciesGoodwillAndIntangibleAssetsDetails</Role>
      <ShortName>Organization and Summary of Significant Accounting Policies - Goodwill and Intangible Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>99940207 - Disclosure - Organization and Summary of Significant Accounting Policies - Non-Employee Directors' Retirement (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureOrganizationAndSummaryOfSignificantAccountingPoliciesNonEmployeeDirectorsRetirementDetails</Role>
      <ShortName>Organization and Summary of Significant Accounting Policies - Non-Employee Directors' Retirement (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>99940208 - Disclosure - Organization and Summary of Significant Accounting Policies - Wealth Management Assets and Fees (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureOrganizationAndSummaryOfSignificantAccountingPoliciesWealthManagementAssetsAndFeesDetails</Role>
      <ShortName>Organization and Summary of Significant Accounting Policies - Wealth Management Assets and Fees (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>99940301 - Disclosure - Available for Sale Securities - Amortized cost, gross unrealized gains, gross unrealized losses, ACL, and approximate fair value of securities available for sale (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureAvailableForSaleSecuritiesAmortizedCostGrossUnrealizedGainsGrossUnrealizedLossesAclAndApproximateFairValueOfSecuritiesAvailableForSaleDetails</Role>
      <ShortName>Available for Sale Securities - Amortized cost, gross unrealized gains, gross unrealized losses, ACL, and approximate fair value of securities available for sale (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>99940302 - Disclosure - Available for Sale Securities - Amortized Cost and Fair Value of Available-for-sale Securities, by Contractual Maturity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureAvailableForSaleSecuritiesAmortizedCostAndFairValueOfAvailableForSaleSecuritiesByContractualMaturityDetails</Role>
      <ShortName>Available for Sale Securities - Amortized Cost and Fair Value of Available-for-sale Securities, by Contractual Maturity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>99940303 - Disclosure - Available for Sale Securities - Investments Pledged as Collateral to Secure Public Deposits and Securities Sold Under Agreements to Repurchase (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureAvailableForSaleSecuritiesInvestmentsPledgedAsCollateralToSecurePublicDepositsAndSecuritiesSoldUnderAgreementsToRepurchaseDetails</Role>
      <ShortName>Available for Sale Securities - Investments Pledged as Collateral to Secure Public Deposits and Securities Sold Under Agreements to Repurchase (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>99940305 - Disclosure - Available for Sale Securities - Gains and Losses from Sales of Available-for-sale Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureAvailableForSaleSecuritiesGainsAndLossesFromSalesOfAvailableForSaleSecuritiesDetails</Role>
      <ShortName>Available for Sale Securities - Gains and Losses from Sales of Available-for-sale Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>99940306 - Disclosure - Available for Sale Securities - Gross Unrealized Losses and Fair Value, Continuous Unrealized Loss Position (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureAvailableForSaleSecuritiesGrossUnrealizedLossesAndFairValueContinuousUnrealizedLossPositionDetails</Role>
      <ShortName>Available for Sale Securities - Gross Unrealized Losses and Fair Value, Continuous Unrealized Loss Position (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>99940307 - Disclosure - Available for Sale Securities - Other Securities Policy: Pooled Trust Preferred Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureAvailableForSaleSecuritiesOtherSecuritiesPolicyPooledTrustPreferredSecuritiesDetails</Role>
      <ShortName>Available for Sale Securities - Other Securities Policy: Pooled Trust Preferred Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>99940401 - Disclosure - Loans and Allowance for Credit Losses - Classes of loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureLoansAndAllowanceForCreditLossesClassesOfLoansDetails</Role>
      <ShortName>Loans and Allowance for Credit Losses - Classes of loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>99940402 - Disclosure - Loans and Allowance for Credit Losses - Risk characteristics applicable to each class of the loan portfolio (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureLoansAndAllowanceForCreditLossesRiskCharacteristicsApplicableToEachClassOfLoanPortfolioDetails</Role>
      <ShortName>Loans and Allowance for Credit Losses - Risk characteristics applicable to each class of the loan portfolio (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>99940403 - Disclosure - Loans and Allowance for Credit Losses - Activity in the Allowance for credit losses (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureLoansAndAllowanceForCreditLossesActivityInAllowanceForCreditLossesDetails</Role>
      <ShortName>Loans and Allowance for Credit Losses - Activity in the Allowance for credit losses (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>99940404 - Disclosure - Loans and Allowance for Credit Losses - Allowance for off-balance credit exposure (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureLoansAndAllowanceForCreditLossesAllowanceForOffBalanceCreditExposureDetails</Role>
      <ShortName>Loans and Allowance for Credit Losses - Allowance for off-balance credit exposure (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>99940405 - Disclosure - Loans and Allowance for Credit Losses - Gross Charge-offs by Loan Class and Year of Origination (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureLoansAndAllowanceForCreditLossesGrossChargeOffsByLoanClassAndYearOfOriginationDetails</Role>
      <ShortName>Loans and Allowance for Credit Losses - Gross Charge-offs by Loan Class and Year of Origination (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>99940406 - Disclosure - Loans and Allowance for Credit Losses - Credit risk profile based on rating category and year of origination (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureLoansAndAllowanceForCreditLossesCreditRiskProfileBasedOnRatingCategoryAndYearOfOriginationDetails</Role>
      <ShortName>Loans and Allowance for Credit Losses - Credit risk profile based on rating category and year of origination (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>99940407 - Disclosure - Loans and Allowance for Credit Losses - Loan portfolio aging analysis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureLoansAndAllowanceForCreditLossesLoanPortfolioAgingAnalysisDetails</Role>
      <ShortName>Loans and Allowance for Credit Losses - Loan portfolio aging analysis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>99940408 - Disclosure - Loans and Allowance for Credit Losses - Collateral dependent loans and related ACL (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureLoansAndAllowanceForCreditLossesCollateralDependentLoansAndRelatedAclDetails</Role>
      <ShortName>Loans and Allowance for Credit Losses - Collateral dependent loans and related ACL (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>99940409 - Disclosure - Loans and Allowance for Credit Losses - Nonaccrual Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureLoansAndAllowanceForCreditLossesNonaccrualLoansDetails</Role>
      <ShortName>Loans and Allowance for Credit Losses - Nonaccrual Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>99940410 - Disclosure - Loans and Allowance for Credit Losses - Modifications to Borrowers Experiencing Financial Difficulty (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureLoansAndAllowanceForCreditLossesModificationsToBorrowersExperiencingFinancialDifficultyDetails</Role>
      <ShortName>Loans and Allowance for Credit Losses - Modifications to Borrowers Experiencing Financial Difficulty (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>99940412 - Disclosure - Loans and Allowance for Credit Losses - Real Estate Foreclosures (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureLoansAndAllowanceForCreditLossesRealEstateForeclosuresDetails</Role>
      <ShortName>Loans and Allowance for Credit Losses - Real Estate Foreclosures (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>99940501 - Disclosure - Premises and Equipment - Summary of premises and equipment (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosurePremisesAndEquipmentSummaryOfPremisesAndEquipmentDetails</Role>
      <ShortName>Premises and Equipment - Summary of premises and equipment (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>99940502 - Disclosure - Premises and Equipment - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosurePremisesAndEquipmentAdditionalInformationDetails</Role>
      <ShortName>Premises and Equipment - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>99940503 - Disclosure - Premises and Equipment - Calculated amount of right of use assets and lease liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosurePremisesAndEquipmentCalculatedAmountOfRightOfUseAssetsAndLeaseLiabilitiesDetails</Role>
      <ShortName>Premises and Equipment - Calculated amount of right of use assets and lease liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>99940504 - Disclosure - Premises and Equipment - Future expected lease payments for leases (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosurePremisesAndEquipmentFutureExpectedLeasePaymentsForLeasesDetails</Role>
      <ShortName>Premises and Equipment - Future expected lease payments for leases (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>99940601 - Disclosure - Deposits (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureDepositsDetails</Role>
      <ShortName>Deposits (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bankwithsouthern.com/role/DisclosureDepositsTables</ParentRole>
      <Position>64</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>99940602 - Disclosure - Deposits - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureDepositsAdditionalInformationDetails</Role>
      <ShortName>Deposits - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>99940701 - Disclosure - Repurchase Agreements - Outstanding Amounts and Interest Rates (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureRepurchaseAgreementsOutstandingAmountsAndInterestRatesDetails</Role>
      <ShortName>Repurchase Agreements - Outstanding Amounts and Interest Rates (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>99940702 - Disclosure - Repurchase Agreements - Collateral Pledged (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureRepurchaseAgreementsCollateralPledgedDetails</Role>
      <ShortName>Repurchase Agreements - Collateral Pledged (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>99940801 - Disclosure - Earnings Per Share - Schedule of computation of basic and diluted earnings per share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureEarningsPerShareScheduleOfComputationOfBasicAndDilutedEarningsPerShareDetails</Role>
      <ShortName>Earnings Per Share - Schedule of computation of basic and diluted earnings per share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>99940802 - Disclosure - Earnings Per Share - Additional information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureEarningsPerShareAdditionalInformationDetails</Role>
      <ShortName>Earnings Per Share - Additional information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>99940901 - Disclosure - Income Taxes - Income tax provision (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureIncomeTaxesIncomeTaxProvisionDetails</Role>
      <ShortName>Income Taxes - Income tax provision (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>99940902 - Disclosure - Income Taxes - Schedule of net deferred tax assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureIncomeTaxesScheduleOfNetDeferredTaxAssetsDetails</Role>
      <ShortName>Income Taxes - Schedule of net deferred tax assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>99940903 - Disclosure - Income Taxes - Reconciliation of income tax expense at statutory rate (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureIncomeTaxesReconciliationOfIncomeTaxExpenseAtStatutoryRateDetails</Role>
      <ShortName>Income Taxes - Reconciliation of income tax expense at statutory rate (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>99940904 - Disclosure - Income Taxes - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureIncomeTaxesAdditionalInformationDetails</Role>
      <ShortName>Income Taxes - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>99941001 - Disclosure - 401(k) Retirement Plan (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/Disclosure401KRetirementPlanDetails</Role>
      <ShortName>401(k) Retirement Plan (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bankwithsouthern.com/role/Disclosure401KRetirementPlan</ParentRole>
      <Position>74</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>99941101 - Disclosure - Subordinated Debt (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureSubordinatedDebtDetails</Role>
      <ShortName>Subordinated Debt (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bankwithsouthern.com/role/DisclosureSubordinatedDebt</ParentRole>
      <Position>75</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>99941201 - Disclosure - Fair Value Measurements - Fair value of Assets Measured on a Recurring Basis and Nonrecurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureFairValueMeasurementsFairValueOfAssetsMeasuredOnRecurringBasisAndNonrecurringBasisDetails</Role>
      <ShortName>Fair Value Measurements - Fair value of Assets Measured on a Recurring Basis and Nonrecurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>99941202 - Disclosure - Fair Value Measurements - Change in Fair Value of Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureFairValueMeasurementsChangeInFairValueOfAssetsDetails</Role>
      <ShortName>Fair Value Measurements - Change in Fair Value of Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>99941203 - Disclosure - Fair Value Measurements - Losses Recognized on Assets Measured on a Nonrecurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureFairValueMeasurementsLossesRecognizedOnAssetsMeasuredOnNonrecurringBasisDetails</Role>
      <ShortName>Fair Value Measurements - Losses Recognized on Assets Measured on a Nonrecurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>99941204 - Disclosure - Fair Value Measurements - Unobservable (Level 3) inputs (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureFairValueMeasurementsUnobservableLevel3InputsDetails</Role>
      <ShortName>Fair Value Measurements - Unobservable (Level 3) inputs (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>99941205 - Disclosure - Fair Value Measurements - Schedule of financial instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureFairValueMeasurementsScheduleOfFinancialInstrumentsDetails</Role>
      <ShortName>Fair Value Measurements - Schedule of financial instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>99941301 - Disclosure - Derivative Financial Instruments - Additional Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureDerivativeFinancialInstrumentsAdditionalInformationDetails</Role>
      <ShortName>Derivative Financial Instruments - Additional Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>99941302 - Disclosure - Derivative Financial Instruments - Notional amounts and estimated fair values of interest rate swaps (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureDerivativeFinancialInstrumentsNotionalAmountsAndEstimatedFairValuesOfInterestRateSwapsDetails</Role>
      <ShortName>Derivative Financial Instruments - Notional amounts and estimated fair values of interest rate swaps (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>99941303 - Disclosure - Derivative Financial Instruments - Carrying amount of the hedged assets, located in loans receivable, net (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureDerivativeFinancialInstrumentsCarryingAmountOfHedgedAssetsLocatedInLoansReceivableNetDetails</Role>
      <ShortName>Derivative Financial Instruments - Carrying amount of the hedged assets, located in loans receivable, net (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>99941304 - Disclosure - Derivative Financial Instruments - Interest Rate Swaps Not Designated as Hedging Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureDerivativeFinancialInstrumentsInterestRateSwapsNotDesignatedAsHedgingInstrumentsDetails</Role>
      <ShortName>Derivative Financial Instruments - Interest Rate Swaps Not Designated as Hedging Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>84</Position>
    </Report>
    <Report instance="smbc-20260331x10q.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>99941401 - Disclosure - Segment Reporting (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.bankwithsouthern.com/role/DisclosureSegmentReportingDetails</Role>
      <ShortName>Segment Reporting (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.bankwithsouthern.com/role/DisclosureSegmentReporting</ParentRole>
      <Position>85</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File>smbc-20260331.xsd</File>
    <File>smbc-20260331_cal.xml</File>
    <File>smbc-20260331_def.xml</File>
    <File>smbc-20260331_lab.xml</File>
    <File>smbc-20260331_pre.xml</File>
    <File doctype="10-Q" isDefinitelyFs="true" isUsgaap="true" original="smbc-20260331x10q.htm">smbc-20260331x10q.htm</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy items="1936">http://fasb.org/us-gaap/2025</BaseTaxonomy>
    <BaseTaxonomy items="29">http://xbrl.sec.gov/dei/2025</BaseTaxonomy>
    <BaseTaxonomy items="4">http://xbrl.sec.gov/ecd/2025</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
