<?xml version='1.0' encoding='UTF-8'?>
<FilingSummary>
  <Version>3.4.0.3</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>356</ContextCount>
  <ElementCount>381</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>71</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>5</UnitCount>
  <MyReports>
    <Report instance="msfg-20160331.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.mainsourcefinancial.com/role/DocumentDocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>00100 - Statement - CONSOLIDATED BALANCE SHEETS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/StatementConsolidatedBalanceSheets</Role>
      <ShortName>CONSOLIDATED BALANCE SHEETS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="msfg-20160331.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.mainsourcefinancial.com/role/StatementConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>CONSOLIDATED BALANCE SHEETS (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>00200 - Statement - CONSOLIDATED STATEMENTS OF INCOME</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/StatementConsolidatedStatementsOfIncome</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF INCOME</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>00205 - Statement - CONSOLIDATED STATEMENTS OF INCOME (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/StatementConsolidatedStatementsOfIncomeParenthetical</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF INCOME (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>00300 - Statement - CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/StatementConsolidatedStatementsOfComprehensiveIncome</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>00400 - Statement - CONSOLIDATED STATEMENTS OF CASH FLOWS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/StatementConsolidatedStatementsOfCashFlows</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CASH FLOWS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>10101 - Disclosure - BASIS OF PRESENTATION</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureBasisOfPresentation</Role>
      <ShortName>BASIS OF PRESENTATION</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>10201 - Disclosure - STOCK PLANS AND STOCK-BASED COMPENSATION</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureStockPlansAndStockBasedCompensation</Role>
      <ShortName>STOCK PLANS AND STOCK-BASED COMPENSATION</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>10301 - Disclosure - SECURITIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureSecurities</Role>
      <ShortName>SECURITIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>10401 - Disclosure - LOANS AND ALLOWANCE</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureLoansAndAllowance</Role>
      <ShortName>LOANS AND ALLOWANCE</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>10501 - Disclosure - OTHER REAL ESTATE OWNED</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureOtherRealEstateOwned</Role>
      <ShortName>OTHER REAL ESTATE OWNED</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>10601 - Disclosure - DEPOSITS &amp; REPURCHASE AGREEMENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureDepositsRepurchaseAgreements</Role>
      <ShortName>DEPOSITS &amp; REPURCHASE AGREEMENTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>10701 - Disclosure - EARNINGS PER COMMON SHARE</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureEarningsPerCommonShare</Role>
      <ShortName>EARNINGS PER COMMON SHARE</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>10801 - Disclosure - FAIR VALUE</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureFairValue</Role>
      <ShortName>FAIR VALUE</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>10901 - Disclosure - REGULATORY CAPITAL</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureRegulatoryCapital</Role>
      <ShortName>REGULATORY CAPITAL</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>11001 - Disclosure - INCOME TAXES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureIncomeTaxes</Role>
      <ShortName>INCOME TAXES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>11101 - Disclosure - DERIVATIVES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureDerivatives</Role>
      <ShortName>DERIVATIVES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>11201 - Disclosure - ACQUISITION</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureAcquisition</Role>
      <ShortName>ACQUISITION</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>11301 - Disclosure - BRANCH CLOSURES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureBranchClosures</Role>
      <ShortName>BRANCH CLOSURES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>30203 - Disclosure - STOCK-BASED COMPENSATION (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureStockBasedCompensationTables</Role>
      <ShortName>STOCK-BASED COMPENSATION (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>30303 - Disclosure - SECURITIES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureSecuritiesTables</Role>
      <ShortName>SECURITIES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mainsourcefinancial.com/role/DisclosureSecurities</ParentRole>
      <Position>22</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>30403 - Disclosure - LOANS AND ALLOWANCE (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureLoansAndAllowanceTables</Role>
      <ShortName>LOANS AND ALLOWANCE (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mainsourcefinancial.com/role/DisclosureLoansAndAllowance</ParentRole>
      <Position>23</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>30503 - Disclosure - OTHER REAL ESTATE OWNED (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureOtherRealEstateOwnedTables</Role>
      <ShortName>OTHER REAL ESTATE OWNED (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mainsourcefinancial.com/role/DisclosureOtherRealEstateOwned</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>30603 - Disclosure - DEPOSITS &amp; REPURCHASE AGREEMENTS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureDepositsRepurchaseAgreementsTables</Role>
      <ShortName>DEPOSITS &amp; REPURCHASE AGREEMENTS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mainsourcefinancial.com/role/DisclosureDepositsRepurchaseAgreements</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>30703 - Disclosure - EARNINGS PER SHARE (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureEarningsPerShareTables</Role>
      <ShortName>EARNINGS PER SHARE (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>30803 - Disclosure - FAIR VALUE (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureFairValueTables</Role>
      <ShortName>FAIR VALUE (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mainsourcefinancial.com/role/DisclosureFairValue</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>30903 - Disclosure - REGULATORY CAPITAL (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureRegulatoryCapitalTables</Role>
      <ShortName>REGULATORY CAPITAL (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mainsourcefinancial.com/role/DisclosureRegulatoryCapital</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>31103 - Disclosure - DERIVATIVES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureDerivativesTables</Role>
      <ShortName>DERIVATIVES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.mainsourcefinancial.com/role/DisclosureDerivatives</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>40201 - Disclosure - STOCK PLANS AND STOCK BASED COMPENSATION - Stock Incentive Plans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureStockPlansAndStockBasedCompensationStockIncentivePlansDetails</Role>
      <ShortName>STOCK PLANS AND STOCK BASED COMPENSATION - Stock Incentive Plans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>40202 - Disclosure - STOCK PLANS AND STOCK BASED COMPENSATION - Activity in Stock Options (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureStockPlansAndStockBasedCompensationActivityInStockOptionsDetails</Role>
      <ShortName>STOCK PLANS AND STOCK BASED COMPENSATION - Activity in Stock Options (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>40203 - Disclosure - STOCK PLANS AND STOCK BASED COMPENSATION - Intrinsic Value of Stock Options and Additional Disclosures (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureStockPlansAndStockBasedCompensationIntrinsicValueOfStockOptionsAndAdditionalDisclosuresDetails</Role>
      <ShortName>STOCK PLANS AND STOCK BASED COMPENSATION - Intrinsic Value of Stock Options and Additional Disclosures (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>40204 - Disclosure - STOCK PLANS AND STOCK BASED COMPENSATION - Restricted Stock Awards (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureStockPlansAndStockBasedCompensationRestrictedStockAwardsDetails</Role>
      <ShortName>STOCK PLANS AND STOCK BASED COMPENSATION - Restricted Stock Awards (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>40205 - Disclosure - STOCK PLANS AND STOCK BASED COMPENSATION - Performance Measures and Additional Disclosures (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureStockPlansAndStockBasedCompensationPerformanceMeasuresAndAdditionalDisclosuresDetails</Role>
      <ShortName>STOCK PLANS AND STOCK BASED COMPENSATION - Performance Measures and Additional Disclosures (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>40206 - Disclosure - STOCK PLANS AND STOCK BASED COMPENSATION - Other Expense and Stock Repurchase Program (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureStockPlansAndStockBasedCompensationOtherExpenseAndStockRepurchaseProgramDetails</Role>
      <ShortName>STOCK PLANS AND STOCK BASED COMPENSATION - Other Expense and Stock Repurchase Program (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>40301 - Disclosure - SECURITIES - Fair Value and Related Gross Unrealized Gains and Losses (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureSecuritiesFairValueAndRelatedGrossUnrealizedGainsAndLossesDetails</Role>
      <ShortName>SECURITIES - Fair Value and Related Gross Unrealized Gains and Losses (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>40302 - Disclosure - SECURITIES - Contractual Maturities of Securities Available for Sale (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureSecuritiesContractualMaturitiesOfSecuritiesAvailableForSaleDetails</Role>
      <ShortName>SECURITIES - Contractual Maturities of Securities Available for Sale (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>40303 - Disclosure - SECURITIES - Additional Information - (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureSecuritiesAdditionalInformationDetails</Role>
      <ShortName>SECURITIES - Additional Information - (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>40304 - Disclosure - SECURITIES - Securities in an Unrealized Loss Position (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureSecuritiesSecuritiesInUnrealizedLossPositionDetails</Role>
      <ShortName>SECURITIES - Securities in an Unrealized Loss Position (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>40305 - Disclosure - SECURITIES - Other-Than-Temporary Impairment and Other Disclosures (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureSecuritiesOtherThanTemporaryImpairmentAndOtherDisclosuresDetails</Role>
      <ShortName>SECURITIES - Other-Than-Temporary Impairment and Other Disclosures (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>40401 - Disclosure - LOANS AND ALLOWANCE FOR LOAN LOSSES - Loans and Allowance For Loan Losses - (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureLoansAndAllowanceForLoanLossesLoansAndAllowanceForLoanLossesDetails</Role>
      <ShortName>LOANS AND ALLOWANCE FOR LOAN LOSSES - Loans and Allowance For Loan Losses - (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>40402 - Disclosure - LOANS AND ALLOWANCE FOR LOAN LOSSES - Allowance for Loan Losses and Recorded Investment By Portfolio Segment - (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureLoansAndAllowanceForLoanLossesAllowanceForLoanLossesAndRecordedInvestmentByPortfolioSegmentDetails</Role>
      <ShortName>LOANS AND ALLOWANCE FOR LOAN LOSSES - Allowance for Loan Losses and Recorded Investment By Portfolio Segment - (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>40403 - Disclosure - LOANS AND ALLOWANCE FOR LOAN LOSSES - Nonperforming Loans - (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureLoansAndAllowanceForLoanLossesNonperformingLoansDetails</Role>
      <ShortName>LOANS AND ALLOWANCE FOR LOAN LOSSES - Nonperforming Loans - (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>40404 - Disclosure - LOANS AND ALLOWANCE FOR LOAN LOSSES - Loan Impairment - (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureLoansAndAllowanceForLoanLossesLoanImpairmentDetails</Role>
      <ShortName>LOANS AND ALLOWANCE FOR LOAN LOSSES - Loan Impairment - (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>40405 - Disclosure - LOANS AND ALLOWANCE FOR LOAN LOSSES - Aging of the Recorded Investment in Past Due Loans - (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureLoansAndAllowanceForLoanLossesAgingOfRecordedInvestmentInPastDueLoansDetails</Role>
      <ShortName>LOANS AND ALLOWANCE FOR LOAN LOSSES - Aging of the Recorded Investment in Past Due Loans - (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>40406 - Disclosure - LOANS AND ALLOWANCE FOR LOAN LOSSES - Troubled Debt Restructurings - (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureLoansAndAllowanceForLoanLossesTroubledDebtRestructuringsDetails</Role>
      <ShortName>LOANS AND ALLOWANCE FOR LOAN LOSSES - Troubled Debt Restructurings - (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>40407 - Disclosure - LOANS AND ALLOWANCE FOR LOAN LOSSES - Risk Categories of Loans by Class of Loans - (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureLoansAndAllowanceForLoanLossesRiskCategoriesOfLoansByClassOfLoansDetails</Role>
      <ShortName>LOANS AND ALLOWANCE FOR LOAN LOSSES - Risk Categories of Loans by Class of Loans - (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>40501 - Disclosure - OTHER REAL ESTATE OWNED - Information Regarding Other Real Estate Owned (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureOtherRealEstateOwnedInformationRegardingOtherRealEstateOwnedDetails</Role>
      <ShortName>OTHER REAL ESTATE OWNED - Information Regarding Other Real Estate Owned (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>40601 - Disclosure - DEPOSITS &amp; REPURCHASE AGREEMENTS - Deposit Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureDepositsRepurchaseAgreementsDepositLiabilitiesDetails</Role>
      <ShortName>DEPOSITS &amp; REPURCHASE AGREEMENTS - Deposit Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>40701 - Disclosure - EARNINGS PER SHARE (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureEarningsPerShareDetails</Role>
      <ShortName>EARNINGS PER SHARE (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mainsourcefinancial.com/role/DisclosureEarningsPerShareTables</ParentRole>
      <Position>50</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>40801 - Disclosure - FAIR VALUE - Valuation Inputs - (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureFairValueValuationInputsDetails</Role>
      <ShortName>FAIR VALUE - Valuation Inputs - (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>40802 - Disclosure - FAIR VALUE - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureFairValueAssetsAndLiabilitiesMeasuredAtFairValueOnRecurringBasisDetails</Role>
      <ShortName>FAIR VALUE - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>40803 - Disclosure - FAIR VALUE - Reconciliation Of Gains and Losses on Level 3 Assets - (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureFairValueReconciliationOfGainsAndLossesOnLevel3AssetsDetails</Role>
      <ShortName>FAIR VALUE - Reconciliation Of Gains and Losses on Level 3 Assets - (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>40804 - Disclosure - FAIR VALUE - Fair Value of Assets and Liabilities Measured on a Nonrecurring Basis - (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureFairValueFairValueOfAssetsAndLiabilitiesMeasuredOnNonrecurringBasisDetails</Role>
      <ShortName>FAIR VALUE - Fair Value of Assets and Liabilities Measured on a Nonrecurring Basis - (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>40805 - Disclosure - FAIR VALUE - Quantitative Information about Level 3 Value Measurements - (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureFairValueQuantitativeInformationAboutLevel3ValueMeasurementsDetails</Role>
      <ShortName>FAIR VALUE - Quantitative Information about Level 3 Value Measurements - (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>40806 - Disclosure - FAIR VALUE - Carrying Amounts and Estimated Fair Values of Financial Instruments - (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureFairValueCarryingAmountsAndEstimatedFairValuesOfFinancialInstrumentsDetails</Role>
      <ShortName>FAIR VALUE - Carrying Amounts and Estimated Fair Values of Financial Instruments - (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>40901 - Disclosure - REGULATORY CAPITAL (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureRegulatoryCapitalDetails</Role>
      <ShortName>REGULATORY CAPITAL (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mainsourcefinancial.com/role/DisclosureRegulatoryCapitalTables</ParentRole>
      <Position>57</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>41001 - Disclosure - INCOME TAXES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureIncomeTaxesDetails</Role>
      <ShortName>INCOME TAXES (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mainsourcefinancial.com/role/DisclosureIncomeTaxes</ParentRole>
      <Position>58</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>41101 - Disclosure - DERIVATIVES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureDerivativesDetails</Role>
      <ShortName>DERIVATIVES (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mainsourcefinancial.com/role/DisclosureDerivativesTables</ParentRole>
      <Position>59</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>41201 - Disclosure - ACQUISITIONS - Acquisition of Cheviot Financial Corp. (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureAcquisitionsAcquisitionOfCheviotFinancialCorpDetails</Role>
      <ShortName>ACQUISITIONS - Acquisition of Cheviot Financial Corp. (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>60</Position>
    </Report>
    <Report instance="msfg-20160331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>41301 - Disclosure - BRANCH CLOSURES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.mainsourcefinancial.com/role/DisclosureBranchClosuresDetails</Role>
      <ShortName>BRANCH CLOSURES (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.mainsourcefinancial.com/role/DisclosureBranchClosures</ParentRole>
      <Position>61</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File>msfg-20160331.xml</File>
    <File>msfg-20160331.xsd</File>
    <File>msfg-20160331_cal.xml</File>
    <File>msfg-20160331_def.xml</File>
    <File>msfg-20160331_lab.xml</File>
    <File>msfg-20160331_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies/>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
