<?xml version='1.0' encoding='utf-8'?>
<FilingSummary>
  <Version>3.20.2</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>975</ContextCount>
  <ElementCount>529</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>false</FootnotesReported>
  <SegmentCount>214</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>13</UnitCount>
  <MyReports>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0001001 - Document - Cover page</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/Coverpage</Role>
      <ShortName>Cover page</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>1001002 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF INCOME/(LOSS) (UNAUDITED)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFINCOMELOSSUNAUDITED</Role>
      <ShortName>CONDENSED CONSOLIDATED STATEMENTS OF INCOME/(LOSS) (UNAUDITED)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>1002003 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) (UNAUDITED)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCOMPREHENSIVEINCOMELOSSUNAUDITED</Role>
      <ShortName>CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) (UNAUDITED)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>1003004 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCASHFLOWSUNAUDITED</Role>
      <ShortName>CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>1004005 - Statement - CONDENSED CONSOLIDATED BALANCE SHEETS (UNAUDITED)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/CONDENSEDCONSOLIDATEDBALANCESHEETSUNAUDITED</Role>
      <ShortName>CONDENSED CONSOLIDATED BALANCE SHEETS (UNAUDITED)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>1005006 - Statement - CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY (UNAUDITED)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFCHANGESINEQUITYUNAUDITED</Role>
      <ShortName>CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY (UNAUDITED)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>2101101 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIES</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>2108102 - Disclosure - STOCK-BASED COMPENSATION PLAN</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/STOCKBASEDCOMPENSATIONPLAN</Role>
      <ShortName>STOCK-BASED COMPENSATION PLAN</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>2114103 - Disclosure - AFFILIATIONS, DISCONTINUED OPERATIONS AND RELATED-PARTY TRANSACTIONS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/AFFILIATIONSDISCONTINUEDOPERATIONSANDRELATEDPARTYTRANSACTIONS</Role>
      <ShortName>AFFILIATIONS, DISCONTINUED OPERATIONS AND RELATED-PARTY TRANSACTIONS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>2119104 - Disclosure - COMMON STOCK</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/COMMONSTOCK</Role>
      <ShortName>COMMON STOCK</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>2124105 - Disclosure - FINANCIAL INSTRUMENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/FINANCIALINSTRUMENTS</Role>
      <ShortName>FINANCIAL INSTRUMENTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>2129106 - Disclosure - SEGMENTS OF BUSINESS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/SEGMENTSOFBUSINESS</Role>
      <ShortName>SEGMENTS OF BUSINESS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>2132107 - Disclosure - RATES AND REGULATORY ACTIONS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/RATESANDREGULATORYACTIONS</Role>
      <ShortName>RATES AND REGULATORY ACTIONS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>2134108 - Disclosure - REGULATORY ASSETS AND REGULATORY LIABILITIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/REGULATORYASSETSANDREGULATORYLIABILITIES</Role>
      <ShortName>REGULATORY ASSETS AND REGULATORY LIABILITIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>2139109 - Disclosure - PENSION AND OTHER POSTRETIREMENT BENEFITS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/PENSIONANDOTHERPOSTRETIREMENTBENEFITS</Role>
      <ShortName>PENSION AND OTHER POSTRETIREMENT BENEFITS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>2143110 - Disclosure - LINES OF CREDIT AND SHORT-TERM BORROWINGS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/LINESOFCREDITANDSHORTTERMBORROWINGS</Role>
      <ShortName>LINES OF CREDIT AND SHORT-TERM BORROWINGS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>2147111 - Disclosure - COMMITMENTS AND CONTINGENCIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/COMMITMENTSANDCONTINGENCIES</Role>
      <ShortName>COMMITMENTS AND CONTINGENCIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>2149112 - Disclosure - DERIVATIVE INSTRUMENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/DERIVATIVEINSTRUMENTS</Role>
      <ShortName>DERIVATIVE INSTRUMENTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>2155113 - Disclosure - FAIR VALUE OF FINANCIAL ASSETS AND FINANCIAL LIABILITIES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/FAIRVALUEOFFINANCIALASSETSANDFINANCIALLIABILITIES</Role>
      <ShortName>FAIR VALUE OF FINANCIAL ASSETS AND FINANCIAL LIABILITIES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>2160114 - Disclosure - LONG-TERM DEBT</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/LONGTERMDEBT</Role>
      <ShortName>LONG-TERM DEBT</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>2162115 - Disclosure - ACCUMULATED OTHER COMPREHENSIVE LOSS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/ACCUMULATEDOTHERCOMPREHENSIVELOSS</Role>
      <ShortName>ACCUMULATED OTHER COMPREHENSIVE LOSS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>2165116 - Disclosure - REVENUE</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/REVENUE</Role>
      <ShortName>REVENUE</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>2170117 - Disclosure - BUSINESS COMBINATIONS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/BUSINESSCOMBINATIONS</Role>
      <ShortName>BUSINESS COMBINATIONS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>2172118 - Disclosure - GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/GOODWILLANDIDENTIFIABLEINTANGIBLEASSETS</Role>
      <ShortName>GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>2177119 - Disclosure - LEASES</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/LEASES</Role>
      <ShortName>LEASES</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>2181120 - Disclosure - SUBSEQUENT EVENTS</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/SUBSEQUENTEVENTS</Role>
      <ShortName>SUBSEQUENT EVENTS</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>2202201 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESPolicies</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>2303301 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESTables</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sjindustries.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIES</ParentRole>
      <Position>28</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>2309302 - Disclosure - STOCK-BASED COMPENSATION PLAN (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/STOCKBASEDCOMPENSATIONPLANTables</Role>
      <ShortName>STOCK-BASED COMPENSATION PLAN (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sjindustries.com/role/STOCKBASEDCOMPENSATIONPLAN</ParentRole>
      <Position>29</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>2315303 - Disclosure - AFFILIATIONS, DISCONTINUED OPERATIONS AND RELATED-PARTY TRANSACTIONS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/AFFILIATIONSDISCONTINUEDOPERATIONSANDRELATEDPARTYTRANSACTIONSTables</Role>
      <ShortName>AFFILIATIONS, DISCONTINUED OPERATIONS AND RELATED-PARTY TRANSACTIONS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sjindustries.com/role/AFFILIATIONSDISCONTINUEDOPERATIONSANDRELATEDPARTYTRANSACTIONS</ParentRole>
      <Position>30</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>2320304 - Disclosure - COMMON STOCK (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/COMMONSTOCKTables</Role>
      <ShortName>COMMON STOCK (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sjindustries.com/role/COMMONSTOCK</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>2325305 - Disclosure - FINANCIAL INSTRUMENTS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/FINANCIALINSTRUMENTSTables</Role>
      <ShortName>FINANCIAL INSTRUMENTS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sjindustries.com/role/FINANCIALINSTRUMENTS</ParentRole>
      <Position>32</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>2330306 - Disclosure - SEGMENTS OF BUSINESS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/SEGMENTSOFBUSINESSTables</Role>
      <ShortName>SEGMENTS OF BUSINESS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sjindustries.com/role/SEGMENTSOFBUSINESS</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>2335307 - Disclosure - REGULATORY ASSETS AND REGULATORY LIABILITIES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/REGULATORYASSETSANDREGULATORYLIABILITIESTables</Role>
      <ShortName>REGULATORY ASSETS AND REGULATORY LIABILITIES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sjindustries.com/role/REGULATORYASSETSANDREGULATORYLIABILITIES</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>2340308 - Disclosure - PENSION AND OTHER POSTRETIREMENT BENEFITS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/PENSIONANDOTHERPOSTRETIREMENTBENEFITSTables</Role>
      <ShortName>PENSION AND OTHER POSTRETIREMENT BENEFITS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sjindustries.com/role/PENSIONANDOTHERPOSTRETIREMENTBENEFITS</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>2344309 - Disclosure - LINES OF CREDIT AND SHORT-TERM BORROWINGS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/LINESOFCREDITANDSHORTTERMBORROWINGSTables</Role>
      <ShortName>LINES OF CREDIT AND SHORT-TERM BORROWINGS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sjindustries.com/role/LINESOFCREDITANDSHORTTERMBORROWINGS</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>2350310 - Disclosure - DERIVATIVE INSTRUMENTS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/DERIVATIVEINSTRUMENTSTables</Role>
      <ShortName>DERIVATIVE INSTRUMENTS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sjindustries.com/role/DERIVATIVEINSTRUMENTS</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>2356311 - Disclosure - FAIR VALUE OF FINANCIAL ASSETS AND FINANCIAL LIABILITIES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/FAIRVALUEOFFINANCIALASSETSANDFINANCIALLIABILITIESTables</Role>
      <ShortName>FAIR VALUE OF FINANCIAL ASSETS AND FINANCIAL LIABILITIES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sjindustries.com/role/FAIRVALUEOFFINANCIALASSETSANDFINANCIALLIABILITIES</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>2363312 - Disclosure - ACCUMULATED OTHER COMPREHENSIVE LOSS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/ACCUMULATEDOTHERCOMPREHENSIVELOSSTables</Role>
      <ShortName>ACCUMULATED OTHER COMPREHENSIVE LOSS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sjindustries.com/role/ACCUMULATEDOTHERCOMPREHENSIVELOSS</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>2366313 - Disclosure - REVENUE (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/REVENUETables</Role>
      <ShortName>REVENUE (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sjindustries.com/role/REVENUE</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>2373314 - Disclosure - GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/GOODWILLANDIDENTIFIABLEINTANGIBLEASSETSTables</Role>
      <ShortName>GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sjindustries.com/role/GOODWILLANDIDENTIFIABLEINTANGIBLEASSETS</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>2378315 - Disclosure - LEASES (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/LEASESTables</Role>
      <ShortName>LEASES (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.sjindustries.com/role/LEASES</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>2404401 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESDetails</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.sjindustries.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESTables</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>2405402 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Assets and Liabilities Held For Sale (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESAssetsandLiabilitiesHeldForSaleDetails</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Assets and Liabilities Held For Sale (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>44</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>2406403 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Asset Retirement Obligations Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESAssetRetirementObligationsActivityDetails</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Asset Retirement Obligations Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>45</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>2407404 - Disclosure - SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Asset Management Agreement (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/SUMMARYOFSIGNIFICANTACCOUNTINGPOLICIESAssetManagementAgreementDetails</Role>
      <ShortName>SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Asset Management Agreement (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>46</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>2410405 - Disclosure - STOCK-BASED COMPENSATION PLAN (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/STOCKBASEDCOMPENSATIONPLANDetails</Role>
      <ShortName>STOCK-BASED COMPENSATION PLAN (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.sjindustries.com/role/STOCKBASEDCOMPENSATIONPLANTables</ParentRole>
      <Position>47</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>2411406 - Disclosure - STOCK-BASED COMPENSATION PLAN - Nonvested Restricted Stock Awards and Fair Value Assumptions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/STOCKBASEDCOMPENSATIONPLANNonvestedRestrictedStockAwardsandFairValueAssumptionsDetails</Role>
      <ShortName>STOCK-BASED COMPENSATION PLAN - Nonvested Restricted Stock Awards and Fair Value Assumptions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>48</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>2412407 - Disclosure - STOCK-BASED COMPENSATION PLAN - Schedule of Stock Based Compensation Cost (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/STOCKBASEDCOMPENSATIONPLANScheduleofStockBasedCompensationCostDetails</Role>
      <ShortName>STOCK-BASED COMPENSATION PLAN - Schedule of Stock Based Compensation Cost (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>49</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>2413408 - Disclosure - STOCK-BASED COMPENSATION PLAN - Restricted Stock Activity (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/STOCKBASEDCOMPENSATIONPLANRestrictedStockActivityDetails</Role>
      <ShortName>STOCK-BASED COMPENSATION PLAN - Restricted Stock Activity (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>50</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>2416409 - Disclosure - AFFILIATIONS, DISCONTINUED OPERATIONS AND RELATED-PARTY TRANSACTIONS - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/AFFILIATIONSDISCONTINUEDOPERATIONSANDRELATEDPARTYTRANSACTIONSNarrativeDetails</Role>
      <ShortName>AFFILIATIONS, DISCONTINUED OPERATIONS AND RELATED-PARTY TRANSACTIONS - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.sjindustries.com/role/AFFILIATIONSDISCONTINUEDOPERATIONSANDRELATEDPARTYTRANSACTIONSTables</ParentRole>
      <Position>51</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>2417410 - Disclosure - AFFILIATIONS, DISCONTINUED OPERATIONS AND RELATED-PARTY TRANSACTIONS - Summarized Operating Results (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/AFFILIATIONSDISCONTINUEDOPERATIONSANDRELATEDPARTYTRANSACTIONSSummarizedOperatingResultsDetails</Role>
      <ShortName>AFFILIATIONS, DISCONTINUED OPERATIONS AND RELATED-PARTY TRANSACTIONS - Summarized Operating Results (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>52</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>2418411 - Disclosure - AFFILIATIONS, DISCONTINUED OPERATIONS AND RELATED-PARTY TRANSACTIONS - Related Party Transactions (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/AFFILIATIONSDISCONTINUEDOPERATIONSANDRELATEDPARTYTRANSACTIONSRelatedPartyTransactionsDetails</Role>
      <ShortName>AFFILIATIONS, DISCONTINUED OPERATIONS AND RELATED-PARTY TRANSACTIONS - Related Party Transactions (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>53</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>2421412 - Disclosure - COMMON STOCK - Summary of Shares Issued and Outstanding (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/COMMONSTOCKSummaryofSharesIssuedandOutstandingDetails</Role>
      <ShortName>COMMON STOCK - Summary of Shares Issued and Outstanding (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>2422413 - Disclosure - COMMON STOCK - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/COMMONSTOCKNarrativeDetails</Role>
      <ShortName>COMMON STOCK - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>55</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>2423414 - Disclosure - COMMON STOCK - Summary of Convertible Units (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/COMMONSTOCKSummaryofConvertibleUnitsDetails</Role>
      <ShortName>COMMON STOCK - Summary of Convertible Units (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>56</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>2426415 - Disclosure - FINANCIAL INSTRUMENTS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/FINANCIALINSTRUMENTSDetails</Role>
      <ShortName>FINANCIAL INSTRUMENTS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.sjindustries.com/role/FINANCIALINSTRUMENTSTables</ParentRole>
      <Position>57</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>2427416 - Disclosure - FINANCIAL INSTRUMENTS - Cash, Cash Equivalents and Restricted Investments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/FINANCIALINSTRUMENTSCashCashEquivalentsandRestrictedInvestmentsDetails</Role>
      <ShortName>FINANCIAL INSTRUMENTS - Cash, Cash Equivalents and Restricted Investments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>58</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>2428417 - Disclosure - FINANCIAL INSTRUMENTS - Schedule of Allowance for Credit Losses (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/FINANCIALINSTRUMENTSScheduleofAllowanceforCreditLossesDetails</Role>
      <ShortName>FINANCIAL INSTRUMENTS - Schedule of Allowance for Credit Losses (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>2431418 - Disclosure - SEGMENTS OF BUSINESS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/SEGMENTSOFBUSINESSDetails</Role>
      <ShortName>SEGMENTS OF BUSINESS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.sjindustries.com/role/SEGMENTSOFBUSINESSTables</ParentRole>
      <Position>60</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>2433419 - Disclosure - RATES AND REGULATORY ACTIONS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/RATESANDREGULATORYACTIONSDetails</Role>
      <ShortName>RATES AND REGULATORY ACTIONS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.sjindustries.com/role/RATESANDREGULATORYACTIONS</ParentRole>
      <Position>61</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>2436420 - Disclosure - REGULATORY ASSETS AND REGULATORY LIABILITIES - Regulatory Assets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/REGULATORYASSETSANDREGULATORYLIABILITIESRegulatoryAssetsDetails</Role>
      <ShortName>REGULATORY ASSETS AND REGULATORY LIABILITIES - Regulatory Assets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>62</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>2437421 - Disclosure - REGULATORY ASSETS AND REGULATORY LIABILITIES - Narrative and Regulatory Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/REGULATORYASSETSANDREGULATORYLIABILITIESNarrativeandRegulatoryLiabilitiesDetails</Role>
      <ShortName>REGULATORY ASSETS AND REGULATORY LIABILITIES - Narrative and Regulatory Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>63</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>2438422 - Disclosure - REGULATORY ASSETS AND REGULATORY LIABILITIES - Regulatory Liabilities (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/REGULATORYASSETSANDREGULATORYLIABILITIESRegulatoryLiabilitiesDetails</Role>
      <ShortName>REGULATORY ASSETS AND REGULATORY LIABILITIES - Regulatory Liabilities (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>64</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>2441423 - Disclosure - PENSION AND OTHER POSTRETIREMENT BENEFITS - Schedule of Defined Benefit Plans Disclosure (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/PENSIONANDOTHERPOSTRETIREMENTBENEFITSScheduleofDefinedBenefitPlansDisclosureDetails</Role>
      <ShortName>PENSION AND OTHER POSTRETIREMENT BENEFITS - Schedule of Defined Benefit Plans Disclosure (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>65</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>2442424 - Disclosure - PENSION AND OTHER POSTRETIREMENT BENEFITS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/PENSIONANDOTHERPOSTRETIREMENTBENEFITSDetails</Role>
      <ShortName>PENSION AND OTHER POSTRETIREMENT BENEFITS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.sjindustries.com/role/PENSIONANDOTHERPOSTRETIREMENTBENEFITSTables</ParentRole>
      <Position>66</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>2445425 - Disclosure - LINES OF CREDIT AND SHORT-TERM BORROWINGS - Schedule of Lines of Credit (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/LINESOFCREDITANDSHORTTERMBORROWINGSScheduleofLinesofCreditDetails</Role>
      <ShortName>LINES OF CREDIT AND SHORT-TERM BORROWINGS - Schedule of Lines of Credit (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>67</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>2446426 - Disclosure - LINES OF CREDIT AND SHORT-TERM BORROWINGS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/LINESOFCREDITANDSHORTTERMBORROWINGSDetails</Role>
      <ShortName>LINES OF CREDIT AND SHORT-TERM BORROWINGS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.sjindustries.com/role/LINESOFCREDITANDSHORTTERMBORROWINGSTables</ParentRole>
      <Position>68</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>2448427 - Disclosure - COMMITMENTS AND CONTINGENCIES (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/COMMITMENTSANDCONTINGENCIESDetails</Role>
      <ShortName>COMMITMENTS AND CONTINGENCIES (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.sjindustries.com/role/COMMITMENTSANDCONTINGENCIES</ParentRole>
      <Position>69</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>2451428 - Disclosure - DERIVATIVE INSTRUMENTS - Outstanding Contracts (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/DERIVATIVEINSTRUMENTSOutstandingContractsDetails</Role>
      <ShortName>DERIVATIVE INSTRUMENTS - Outstanding Contracts (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>70</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>2452429 - Disclosure - DERIVATIVE INSTRUMENTS - Fair Value of all Derivative Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/DERIVATIVEINSTRUMENTSFairValueofallDerivativeInstrumentsDetails</Role>
      <ShortName>DERIVATIVE INSTRUMENTS - Fair Value of all Derivative Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>71</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>2453430 - Disclosure - DERIVATIVE INSTRUMENTS - Offsetting Arrangements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/DERIVATIVEINSTRUMENTSOffsettingArrangementsDetails</Role>
      <ShortName>DERIVATIVE INSTRUMENTS - Offsetting Arrangements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>72</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>2454431 - Disclosure - DERIVATIVE INSTRUMENTS - Effect of Instruments (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/DERIVATIVEINSTRUMENTSEffectofInstrumentsDetails</Role>
      <ShortName>DERIVATIVE INSTRUMENTS - Effect of Instruments (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>73</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>2457432 - Disclosure - FAIR VALUE OF FINANCIAL ASSETS AND FINANCIAL LIABILITIES - Measured on a Recurring Basis (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/FAIRVALUEOFFINANCIALASSETSANDFINANCIALLIABILITIESMeasuredonaRecurringBasisDetails</Role>
      <ShortName>FAIR VALUE OF FINANCIAL ASSETS AND FINANCIAL LIABILITIES - Measured on a Recurring Basis (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>74</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>2458433 - Disclosure - FAIR VALUE OF FINANCIAL ASSETS AND FINANCIAL LIABILITIES - Quantitative Information Regarding Significant Unobservable Inputs (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/FAIRVALUEOFFINANCIALASSETSANDFINANCIALLIABILITIESQuantitativeInformationRegardingSignificantUnobservableInputsDetails</Role>
      <ShortName>FAIR VALUE OF FINANCIAL ASSETS AND FINANCIAL LIABILITIES - Quantitative Information Regarding Significant Unobservable Inputs (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>75</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>2459434 - Disclosure - FAIR VALUE OF FINANCIAL ASSETS AND FINANCIAL LIABILITIES - Changes in Measurements (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/FAIRVALUEOFFINANCIALASSETSANDFINANCIALLIABILITIESChangesinMeasurementsDetails</Role>
      <ShortName>FAIR VALUE OF FINANCIAL ASSETS AND FINANCIAL LIABILITIES - Changes in Measurements (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>76</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>2461435 - Disclosure - LONG-TERM DEBT (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/LONGTERMDEBTDetails</Role>
      <ShortName>LONG-TERM DEBT (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.sjindustries.com/role/LONGTERMDEBT</ParentRole>
      <Position>77</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>2464436 - Disclosure - ACCUMULATED OTHER COMPREHENSIVE LOSS - Summary of Changes (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/ACCUMULATEDOTHERCOMPREHENSIVELOSSSummaryofChangesDetails</Role>
      <ShortName>ACCUMULATED OTHER COMPREHENSIVE LOSS - Summary of Changes (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>78</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>2467437 - Disclosure - REVENUE (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/REVENUEDetails</Role>
      <ShortName>REVENUE (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.sjindustries.com/role/REVENUETables</ParentRole>
      <Position>79</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>2468438 - Disclosure - REVENUE - Disaggregated Revenue (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/REVENUEDisaggregatedRevenueDetails</Role>
      <ShortName>REVENUE - Disaggregated Revenue (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>80</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>2469439 - Disclosure - REVENUE - Accounts Receivable (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/REVENUEAccountsReceivableDetails</Role>
      <ShortName>REVENUE - Accounts Receivable (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>81</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>2471440 - Disclosure - BUSINESS COMBINATIONS - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/BUSINESSCOMBINATIONSNarrativeDetails</Role>
      <ShortName>BUSINESS COMBINATIONS - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>82</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>2474441 - Disclosure - GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/GOODWILLANDIDENTIFIABLEINTANGIBLEASSETSNarrativeDetails</Role>
      <ShortName>GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>2475442 - Disclosure - GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS - SUMMARY OF INTANGIBLE ASSETS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/GOODWILLANDIDENTIFIABLEINTANGIBLEASSETSSUMMARYOFINTANGIBLEASSETSDetails</Role>
      <ShortName>GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS - SUMMARY OF INTANGIBLE ASSETS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>84</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>2476443 - Disclosure - GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS - FUTURE AMORTIZATION EXPENSE (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/GOODWILLANDIDENTIFIABLEINTANGIBLEASSETSFUTUREAMORTIZATIONEXPENSEDetails</Role>
      <ShortName>GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS - FUTURE AMORTIZATION EXPENSE (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>85</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>2479444 - Disclosure - LEASES - Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/LEASESNarrativeDetails</Role>
      <ShortName>LEASES - Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>86</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>2480445 - Disclosure - LEASES - Lease Maturity and Balance Sheet Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/LEASESLeaseMaturityandBalanceSheetInformationDetails</Role>
      <ShortName>LEASES - Lease Maturity and Balance Sheet Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>87</Position>
    </Report>
    <Report instance="sji-20200630.htm">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>2482446 - Disclosure - SUBSEQUENT EVENTS (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.sjindustries.com/role/SUBSEQUENTEVENTSDetails</Role>
      <ShortName>SUBSEQUENT EVENTS (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.sjindustries.com/role/SUBSEQUENTEVENTS</ParentRole>
      <Position>88</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File doctype="10-Q" original="sji-20200630.htm">sji-20200630.htm</File>
    <File>sjg-63020ex313.htm</File>
    <File>sjg-63020ex314.htm</File>
    <File>sjg-63020ex323.htm</File>
    <File>sjg-63020ex324.htm</File>
    <File>sji-20200630.xsd</File>
    <File>sji-20200630_cal.xml</File>
    <File>sji-20200630_def.xml</File>
    <File>sji-20200630_lab.xml</File>
    <File>sji-20200630_pre.xml</File>
    <File>sji-63020xex311.htm</File>
    <File>sji-63020xex312.htm</File>
    <File>sji-63020xex321.htm</File>
    <File>sji-63020xex322.htm</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2019-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/srt/2020-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/us-gaap/2020-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
