<?xml version='1.0' encoding='UTF-8'?>
<FilingSummary>
  <Version>3.8.0.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>435</ContextCount>
  <ElementCount>286</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>86</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>15</UnitCount>
  <MyReports>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0001000 - Document - DOCUMENT AND ENTITY INFORMATION</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/DocumentAndEntityInformation</Role>
      <ShortName>DOCUMENT AND ENTITY INFORMATION</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>1001000 - Statement - CONDENSED CONSOLIDATED BALANCE SHEETS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/CondensedConsolidatedBalanceSheets</Role>
      <ShortName>CONDENSED CONSOLIDATED BALANCE SHEETS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>1002000 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF INCOME</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/CondensedConsolidatedStatementsOfIncome</Role>
      <ShortName>CONDENSED CONSOLIDATED STATEMENTS OF INCOME</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>1003000 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/CondensedConsolidatedStatementsOfComprehensiveIncome</Role>
      <ShortName>CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>1005000 - Statement - CONDENSED CONSOLIDATED STATEMENT OF CASH FLOWS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/CondensedConsolidatedStatementOfCashFlows</Role>
      <ShortName>CONDENSED CONSOLIDATED STATEMENT OF CASH FLOWS</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>2101100 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/SummaryOfSignificantAccountingPolicies</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>2102100 - Disclosure - REGULATORY ACCOUNTING</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/RegulatoryAccounting</Role>
      <ShortName>REGULATORY ACCOUNTING</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>2103100 - Disclosure - PROPERTY, PLANT AND EQUIPMENT AND ACCUMULATED DEPRECIATION</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/PropertyPlantAndEquipmentAndAccumulatedDepreciation</Role>
      <ShortName>PROPERTY, PLANT AND EQUIPMENT AND ACCUMULATED DEPRECIATION</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>2104100 - Disclosure - DERIVATIVE INSTRUMENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/DerivativeInstruments</Role>
      <ShortName>DERIVATIVE INSTRUMENTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>2105100 - Disclosure - MARKETABLE SECURITIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/MarketableSecurities</Role>
      <ShortName>MARKETABLE SECURITIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>2106100 - Disclosure - SHORT-TERM AND LONG-TERM DEBT</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/ShortTermAndLongTermDebt</Role>
      <ShortName>SHORT-TERM AND LONG-TERM DEBT</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>2107100 - Disclosure - PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/PensionBenefitsAndPostretirementBenefitsOtherThanPensions</Role>
      <ShortName>PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>2108100 - Disclosure - COMMITMENTS AND CONTINGENCIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/CommitmentsAndContingencies</Role>
      <ShortName>COMMITMENTS AND CONTINGENCIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>2109100 - Disclosure - PSNH GENERATION ASSET SALE</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/PsnhGenerationAssetSale</Role>
      <ShortName>PSNH GENERATION ASSET SALE</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>2110100 - Disclosure - FAIR VALUE OF FINANCIAL INSTRUMENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/FairValueOfFinancialInstruments</Role>
      <ShortName>FAIR VALUE OF FINANCIAL INSTRUMENTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>2111100 - Disclosure - ACCUMULATED OTHER COMPREHENSIVE INCOME/(LOSS)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/AccumulatedOtherComprehensiveIncomeLoss</Role>
      <ShortName>ACCUMULATED OTHER COMPREHENSIVE INCOME/(LOSS)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>2112100 - Disclosure - COMMON SHARES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/CommonShares</Role>
      <ShortName>COMMON SHARES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>2113100 - Disclosure - COMMON SHAREHOLDERS' EQUITY AND NONCONTROLLING INTERESTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/CommonShareholdersEquityAndNoncontrollingInterests</Role>
      <ShortName>COMMON SHAREHOLDERS' EQUITY AND NONCONTROLLING INTERESTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>2114100 - Disclosure - EARNINGS PER SHARE</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/EarningsPerShare</Role>
      <ShortName>EARNINGS PER SHARE</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>2116100 - Disclosure - SEGMENT INFORMATION</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/SegmentInformation</Role>
      <ShortName>SEGMENT INFORMATION</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>2201201 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/SummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>2301302 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/SummaryOfSignificantAccountingPoliciesTables</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://eversource.com/role/SummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>22</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>2302301 - Disclosure - REGULATORY ACCOUNTING (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/RegulatoryAccountingTables</Role>
      <ShortName>REGULATORY ACCOUNTING (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://eversource.com/role/RegulatoryAccounting</ParentRole>
      <Position>23</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>2303301 - Disclosure - PROPERTY, PLANT AND EQUIPMENT AND ACCUMULATED DEPRECIATION (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/PropertyPlantAndEquipmentAndAccumulatedDepreciationTables</Role>
      <ShortName>PROPERTY, PLANT AND EQUIPMENT AND ACCUMULATED DEPRECIATION (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://eversource.com/role/PropertyPlantAndEquipmentAndAccumulatedDepreciation</ParentRole>
      <Position>24</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>2304301 - Disclosure - DERIVATIVE INSTRUMENTS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/DerivativeInstrumentsTables</Role>
      <ShortName>DERIVATIVE INSTRUMENTS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://eversource.com/role/DerivativeInstruments</ParentRole>
      <Position>25</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>2305301 - Disclosure - MARKETABLE SECURITIES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/MarketableSecuritiesTables</Role>
      <ShortName>MARKETABLE SECURITIES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://eversource.com/role/MarketableSecurities</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>2307301 - Disclosure - PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/PensionBenefitsAndPostretirementBenefitsOtherThanPensionsTables</Role>
      <ShortName>PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://eversource.com/role/PensionBenefitsAndPostretirementBenefitsOtherThanPensions</ParentRole>
      <Position>27</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>2308301 - Disclosure - COMMITMENTS AND CONTINGENCIES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/CommitmentsAndContingenciesTables</Role>
      <ShortName>COMMITMENTS AND CONTINGENCIES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://eversource.com/role/CommitmentsAndContingencies</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>2309301 - Disclosure - PSNH GENERATION ASSET SALE - (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/PsnhGenerationAssetSaleTables</Role>
      <ShortName>PSNH GENERATION ASSET SALE - (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://eversource.com/role/PsnhGenerationAssetSale</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>2310301 - Disclosure - FAIR VALUE OF FINANCIAL INSTRUMENTS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/FairValueOfFinancialInstrumentsTables</Role>
      <ShortName>FAIR VALUE OF FINANCIAL INSTRUMENTS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://eversource.com/role/FairValueOfFinancialInstruments</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>2311301 - Disclosure - ACCUMULATED OTHER COMPREHENSIVE INCOME/(LOSS) (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/AccumulatedOtherComprehensiveIncomeLossTables</Role>
      <ShortName>ACCUMULATED OTHER COMPREHENSIVE INCOME/(LOSS) (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://eversource.com/role/AccumulatedOtherComprehensiveIncomeLoss</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>2312301 - Disclosure - COMMON SHARES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/CommonSharesTables</Role>
      <ShortName>COMMON SHARES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://eversource.com/role/CommonShares</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>2314301 - Disclosure - EARNINGS PER SHARE (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/EarningsPerShareTables</Role>
      <ShortName>EARNINGS PER SHARE (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://eversource.com/role/EarningsPerShare</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>2316301 - Disclosure - SEGMENT INFORMATION (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/SegmentInformationTables</Role>
      <ShortName>SEGMENT INFORMATION (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://eversource.com/role/SegmentInformation</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>2401403 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Basis of Presentation (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/SummaryOfSignificantAccountingPoliciesBasisOfPresentationDetails</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Basis of Presentation (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>35</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>2401404 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accounting Standards (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/SummaryOfSignificantAccountingPoliciesAccountingStandardsDetails</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accounting Standards (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>36</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>2401405 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/SummaryOfSignificantAccountingPoliciesProvisionForUncollectibleAccountsNarrativeDetails</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>37</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>2401406 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/SummaryOfSignificantAccountingPoliciesProvisionForUncollectibleAccountsDetails</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Provision for Uncollectible Accounts (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>38</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>2401407 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Other Income, Net Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/SummaryOfSignificantAccountingPoliciesOtherIncomeNetNarrativeDetails</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Other Income, Net Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>39</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>2401408 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Other Taxes (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/SummaryOfSignificantAccountingPoliciesOtherTaxesDetails</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Other Taxes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>40</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>2401409 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Supplemental Cash Flow (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/SummaryOfSignificantAccountingPoliciesSupplementalCashFlowDetails</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Supplemental Cash Flow (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>41</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>2402402 - Disclosure - REGULATORY ACCOUNTING - Components of Regulatory Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/RegulatoryAccountingComponentsOfRegulatoryAssetsDetails</Role>
      <ShortName>REGULATORY ACCOUNTING - Components of Regulatory Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>42</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>2402403 - Disclosure - REGULATORY ACCOUNTING - Regulatory Costs in Other Long-Term Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/RegulatoryAccountingRegulatoryCostsInOtherLongTermAssetsDetails</Role>
      <ShortName>REGULATORY ACCOUNTING - Regulatory Costs in Other Long-Term Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>43</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>2402404 - Disclosure - REGULATORY ACCOUNTING - Components of Regulatory Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/RegulatoryAccountingComponentsOfRegulatoryLiabilitiesDetails</Role>
      <ShortName>REGULATORY ACCOUNTING - Components of Regulatory Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>2403402 - Disclosure - PROPERTY, PLANT AND EQUIPMENT AND ACCUMULATED DEPRECIATION - Investment in Utility Property, plant and equipment by asset (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/PropertyPlantAndEquipmentAndAccumulatedDepreciationInvestmentInUtilityPropertyPlantAndEquipmentByAssetDetails</Role>
      <ShortName>PROPERTY, PLANT AND EQUIPMENT AND ACCUMULATED DEPRECIATION - Investment in Utility Property, plant and equipment by asset (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>2404402 - Disclosure - DERIVATIVE INSTRUMENTS - Gross Fair Value of Contracts (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/DerivativeInstrumentsGrossFairValueOfContractsDetails</Role>
      <ShortName>DERIVATIVE INSTRUMENTS - Gross Fair Value of Contracts (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>2404403 - Disclosure - DERIVATIVE INSTRUMENTS - Derivative Contracts at Fair Value (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/DerivativeInstrumentsDerivativeContractsAtFairValueDetails</Role>
      <ShortName>DERIVATIVE INSTRUMENTS - Derivative Contracts at Fair Value (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>47</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>2404404 - Disclosure - DERIVATIVE INSTRUMENTS - Unobservable Inputs Utilized (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/DerivativeInstrumentsUnobservableInputsUtilizedDetails</Role>
      <ShortName>DERIVATIVE INSTRUMENTS - Unobservable Inputs Utilized (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>2404405 - Disclosure - DERIVATIVE INSTRUMENTS - Unobservable Inputs Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/DerivativeInstrumentsUnobservableInputsNarrativeDetails</Role>
      <ShortName>DERIVATIVE INSTRUMENTS - Unobservable Inputs Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>2404406 - Disclosure - DERIVATIVE INSTRUMENTS - Derivative Assets and Liabilities Measured at Fair Value (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/DerivativeInstrumentsDerivativeAssetsAndLiabilitiesMeasuredAtFairValueDetails</Role>
      <ShortName>DERIVATIVE INSTRUMENTS - Derivative Assets and Liabilities Measured at Fair Value (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>2405402 - Disclosure - MARKETABLE SECURITIES - Trading Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/MarketableSecuritiesTradingSecuritiesDetails</Role>
      <ShortName>MARKETABLE SECURITIES - Trading Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>51</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>2405403 - Disclosure - MARKETABLE SECURITIES - Schedule of Available for Sale Securities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/MarketableSecuritiesScheduleOfAvailableForSaleSecuritiesDetails</Role>
      <ShortName>MARKETABLE SECURITIES - Schedule of Available for Sale Securities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>2405404 - Disclosure - MARKETABLE SECURITIES - Available for Sale Securities Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/MarketableSecuritiesAvailableForSaleSecuritiesNarrativeDetails</Role>
      <ShortName>MARKETABLE SECURITIES - Available for Sale Securities Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>2405405 - Disclosure - MARKETABLE SECURITIES - Schedule of Contractual Maturities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/MarketableSecuritiesScheduleOfContractualMaturitiesDetails</Role>
      <ShortName>MARKETABLE SECURITIES - Schedule of Contractual Maturities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>2405406 - Disclosure - MARKETABLE SECURITIES - Schedule of Fair Value Measurements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/MarketableSecuritiesScheduleOfFairValueMeasurementsDetails</Role>
      <ShortName>MARKETABLE SECURITIES - Schedule of Fair Value Measurements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>2406401 - Disclosure - SHORT-TERM AND LONG-TERM DEBT - Commercial Paper Programs and Credit Agreements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/ShortTermAndLongTermDebtCommercialPaperProgramsAndCreditAgreementsDetails</Role>
      <ShortName>SHORT-TERM AND LONG-TERM DEBT - Commercial Paper Programs and Credit Agreements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>2406402 - Disclosure - SHORT-TERM AND LONG-TERM DEBT - Long-Term Debt Issuances (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/ShortTermAndLongTermDebtLongTermDebtIssuancesDetails</Role>
      <ShortName>SHORT-TERM AND LONG-TERM DEBT - Long-Term Debt Issuances (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>57</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>2406403 - Disclosure - SHORT-TERM AND LONG-TERM DEBT - Long-Term Debt Repayments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/ShortTermAndLongTermDebtLongTermDebtRepaymentsDetails</Role>
      <ShortName>SHORT-TERM AND LONG-TERM DEBT - Long-Term Debt Repayments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>2406404 - Disclosure - SHORT-TERM AND LONG - Long-Term Debt Issuance Authorization (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/ShortTermAndLongLongTermDebtIssuanceAuthorizationDetails</Role>
      <ShortName>SHORT-TERM AND LONG - Long-Term Debt Issuance Authorization (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>2407402 - Disclosure - PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/PensionBenefitsAndPostretirementBenefitsOtherThanPensionsNarrativeDetails</Role>
      <ShortName>PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://eversource.com/role/PensionBenefitsAndPostretirementBenefitsOtherThanPensionsTables</ParentRole>
      <Position>60</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>2407403 - Disclosure - PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS - Components of Periodic Benefit Expense (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/PensionBenefitsAndPostretirementBenefitsOtherThanPensionsComponentsOfPeriodicBenefitExpenseDetails</Role>
      <ShortName>PENSION BENEFITS AND POSTRETIREMENT BENEFITS OTHER THAN PENSIONS - Components of Periodic Benefit Expense (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>61</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>2408402 - Disclosure - COMMITMENTS AND CONTINGENCIES - Schedule of Environmental Matters (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/CommitmentsAndContingenciesScheduleOfEnvironmentalMattersDetails</Role>
      <ShortName>COMMITMENTS AND CONTINGENCIES - Schedule of Environmental Matters (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>2408403 - Disclosure - COMMITMENTS AND CONTINGENCIES - Environmental Matters Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/CommitmentsAndContingenciesEnvironmentalMattersNarrativeDetails</Role>
      <ShortName>COMMITMENTS AND CONTINGENCIES - Environmental Matters Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>2408404 - Disclosure - COMMITMENTS AND CONTINGENCIES - Guarantees and Indemnifications Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/CommitmentsAndContingenciesGuaranteesAndIndemnificationsNarrativeDetails</Role>
      <ShortName>COMMITMENTS AND CONTINGENCIES - Guarantees and Indemnifications Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>2408405 - Disclosure - COMMITMENTS AND CONTINGENCIES - Spent Nuclear Fuel Obligations , Yankee Companies (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/CommitmentsAndContingenciesSpentNuclearFuelObligationsYankeeCompaniesDetails</Role>
      <ShortName>COMMITMENTS AND CONTINGENCIES - Spent Nuclear Fuel Obligations , Yankee Companies (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>2408406 - Disclosure - COMMITMENTS AND CONTINGENCIES - Schedule of Guarantees and Indemnifications (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/CommitmentsAndContingenciesScheduleOfGuaranteesAndIndemnificationsDetails</Role>
      <ShortName>COMMITMENTS AND CONTINGENCIES - Schedule of Guarantees and Indemnifications (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>66</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>2408407 - Disclosure - COMMITMENTS AND CONTINGENCIES - FERC ROE Complaints (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/CommitmentsAndContingenciesFercRoeComplaintsDetails</Role>
      <ShortName>COMMITMENTS AND CONTINGENCIES - FERC ROE Complaints (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>2408408 - Disclosure - COMMITMENTS AND CONTINGENCIES - Schedule of Complaints and Base ROEs (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/CommitmentsAndContingenciesScheduleOfComplaintsAndBaseRoesDetails</Role>
      <ShortName>COMMITMENTS AND CONTINGENCIES - Schedule of Complaints and Base ROEs (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>68</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>2408409 - Disclosure - COMMITMENTS AND CONTINGENCIES - Harbor Civil Action (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/CommitmentsAndContingenciesHarborCivilActionDetails</Role>
      <ShortName>COMMITMENTS AND CONTINGENCIES - Harbor Civil Action (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>69</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>2409402 - Disclosure - PSNH GENERATION ASSET SALE - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/PsnhGenerationAssetSaleNarrativeDetails</Role>
      <ShortName>PSNH GENERATION ASSET SALE - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>2409403 - Disclosure - PSNH GENERATION ASSET SALE - PSNH Generation Assets and Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/PsnhGenerationAssetSalePsnhGenerationAssetsAndLiabilitiesDetails</Role>
      <ShortName>PSNH GENERATION ASSET SALE - PSNH Generation Assets and Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>2410402 - Disclosure - FAIR VALUE OF FINANCIAL INSTRUMENTS - Carrying Amounts and Fair Value Hierarchy (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/FairValueOfFinancialInstrumentsCarryingAmountsAndFairValueHierarchyDetails</Role>
      <ShortName>FAIR VALUE OF FINANCIAL INSTRUMENTS - Carrying Amounts and Fair Value Hierarchy (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>2411402 - Disclosure - ACCUMULATED OTHER COMPREHENSIVE INCOME/(LOSS) - Schedule of Accumulated Other Comprehensive Income (loss) by Component (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/AccumulatedOtherComprehensiveIncomeLossScheduleOfAccumulatedOtherComprehensiveIncomeLossByComponentDetails</Role>
      <ShortName>ACCUMULATED OTHER COMPREHENSIVE INCOME/(LOSS) - Schedule of Accumulated Other Comprehensive Income (loss) by Component (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>2412402 - Disclosure - COMMON SHARES - Schedule of Common Shares (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/CommonSharesScheduleOfCommonSharesDetails</Role>
      <ShortName>COMMON SHARES - Schedule of Common Shares (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>2412403 - Disclosure - COMMON SHARES - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/CommonSharesNarrativeDetails</Role>
      <ShortName>COMMON SHARES - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>2413401 - Disclosure - COMMON SHAREHOLDERS' EQUITY AND NONCONTROLLING INTERESTS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/CommonShareholdersEquityAndNoncontrollingInterestsDetails</Role>
      <ShortName>COMMON SHAREHOLDERS' EQUITY AND NONCONTROLLING INTERESTS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://eversource.com/role/CommonShareholdersEquityAndNoncontrollingInterests</ParentRole>
      <Position>76</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>2414402 - Disclosure - EARNINGS PER SHARE - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/EarningsPerShareNarrativeDetails</Role>
      <ShortName>EARNINGS PER SHARE - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>77</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>2414403 - Disclosure - EARNINGS PER SHARE - Schedule of Earnings Per Share (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/EarningsPerShareScheduleOfEarningsPerShareDetails</Role>
      <ShortName>EARNINGS PER SHARE - Schedule of Earnings Per Share (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>2416402 - Disclosure - SEGMENT INFORMATION - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/SegmentInformationNarrativeDetails</Role>
      <ShortName>SEGMENT INFORMATION - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>79</Position>
    </Report>
    <Report instance="es-20170930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>2416403 - Disclosure - SEGMENT INFORMATION - Schedule of Segment Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://eversource.com/role/SegmentInformationScheduleOfSegmentInformationDetails</Role>
      <ShortName>SEGMENT INFORMATION - Schedule of Segment Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File>es-20170930.xml</File>
    <File>es-20170930.xsd</File>
    <File>es-20170930_cal.xml</File>
    <File>es-20170930_def.xml</File>
    <File>es-20170930_lab.xml</File>
    <File>es-20170930_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy>http://fasb.org/us-gaap/2017-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2014-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
