<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.20.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>326</ContextCount>
  <ElementCount>452</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>73</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>9</UnitCount>
  <MyReports>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>000100 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/DocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>010000 - Statement - CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION (UNAUDITED)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/ConsolidatedStatementsOfFinancialConditionUnaudited</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION (UNAUDITED)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>010100 - Statement - CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION (UNAUDITED) (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/ConsolidatedStatementsOfFinancialConditionUnauditedParenthetical</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF FINANCIAL CONDITION (UNAUDITED) (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>020000 - Statement - CONSOLIDATED STATEMENTS OF INCOME (UNAUDITED)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/ConsolidatedStatementsOfIncomeUnaudited</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF INCOME (UNAUDITED)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>030000 - Statement - CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/ConsolidatedStatementsOfComprehensiveIncomeUnaudited</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>040000 - Statement - CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY (UNAUDITED)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/ConsolidatedStatementsOfChangesInStockholdersEquityUnaudited</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY (UNAUDITED)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>050000 - Statement - CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/ConsolidatedStatementsOfCashFlowsUnaudited</Role>
      <ShortName>CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>060100 - Disclosure - NATURE OF OPERATIONS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/NatureOfOperations</Role>
      <ShortName>NATURE OF OPERATIONS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>060200 - Disclosure - SUMMARY OF ACCOUNTING POLICIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/SummaryOfAccountingPolicies</Role>
      <ShortName>SUMMARY OF ACCOUNTING POLICIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>060300 - Disclosure - ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/AccumulatedOtherComprehensiveIncomeLoss</Role>
      <ShortName>ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>060400 - Disclosure - EARNINGS PER COMMON SHARE ("EPS")</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/EarningsPerCommonShareEps</Role>
      <ShortName>EARNINGS PER COMMON SHARE ("EPS")</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>060500 - Disclosure - PREFERRED STOCK</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/PreferredStock</Role>
      <ShortName>PREFERRED STOCK</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>060600 - Disclosure - REVENUE FROM CONTRACTS WITH CUSTOMERS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/RevenueFromContractsWithCustomers</Role>
      <ShortName>REVENUE FROM CONTRACTS WITH CUSTOMERS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>060700 - Disclosure - INVESTMENT AND MORTGAGE-BACKED SECURITIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/InvestmentAndMortgagebackedSecurities</Role>
      <ShortName>INVESTMENT AND MORTGAGE-BACKED SECURITIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>060800 - Disclosure - LOANS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/Loans</Role>
      <ShortName>LOANS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>060900 - Disclosure - LEASES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/Leases</Role>
      <ShortName>LEASES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>061000 - Disclosure - DERIVATIVES AND HEDGING ACTIVITIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/DerivativesAndHedgingActivities</Role>
      <ShortName>DERIVATIVES AND HEDGING ACTIVITIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>061100 - Disclosure - FAIR VALUE OF FINANCIAL INSTRUMENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/FairValueOfFinancialInstruments</Role>
      <ShortName>FAIR VALUE OF FINANCIAL INSTRUMENTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>061200 - Disclosure - RETIREMENT AND POSTRETIREMENT PLANS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/RetirementAndPostretirementPlans</Role>
      <ShortName>RETIREMENT AND POSTRETIREMENT PLANS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>061300 - Disclosure - STOCK-BASED COMPENSATION</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/StockbasedCompensation</Role>
      <ShortName>STOCK-BASED COMPENSATION</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>061400 - Disclosure - OTHER BORROWINGS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/OtherBorrowings</Role>
      <ShortName>OTHER BORROWINGS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>061500 - Disclosure - INCOME TAXES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/IncomeTaxes</Role>
      <ShortName>INCOME TAXES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>061600 - Disclosure - SUBSEQUENT EVENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/SubsequentEvents</Role>
      <ShortName>SUBSEQUENT EVENTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>070100 - Disclosure - NATURE OF OPERATIONS (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/NatureOfOperationsPolicies</Role>
      <ShortName>NATURE OF OPERATIONS (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>070200 - Disclosure - SUMMARY OF ACCOUNTING POLICIES (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/SummaryOfAccountingPoliciesPolicies</Role>
      <ShortName>SUMMARY OF ACCOUNTING POLICIES (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>080300 - Disclosure - ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/AccumulatedOtherComprehensiveIncomeLossTables</Role>
      <ShortName>ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://dime.com/role/AccumulatedOtherComprehensiveIncomeLoss</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>080400 - Disclosure - EARNINGS PER COMMON SHARE ("EPS") (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/EarningsPerCommonShareEpsTables</Role>
      <ShortName>EARNINGS PER COMMON SHARE ("EPS") (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://dime.com/role/EarningsPerCommonShareEps</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>080700 - Disclosure - INVESTMENT AND MORTGAGE-BACKED SECURITIES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/InvestmentAndMortgagebackedSecuritiesTables</Role>
      <ShortName>INVESTMENT AND MORTGAGE-BACKED SECURITIES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://dime.com/role/InvestmentAndMortgagebackedSecurities</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>080800 - Disclosure - LOANS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/LoansTables</Role>
      <ShortName>LOANS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://dime.com/role/Loans</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>080900 - Disclosure - LEASES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/LeasesTables</Role>
      <ShortName>LEASES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://dime.com/role/Leases</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>081000 - Disclosure - DERIVATIVES AND HEDGING ACTIVITIES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/DerivativesAndHedgingActivitiesTables</Role>
      <ShortName>DERIVATIVES AND HEDGING ACTIVITIES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://dime.com/role/DerivativesAndHedgingActivities</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>081100 - Disclosure - FAIR VALUE OF FINANCIAL INSTRUMENTS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/FairValueOfFinancialInstrumentsTables</Role>
      <ShortName>FAIR VALUE OF FINANCIAL INSTRUMENTS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://dime.com/role/FairValueOfFinancialInstruments</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>081200 - Disclosure - RETIREMENT AND POSTRETIREMENT PLANS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/RetirementAndPostretirementPlansTables</Role>
      <ShortName>RETIREMENT AND POSTRETIREMENT PLANS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://dime.com/role/RetirementAndPostretirementPlans</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>081300 - Disclosure - STOCK-BASED COMPENSATION (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/StockbasedCompensationTables</Role>
      <ShortName>STOCK-BASED COMPENSATION (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://dime.com/role/StockbasedCompensation</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>090100 - Disclosure - NATURE OF OPERATIONS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/NatureOfOperationsDetails</Role>
      <ShortName>NATURE OF OPERATIONS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://dime.com/role/NatureOfOperationsPolicies</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>090300 - Disclosure - ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS), Accumulated Other Comprehensive Income (Loss) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/AccumulatedOtherComprehensiveIncomeLossAccumulatedOtherComprehensiveIncomeLossDetails</Role>
      <ShortName>ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS), Accumulated Other Comprehensive Income (Loss) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://dime.com/role/AccumulatedOtherComprehensiveIncomeLossTables</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>090302 - Disclosure - ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS), Before and After Tax Amounts by Component (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/AccumulatedOtherComprehensiveIncomeLossBeforeAndAfterTaxAmountsByComponentDetails</Role>
      <ShortName>ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS), Before and After Tax Amounts by Component (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://dime.com/role/AccumulatedOtherComprehensiveIncomeLossTables</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>090400 - Disclosure - EARNINGS PER COMMON SHARE ("EPS") (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/EarningsPerCommonShareEpsDetails</Role>
      <ShortName>EARNINGS PER COMMON SHARE ("EPS") (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://dime.com/role/EarningsPerCommonShareEpsTables</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>090500 - Disclosure - PREFERRED STOCK (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/PreferredStockDetails</Role>
      <ShortName>PREFERRED STOCK (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://dime.com/role/PreferredStock</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>090600 - Disclosure - REVENUE FROM CONTRACTS WITH CUSTOMERS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/RevenueFromContractsWithCustomersDetails</Role>
      <ShortName>REVENUE FROM CONTRACTS WITH CUSTOMERS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://dime.com/role/RevenueFromContractsWithCustomers</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>090700 - Disclosure - INVESTMENT AND MORTGAGE-BACKED SECURITIES, Major Categories of Securities Owned (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/InvestmentAndMortgagebackedSecuritiesMajorCategoriesOfSecuritiesOwnedDetails</Role>
      <ShortName>INVESTMENT AND MORTGAGE-BACKED SECURITIES, Major Categories of Securities Owned (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>090702 - Disclosure - INVESTMENT AND MORTGAGE-BACKED SECURITIES, Continuous Unrealized Loss Position (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/InvestmentAndMortgagebackedSecuritiesContinuousUnrealizedLossPositionDetails</Role>
      <ShortName>INVESTMENT AND MORTGAGE-BACKED SECURITIES, Continuous Unrealized Loss Position (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>090800 - Disclosure - LOANS, Loan Categories (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/LoansLoanCategoriesDetails</Role>
      <ShortName>LOANS, Loan Categories (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>090802 - Disclosure - LOANS, Allowance by Class of Loan (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/LoansAllowanceByClassOfLoanDetails</Role>
      <ShortName>LOANS, Allowance by Class of Loan (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>090804 - Disclosure - LOANS, Impaired Real Estate Loans (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/LoansImpairedRealEstateLoansDetails</Role>
      <ShortName>LOANS, Impaired Real Estate Loans (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>090806 - Disclosure - LOANS, Past Due (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/LoansPastDueDetails</Role>
      <ShortName>LOANS, Past Due (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>090808 - Disclosure - LOANS, Troubled Debt Restructurings (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/LoansTroubledDebtRestructuringsDetails</Role>
      <ShortName>LOANS, Troubled Debt Restructurings (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>090810 - Disclosure - LOANS, Credit Quality Indicators (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/LoansCreditQualityIndicatorsDetails</Role>
      <ShortName>LOANS, Credit Quality Indicators (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>090900 - Disclosure - LEASES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/LeasesDetails</Role>
      <ShortName>LEASES (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://dime.com/role/LeasesTables</ParentRole>
      <Position>49</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>091000 - Disclosure - DERIVATIVES AND HEDGING ACTIVITIES, Classification on Consolidated Statements of Financial Condition (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/DerivativesAndHedgingActivitiesClassificationOnConsolidatedStatementsOfFinancialConditionDetails</Role>
      <ShortName>DERIVATIVES AND HEDGING ACTIVITIES, Classification on Consolidated Statements of Financial Condition (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>091002 - Disclosure - DERIVATIVES AND HEDGING ACTIVITIES, Effect on Accumulated Other Comprehensive Loss (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/DerivativesAndHedgingActivitiesEffectOnAccumulatedOtherComprehensiveLossDetails</Role>
      <ShortName>DERIVATIVES AND HEDGING ACTIVITIES, Effect on Accumulated Other Comprehensive Loss (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>091004 - Disclosure - DERIVATIVES AND HEDGING ACTIVITIES, Offsetting of Derivative Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/DerivativesAndHedgingActivitiesOffsettingOfDerivativeLiabilitiesDetails</Role>
      <ShortName>DERIVATIVES AND HEDGING ACTIVITIES, Offsetting of Derivative Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>091100 - Disclosure - FAIR VALUE OF FINANCIAL INSTRUMENTS, Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/FairValueOfFinancialInstrumentsAssetsAndLiabilitiesMeasuredAtFairValueOnRecurringBasisDetails</Role>
      <ShortName>FAIR VALUE OF FINANCIAL INSTRUMENTS, Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>091102 - Disclosure - FAIR VALUE OF FINANCIAL INSTRUMENTS, Balance Sheet Groupings (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/FairValueOfFinancialInstrumentsBalanceSheetGroupingsDetails</Role>
      <ShortName>FAIR VALUE OF FINANCIAL INSTRUMENTS, Balance Sheet Groupings (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>091200 - Disclosure - RETIREMENT AND POSTRETIREMENT PLANS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/RetirementAndPostretirementPlansDetails</Role>
      <ShortName>RETIREMENT AND POSTRETIREMENT PLANS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://dime.com/role/RetirementAndPostretirementPlansTables</ParentRole>
      <Position>55</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>091300 - Disclosure - STOCK-BASED COMPENSATION, Stock Option Awards (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/StockbasedCompensationStockOptionAwardsDetails</Role>
      <ShortName>STOCK-BASED COMPENSATION, Stock Option Awards (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>091302 - Disclosure - STOCK-BASED COMPENSATION, Restricted Stock Awards (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/StockbasedCompensationRestrictedStockAwardsDetails</Role>
      <ShortName>STOCK-BASED COMPENSATION, Restricted Stock Awards (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>091304 - Disclosure - STOCK-BASED COMPENSATION, Performance Based Equity Awards (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/StockbasedCompensationPerformanceBasedEquityAwardsDetails</Role>
      <ShortName>STOCK-BASED COMPENSATION, Performance Based Equity Awards (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>091400 - Disclosure - OTHER BORROWINGS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/OtherBorrowingsDetails</Role>
      <ShortName>OTHER BORROWINGS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://dime.com/role/OtherBorrowings</ParentRole>
      <Position>59</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>091500 - Disclosure - INCOME TAXES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/IncomeTaxesDetails</Role>
      <ShortName>INCOME TAXES (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://dime.com/role/IncomeTaxes</ParentRole>
      <Position>60</Position>
    </Report>
    <Report instance="dcom-20200331.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>091600 - Disclosure - SUBSEQUENT EVENTS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://dime.com/role/SubsequentEventsDetails</Role>
      <ShortName>SUBSEQUENT EVENTS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://dime.com/role/SubsequentEvents</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>dcom-20200331.xml</File>
    <File>dcom-20200331.xsd</File>
    <File>dcom-20200331_cal.xml</File>
    <File>dcom-20200331_def.xml</File>
    <File>dcom-20200331_lab.xml</File>
    <File>dcom-20200331_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2019-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/us-gaap/2019-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/srt/2019-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
