<?xml version='1.0' encoding='UTF-8'?>
<FilingSummary>
  <Version>3.8.0.1</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>1426</ContextCount>
  <ElementCount>613</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>226</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>4</UnitCount>
  <MyReports>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R1.htm</HtmlFileName>
      <LongName>0001000 - Document - Document and Entity Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/DocumentAndEntityInformation</Role>
      <ShortName>Document and Entity Information</ShortName>
      <MenuCategory>Cover</MenuCategory>
      <Position>1</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R2.htm</HtmlFileName>
      <LongName>1001000 - Statement - Consolidated Balance Sheets</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ConsolidatedBalanceSheets</Role>
      <ShortName>Consolidated Balance Sheets</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>2</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R3.htm</HtmlFileName>
      <LongName>1001501 - Statement - Consolidated Balance Sheets (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ConsolidatedBalanceSheetsParenthetical</Role>
      <ShortName>Consolidated Balance Sheets (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>3</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R4.htm</HtmlFileName>
      <LongName>1002000 - Statement - Consolidated Statements of Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ConsolidatedStatementsOfIncome</Role>
      <ShortName>Consolidated Statements of Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>4</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>1004000 - Statement - Consolidation Statements of Comprehensive Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ConsolidationStatementsOfComprehensiveIncome</Role>
      <ShortName>Consolidation Statements of Comprehensive Income</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>5</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>1005000 - Statement - Consolidated Statements of Stockholders' Equity</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ConsolidatedStatementsOfStockholdersEquity</Role>
      <ShortName>Consolidated Statements of Stockholders' Equity</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>6</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>1005501 - Statement - Consolidated Statements of Stockholders' Equity (Parenthetical)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ConsolidatedStatementsOfStockholdersEquityParenthetical</Role>
      <ShortName>Consolidated Statements of Stockholders' Equity (Parenthetical)</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>7</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>1006000 - Statement - Consolidated Statements of Cashflows</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ConsolidatedStatementsOfCashflows</Role>
      <ShortName>Consolidated Statements of Cashflows</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>2101100 - Disclosure - Organization</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/Organization</Role>
      <ShortName>Organization</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>2102100 - Disclosure - Summary of Significant Accounting Policies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/SummaryOfSignificantAccountingPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>2103100 - Disclosure - Adoption of Accounting Pronouncements Adoption of Accounting Pronouncements (Notes)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.selective.com/role/AdoptionOfAccountingPronouncementsAdoptionOfAccountingPronouncementsNotes</Role>
      <ShortName>Adoption of Accounting Pronouncements Adoption of Accounting Pronouncements (Notes)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>11</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>2105100 - Disclosure - Statements of Cash Flow</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/StatementsOfCashFlow</Role>
      <ShortName>Statements of Cash Flow</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>2108100 - Disclosure - Investments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/Investments</Role>
      <ShortName>Investments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>2111100 - Disclosure - Comprehensive Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ComprehensiveIncome</Role>
      <ShortName>Comprehensive Income</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>14</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>2112100 - Disclosure - Fair Value Measurements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/FairValueMeasurements</Role>
      <ShortName>Fair Value Measurements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>2113100 - Disclosure - Reinsurance</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/Reinsurance</Role>
      <ShortName>Reinsurance</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>2115100 - Disclosure - Reserve for Loss and Loss Expense</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ReserveForLossAndLossExpense</Role>
      <ShortName>Reserve for Loss and Loss Expense</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>2116100 - Disclosure - Indebtedness</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/Indebtedness</Role>
      <ShortName>Indebtedness</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>2117100 - Disclosure - Segment Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/SegmentInformation</Role>
      <ShortName>Segment Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>2121100 - Disclosure - Earnings Per Share</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/EarningsPerShare</Role>
      <ShortName>Earnings Per Share</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>2126100 - Disclosure - Federal Income Taxes</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/FederalIncomeTaxes</Role>
      <ShortName>Federal Income Taxes</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>21</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>2127100 - Disclosure - Retirement Plans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/RetirementPlans</Role>
      <ShortName>Retirement Plans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>22</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>2128100 - Disclosure - Share-Based Payments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ShareBasedPayments</Role>
      <ShortName>Share-Based Payments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>23</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>2129100 - Disclosure - Related Party Transactions</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/RelatedPartyTransactions</Role>
      <ShortName>Related Party Transactions</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>24</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>2130100 - Disclosure - Commitments and Contingencies</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/CommitmentsAndContingencies</Role>
      <ShortName>Commitments and Contingencies</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>25</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>2131100 - Disclosure - Litigation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/Litigation</Role>
      <ShortName>Litigation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>26</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>2133100 - Disclosure - Statutory Financial Information, Capital Requirements, and Restrictions on Dividends and Transfers of Funds</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/StatutoryFinancialInformationCapitalRequirementsAndRestrictionsOnDividendsAndTransfersOfFunds</Role>
      <ShortName>Statutory Financial Information, Capital Requirements, and Restrictions on Dividends and Transfers of Funds</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>27</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>2136100 - Disclosure - Quarterly Financial Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/QuarterlyFinancialInformation</Role>
      <ShortName>Quarterly Financial Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>28</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>2137100 - Disclosure - Subsequent Events (Notes)</LongName>
      <ReportType>Notes</ReportType>
      <Role>http://www.selective.com/role/SubsequentEventsNotes</Role>
      <ShortName>Subsequent Events (Notes)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>29</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>2138100 - Disclosure - Schedule I - Summay of Investments - Other Than Investments in Related Parties</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ScheduleISummayOfInvestmentsOtherThanInvestmentsInRelatedParties</Role>
      <ShortName>Schedule I - Summay of Investments - Other Than Investments in Related Parties</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>30</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>2139100 - Disclosure - Schedule II - Parent Corporation Finanical Statements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ScheduleIiParentCorporationFinanicalStatements</Role>
      <ShortName>Schedule II - Parent Corporation Finanical Statements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>31</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>2140100 - Disclosure - Schedule III - Supplementary Insurance Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ScheduleIiiSupplementaryInsuranceInformation</Role>
      <ShortName>Schedule III - Supplementary Insurance Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>32</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>2141100 - Disclosure - Schedule IV - Reinsurance</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ScheduleIvReinsurance</Role>
      <ShortName>Schedule IV - Reinsurance</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>33</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>2142100 - Disclosure - Schedule V - Allowance for Uncollectible Premiums and Other Receivables</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ScheduleVAllowanceForUncollectiblePremiumsAndOtherReceivables</Role>
      <ShortName>Schedule V - Allowance for Uncollectible Premiums and Other Receivables</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>34</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>2202201 - Disclosure - Summary of Significant Accounting Policies (Policies)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/SummaryOfSignificantAccountingPoliciesPolicies</Role>
      <ShortName>Summary of Significant Accounting Policies (Policies)</ShortName>
      <MenuCategory>Policies</MenuCategory>
      <ParentRole>http://www.selective.com/role/SummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>35</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>2302302 - Disclosure - Summary of Significant Accounting Policies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/SummaryOfSignificantAccountingPoliciesTables</Role>
      <ShortName>Summary of Significant Accounting Policies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selective.com/role/SummaryOfSignificantAccountingPolicies</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>2305301 - Disclosure - Statements of Cash Flow (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/StatementsOfCashFlowTables</Role>
      <ShortName>Statements of Cash Flow (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selective.com/role/StatementsOfCashFlow</ParentRole>
      <Position>37</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>2308301 - Disclosure - Investments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/InvestmentsTables</Role>
      <ShortName>Investments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selective.com/role/Investments</ParentRole>
      <Position>38</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>2311301 - Disclosure - Comprehensive Income (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ComprehensiveIncomeTables</Role>
      <ShortName>Comprehensive Income (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selective.com/role/ComprehensiveIncome</ParentRole>
      <Position>39</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>2312301 - Disclosure - Fair Value Measurements (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/FairValueMeasurementsTables</Role>
      <ShortName>Fair Value Measurements (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selective.com/role/FairValueMeasurements</ParentRole>
      <Position>40</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>2313301 - Disclosure - Reinsurance (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ReinsuranceTables</Role>
      <ShortName>Reinsurance (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selective.com/role/Reinsurance</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>2315301 - Disclosure - Reserve for Loss and Loss Expense (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ReserveForLossAndLossExpenseTables</Role>
      <ShortName>Reserve for Loss and Loss Expense (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selective.com/role/ReserveForLossAndLossExpense</ParentRole>
      <Position>42</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>2316301 - Disclosure - Indebtedness (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/IndebtednessTables</Role>
      <ShortName>Indebtedness (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selective.com/role/Indebtedness</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>2317301 - Disclosure - Segment Information (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/SegmentInformationTables</Role>
      <ShortName>Segment Information (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selective.com/role/SegmentInformation</ParentRole>
      <Position>44</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>2321301 - Disclosure - Earnings Per Share (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/EarningsPerShareTables</Role>
      <ShortName>Earnings Per Share (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selective.com/role/EarningsPerShare</ParentRole>
      <Position>45</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>2326301 - Disclosure - Federal Income Taxes (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/FederalIncomeTaxesTables</Role>
      <ShortName>Federal Income Taxes (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selective.com/role/FederalIncomeTaxes</ParentRole>
      <Position>46</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>2327301 - Disclosure - Retirement Plans (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/RetirementPlansTables</Role>
      <ShortName>Retirement Plans (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selective.com/role/RetirementPlans</ParentRole>
      <Position>47</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>2328301 - Disclosure - Share-Based Payments (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ShareBasedPaymentsTables</Role>
      <ShortName>Share-Based Payments (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selective.com/role/ShareBasedPayments</ParentRole>
      <Position>48</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>2330301 - Disclosure - Commitments and Contingencies (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/CommitmentsAndContingenciesTables</Role>
      <ShortName>Commitments and Contingencies (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selective.com/role/CommitmentsAndContingencies</ParentRole>
      <Position>49</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>2333301 - Disclosure - Statutory Financial Information, Capital Requirements, and Restrictions on Dividends and Transfers of Funds (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/StatutoryFinancialInformationCapitalRequirementsAndRestrictionsOnDividendsAndTransfersOfFundsTables</Role>
      <ShortName>Statutory Financial Information, Capital Requirements, and Restrictions on Dividends and Transfers of Funds (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selective.com/role/StatutoryFinancialInformationCapitalRequirementsAndRestrictionsOnDividendsAndTransfersOfFunds</ParentRole>
      <Position>50</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>2336301 - Disclosure - Quarterly Financial Information (Tables)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/QuarterlyFinancialInformationTables</Role>
      <ShortName>Quarterly Financial Information (Tables)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selective.com/role/QuarterlyFinancialInformation</ParentRole>
      <Position>51</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>2402403 - Disclosure - Summary of Significant Accounting Policies (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/SummaryOfSignificantAccountingPoliciesNarrativeDetails</Role>
      <ShortName>Summary of Significant Accounting Policies (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/SummaryOfSignificantAccountingPoliciesTables</ParentRole>
      <Position>52</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>2402404 - Disclosure - Summary of Significant Accounting Policies (Property and Equipment) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/SummaryOfSignificantAccountingPoliciesPropertyAndEquipmentDetails</Role>
      <ShortName>Summary of Significant Accounting Policies (Property and Equipment) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/SummaryOfSignificantAccountingPoliciesTables</ParentRole>
      <Position>53</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>2403401 - Disclosure - Adoption of Accounting Pronouncements(Other Information) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/AdoptionOfAccountingPronouncementsotherInformationDetails</Role>
      <ShortName>Adoption of Accounting Pronouncements(Other Information) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>54</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>2405402 - Disclosure - Statements of Cash Flow (Cash Flow Supplemental Disclosures) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/StatementsOfCashFlowCashFlowSupplementalDisclosuresDetails</Role>
      <ShortName>Statements of Cash Flow (Cash Flow Supplemental Disclosures) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/StatementsOfCashFlowTables</ParentRole>
      <Position>55</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>2408402 - Disclosure - Investments (Unrealized Gains on Investments) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/InvestmentsUnrealizedGainsOnInvestmentsDetails</Role>
      <ShortName>Investments (Unrealized Gains on Investments) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/InvestmentsTables</ParentRole>
      <Position>56</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>2408403 - Disclosure - Investments (Held-To-Maturity Securities Disclosure) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/InvestmentsHeldToMaturitySecuritiesDisclosureDetails</Role>
      <ShortName>Investments (Held-To-Maturity Securities Disclosure) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/InvestmentsTables</ParentRole>
      <Position>57</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>2408404 - Disclosure - Investments (Available-For-Sale Securities Disclosure) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/InvestmentsAvailableForSaleSecuritiesDisclosureDetails</Role>
      <ShortName>Investments (Available-For-Sale Securities Disclosure) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/InvestmentsTables</ParentRole>
      <Position>58</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>2408405 - Disclosure - Investments (Number of Securities in an Unrealized Unrecognized Loss Position) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/InvestmentsNumberOfSecuritiesInUnrealizedUnrecognizedLossPositionDetails</Role>
      <ShortName>Investments (Number of Securities in an Unrealized Unrecognized Loss Position) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/InvestmentsTables</ParentRole>
      <Position>59</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R60.htm</HtmlFileName>
      <LongName>2408406 - Disclosure - Investments (Fixed Maturity Securities by Contractual Maturity) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/InvestmentsFixedMaturitySecuritiesByContractualMaturityDetails</Role>
      <ShortName>Investments (Fixed Maturity Securities by Contractual Maturity) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/InvestmentsTables</ParentRole>
      <Position>60</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R61.htm</HtmlFileName>
      <LongName>2408407 - Disclosure - Investments (Other Investment Portfolio by Strategy and the Remaining Commitment Amount Associated With Each Strategy) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/InvestmentsOtherInvestmentPortfolioByStrategyAndRemainingCommitmentAmountAssociatedWithEachStrategyDetails</Role>
      <ShortName>Investments (Other Investment Portfolio by Strategy and the Remaining Commitment Amount Associated With Each Strategy) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/InvestmentsTables</ParentRole>
      <Position>61</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R62.htm</HtmlFileName>
      <LongName>2408408 - Disclosure - Investments (Aggregated Balance Sheet Summarized Financial Information for Partnerships in our Alternative Investment Portfolio) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/InvestmentsAggregatedBalanceSheetSummarizedFinancialInformationForPartnershipsInOurAlternativeInvestmentPortfolioDetails</Role>
      <ShortName>Investments (Aggregated Balance Sheet Summarized Financial Information for Partnerships in our Alternative Investment Portfolio) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/InvestmentsTables</ParentRole>
      <Position>62</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R63.htm</HtmlFileName>
      <LongName>2408409 - Disclosure - Investments (Aggregated Income Statement Summarized Financial Information for Partnerships in our Alternative Investment Portfolio) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/InvestmentsAggregatedIncomeStatementSummarizedFinancialInformationForPartnershipsInOurAlternativeInvestmentPortfolioDetails</Role>
      <ShortName>Investments (Aggregated Income Statement Summarized Financial Information for Partnerships in our Alternative Investment Portfolio) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/InvestmentsTables</ParentRole>
      <Position>63</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R64.htm</HtmlFileName>
      <LongName>2408410 - Disclosure - Investments (Credit Concentration Risk) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/InvestmentsCreditConcentrationRiskDetails</Role>
      <ShortName>Investments (Credit Concentration Risk) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/InvestmentsTables</ParentRole>
      <Position>64</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R65.htm</HtmlFileName>
      <LongName>2408411 - Disclosure - Investments (Investments Pledged as Collateral) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/InvestmentsInvestmentsPledgedAsCollateralDetails</Role>
      <ShortName>Investments (Investments Pledged as Collateral) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/InvestmentsTables</ParentRole>
      <Position>65</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R66.htm</HtmlFileName>
      <LongName>2408412 - Disclosure - Investments (Net Investment Income) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/InvestmentsNetInvestmentIncomeDetails</Role>
      <ShortName>Investments (Net Investment Income) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/InvestmentsTables</ParentRole>
      <Position>66</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R67.htm</HtmlFileName>
      <LongName>2408413 - Disclosure - Investments (OTTI by Asset Type) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/InvestmentsOttiByAssetTypeDetails</Role>
      <ShortName>Investments (OTTI by Asset Type) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/InvestmentsTables</ParentRole>
      <Position>67</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R68.htm</HtmlFileName>
      <LongName>2408414 - Disclosure - Investments (Components Of Net Realized Gains, Excluding OTTI Charges) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/InvestmentsComponentsOfNetRealizedGainsExcludingOttiChargesDetails</Role>
      <ShortName>Investments (Components Of Net Realized Gains, Excluding OTTI Charges) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/InvestmentsTables</ParentRole>
      <Position>68</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R69.htm</HtmlFileName>
      <LongName>2411402 - Disclosure - Comprehensive Income (Components of Comprehensive Income-Gross and Net of Tax) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ComprehensiveIncomeComponentsOfComprehensiveIncomeGrossAndNetOfTaxDetails</Role>
      <ShortName>Comprehensive Income (Components of Comprehensive Income-Gross and Net of Tax) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/ComprehensiveIncomeTables</ParentRole>
      <Position>69</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R70.htm</HtmlFileName>
      <LongName>2411403 - Disclosure - Comprehensive Income (Components of Accumulated Other Comprehensive Income) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ComprehensiveIncomeComponentsOfAccumulatedOtherComprehensiveIncomeDetails</Role>
      <ShortName>Comprehensive Income (Components of Accumulated Other Comprehensive Income) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/ComprehensiveIncomeTables</ParentRole>
      <Position>70</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R71.htm</HtmlFileName>
      <LongName>2411404 - Disclosure - Comprehensive Income (Reclassification out of AOCI) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ComprehensiveIncomeReclassificationOutOfAociDetails</Role>
      <ShortName>Comprehensive Income (Reclassification out of AOCI) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/ComprehensiveIncomeTables</ParentRole>
      <Position>71</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R72.htm</HtmlFileName>
      <LongName>2412402 - Disclosure - Fair Value Measurements (Carrying Value and Estimated Fair Value of Financial Instruments) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/FairValueMeasurementsCarryingValueAndEstimatedFairValueOfFinancialInstrumentsDetails</Role>
      <ShortName>Fair Value Measurements (Carrying Value and Estimated Fair Value of Financial Instruments) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/FairValueMeasurementsTables</ParentRole>
      <Position>72</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R73.htm</HtmlFileName>
      <LongName>2412403 - Disclosure - Fair Value Measurements (Quantitative Disclosures at Fair Value Assets) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/FairValueMeasurementsQuantitativeDisclosuresAtFairValueAssetsDetails</Role>
      <ShortName>Fair Value Measurements (Quantitative Disclosures at Fair Value Assets) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/FairValueMeasurementsTables</ParentRole>
      <Position>73</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R74.htm</HtmlFileName>
      <LongName>2412404 - Disclosure - Fair Value Measurements (Changes in Fair Value of Securities Using Level 3 Inputs) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/FairValueMeasurementsChangesInFairValueOfSecuritiesUsingLevel3InputsDetails</Role>
      <ShortName>Fair Value Measurements (Changes in Fair Value of Securities Using Level 3 Inputs) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/FairValueMeasurementsTables</ParentRole>
      <Position>74</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R75.htm</HtmlFileName>
      <LongName>2412405 - Disclosure - Fair Value Measurements (Quantitative Information of Our Financial Assets and Liabilities That Were Disclosed at Fair Value) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/FairValueMeasurementsQuantitativeInformationOfOurFinancialAssetsAndLiabilitiesThatWereDisclosedAtFairValueDetails</Role>
      <ShortName>Fair Value Measurements (Quantitative Information of Our Financial Assets and Liabilities That Were Disclosed at Fair Value) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/FairValueMeasurementsTables</ParentRole>
      <Position>75</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R76.htm</HtmlFileName>
      <LongName>2413402 - Disclosure - Reinsurance (Total Reinsurance Balances Segregated By Reinsurer) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ReinsuranceTotalReinsuranceBalancesSegregatedByReinsurerDetails</Role>
      <ShortName>Reinsurance (Total Reinsurance Balances Segregated By Reinsurer) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/ReinsuranceTables</ParentRole>
      <Position>76</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R77.htm</HtmlFileName>
      <LongName>2413403 - Disclosure - Reinsurance (List of direct, assumed and ceded Reinsurance Amounts) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ReinsuranceListOfDirectAssumedAndCededReinsuranceAmountsDetails</Role>
      <ShortName>Reinsurance (List of direct, assumed and ceded Reinsurance Amounts) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/ReinsuranceTables</ParentRole>
      <Position>77</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R78.htm</HtmlFileName>
      <LongName>2413404 - Disclosure - Reinsurance (Ceded Premiums and Losses Related to Flood Operations) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ReinsuranceCededPremiumsAndLossesRelatedToFloodOperationsDetails</Role>
      <ShortName>Reinsurance (Ceded Premiums and Losses Related to Flood Operations) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/ReinsuranceTables</ParentRole>
      <Position>78</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R79.htm</HtmlFileName>
      <LongName>2415402 - Disclosure - Reserve for Loss and Loss Expense (Rollforward of Reserve for Loss and Loss Expense) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ReserveForLossAndLossExpenseRollforwardOfReserveForLossAndLossExpenseDetails</Role>
      <ShortName>Reserve for Loss and Loss Expense (Rollforward of Reserve for Loss and Loss Expense) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/ReserveForLossAndLossExpenseTables</ParentRole>
      <Position>79</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R80.htm</HtmlFileName>
      <LongName>2415403 - Disclosure - Reserve for Loss and Loss Expense (Reserve Loss Development) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ReserveForLossAndLossExpenseReserveLossDevelopmentDetails</Role>
      <ShortName>Reserve for Loss and Loss Expense (Reserve Loss Development) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/ReserveForLossAndLossExpenseTables</ParentRole>
      <Position>80</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R81.htm</HtmlFileName>
      <LongName>2415404 - Disclosure - Reserve for Loss and Loss Expense (Exposure to Environmental Claims) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ReserveForLossAndLossExpenseExposureToEnvironmentalClaimsDetails</Role>
      <ShortName>Reserve for Loss and Loss Expense (Exposure to Environmental Claims) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/ReserveForLossAndLossExpenseTables</ParentRole>
      <Position>81</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R82.htm</HtmlFileName>
      <LongName>2415405 - Disclosure - Reserve for Loss and Loss Expense (Roll Forward of Gross and Net Asbestos and Net Environmental Incurred Losses and Loss Expenses and Related Reserves) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ReserveForLossAndLossExpenseRollForwardOfGrossAndNetAsbestosAndNetEnvironmentalIncurredLossesAndLossExpensesAndRelatedReservesDetails</Role>
      <ShortName>Reserve for Loss and Loss Expense (Roll Forward of Gross and Net Asbestos and Net Environmental Incurred Losses and Loss Expenses and Related Reserves) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/ReserveForLossAndLossExpenseTables</ParentRole>
      <Position>82</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R83.htm</HtmlFileName>
      <LongName>2415406 - Disclosure - Reserve for Loss and Loss Expense Reserve for Loss and Loss Expense (Claims Development) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ReserveForLossAndLossExpenseReserveForLossAndLossExpenseClaimsDevelopmentDetails</Role>
      <ShortName>Reserve for Loss and Loss Expense Reserve for Loss and Loss Expense (Claims Development) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>83</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R84.htm</HtmlFileName>
      <LongName>2415407 - Disclosure - Reserve for Loss and Loss Expense Reserve for Loss and Loss Expense (Reconciliation of Claims Development to Liability) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ReserveForLossAndLossExpenseReserveForLossAndLossExpenseReconciliationOfClaimsDevelopmentToLiabilityDetails</Role>
      <ShortName>Reserve for Loss and Loss Expense Reserve for Loss and Loss Expense (Reconciliation of Claims Development to Liability) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>84</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R85.htm</HtmlFileName>
      <LongName>2415408 - Disclosure - Reserve for Loss and Loss Expense Reserve for Loss and Loss Expense (Historical Claims Duration) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ReserveForLossAndLossExpenseReserveForLossAndLossExpenseHistoricalClaimsDurationDetails</Role>
      <ShortName>Reserve for Loss and Loss Expense Reserve for Loss and Loss Expense (Historical Claims Duration) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>85</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R86.htm</HtmlFileName>
      <LongName>2416402 - Disclosure - Indebtedness Schedule of Long-Term Debt (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/IndebtednessScheduleOfLongTermDebtDetails</Role>
      <ShortName>Indebtedness Schedule of Long-Term Debt (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>86</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R87.htm</HtmlFileName>
      <LongName>2416403 - Disclosure - Indebtedness (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/IndebtednessNarrativeDetails</Role>
      <ShortName>Indebtedness (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/IndebtednessTables</ParentRole>
      <Position>87</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R88.htm</HtmlFileName>
      <LongName>2416404 - Disclosure - Indebtedness (Covenants in the Line of Credit) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/IndebtednessCovenantsInLineOfCreditDetails</Role>
      <ShortName>Indebtedness (Covenants in the Line of Credit) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/IndebtednessTables</ParentRole>
      <Position>88</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R89.htm</HtmlFileName>
      <LongName>2417402 - Disclosure - Segment Information (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/SegmentInformationNarrativeDetails</Role>
      <ShortName>Segment Information (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/SegmentInformationTables</ParentRole>
      <Position>89</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R90.htm</HtmlFileName>
      <LongName>2417403 - Disclosure - Segment Information (Revenue From Continuing Operations by Segment) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/SegmentInformationRevenueFromContinuingOperationsBySegmentDetails</Role>
      <ShortName>Segment Information (Revenue From Continuing Operations by Segment) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/SegmentInformationTables</ParentRole>
      <Position>90</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R91.htm</HtmlFileName>
      <LongName>2417404 - Disclosure - Segment Information (Income From Continuing Operations, Before Federal Income Tax) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/SegmentInformationIncomeFromContinuingOperationsBeforeFederalIncomeTaxDetails</Role>
      <ShortName>Segment Information (Income From Continuing Operations, Before Federal Income Tax) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/SegmentInformationTables</ParentRole>
      <Position>91</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R92.htm</HtmlFileName>
      <LongName>2417405 - Disclosure - Segment Information (Reconciliation of Segment Results to Income from Continuing Operations, before Federal Income Tax (Details))</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/SegmentInformationReconciliationOfSegmentResultsToIncomeFromContinuingOperationsBeforeFederalIncomeTaxDetails</Role>
      <ShortName>Segment Information (Reconciliation of Segment Results to Income from Continuing Operations, before Federal Income Tax (Details))</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/SegmentInformationTables</ParentRole>
      <Position>92</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R93.htm</HtmlFileName>
      <LongName>2421402 - Disclosure - Earnings Per Share (Reconciliation of EPS) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/EarningsPerShareReconciliationOfEpsDetails</Role>
      <ShortName>Earnings Per Share (Reconciliation of EPS) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/EarningsPerShareTables</ParentRole>
      <Position>93</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R94.htm</HtmlFileName>
      <LongName>2426402 - Disclosure - Federal Income Taxes (Reconciliation of Federal Income Tax on Income at the Corporate Rate to the Effective Tax Rate) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/FederalIncomeTaxesReconciliationOfFederalIncomeTaxOnIncomeAtCorporateRateToEffectiveTaxRateDetails</Role>
      <ShortName>Federal Income Taxes (Reconciliation of Federal Income Tax on Income at the Corporate Rate to the Effective Tax Rate) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/FederalIncomeTaxesTables</ParentRole>
      <Position>94</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R95.htm</HtmlFileName>
      <LongName>2426403 - Disclosure - Federal Income Taxes (Deferred Tax Assets and Liabilities) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/FederalIncomeTaxesDeferredTaxAssetsAndLiabilitiesDetails</Role>
      <ShortName>Federal Income Taxes (Deferred Tax Assets and Liabilities) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/FederalIncomeTaxesTables</ParentRole>
      <Position>95</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R96.htm</HtmlFileName>
      <LongName>2426404 - Disclosure - Federal Income Tax (Federal Tax Net Operating Loss Carryforwards) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/FederalIncomeTaxFederalTaxNetOperatingLossCarryforwardsDetails</Role>
      <ShortName>Federal Income Tax (Federal Tax Net Operating Loss Carryforwards) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/FederalIncomeTaxesTables</ParentRole>
      <Position>96</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R97.htm</HtmlFileName>
      <LongName>2427402 - Disclosure - Retirement Plans (Retirement Savings Plan) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/RetirementPlansRetirementSavingsPlanDetails</Role>
      <ShortName>Retirement Plans (Retirement Savings Plan) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/RetirementPlansTables</ParentRole>
      <Position>97</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R98.htm</HtmlFileName>
      <LongName>2427403 - Disclosure - Retirement Plans (Deferred Compensation Plan) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/RetirementPlansDeferredCompensationPlanDetails</Role>
      <ShortName>Retirement Plans (Deferred Compensation Plan) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/RetirementPlansTables</ParentRole>
      <Position>98</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R99.htm</HtmlFileName>
      <LongName>2427404 - Disclosure - Retirement Plans (Funded Status of Retirement Income Plan and Retirement Life Plan) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/RetirementPlansFundedStatusOfRetirementIncomePlanAndRetirementLifePlanDetails</Role>
      <ShortName>Retirement Plans (Funded Status of Retirement Income Plan and Retirement Life Plan) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/RetirementPlansTables</ParentRole>
      <Position>99</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R100.htm</HtmlFileName>
      <LongName>2427405 - Disclosure - Retirement Plans (Components of Net Periodic Benefit Cost and Other Amounts Recognized in Other Comprehensive Income) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/RetirementPlansComponentsOfNetPeriodicBenefitCostAndOtherAmountsRecognizedInOtherComprehensiveIncomeDetails</Role>
      <ShortName>Retirement Plans (Components of Net Periodic Benefit Cost and Other Amounts Recognized in Other Comprehensive Income) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/RetirementPlansTables</ParentRole>
      <Position>100</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R101.htm</HtmlFileName>
      <LongName>2427406 - Disclosure - Retirement Plans (Weighted-Average Expense Assumptions) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/RetirementPlansWeightedAverageExpenseAssumptionsDetails</Role>
      <ShortName>Retirement Plans (Weighted-Average Expense Assumptions) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/RetirementPlansTables</ParentRole>
      <Position>101</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R102.htm</HtmlFileName>
      <LongName>2427407 - Disclosure - Retirement Plans (Plan's Allocated Target Ranges and Actual Weighted Average Asset Allocation by Investment Categories) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/RetirementPlansPlansAllocatedTargetRangesAndActualWeightedAverageAssetAllocationByInvestmentCategoriesDetails</Role>
      <ShortName>Retirement Plans (Plan's Allocated Target Ranges and Actual Weighted Average Asset Allocation by Investment Categories) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/RetirementPlansTables</ParentRole>
      <Position>102</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R103.htm</HtmlFileName>
      <LongName>2427408 - Disclosure - Retirement Plans (Quantitative Disclosures of Retirement Income Plan's Invested Assets that are Measured at Fair Value on a Recurring Basis) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/RetirementPlansQuantitativeDisclosuresOfRetirementIncomePlansInvestedAssetsThatAreMeasuredAtFairValueOnRecurringBasisDetails</Role>
      <ShortName>Retirement Plans (Quantitative Disclosures of Retirement Income Plan's Invested Assets that are Measured at Fair Value on a Recurring Basis) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/RetirementPlansTables</ParentRole>
      <Position>103</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R104.htm</HtmlFileName>
      <LongName>2427409 - Disclosure - Retirement Plans (Benefit Payments) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/RetirementPlansBenefitPaymentsDetails</Role>
      <ShortName>Retirement Plans (Benefit Payments) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/RetirementPlansTables</ParentRole>
      <Position>104</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R105.htm</HtmlFileName>
      <LongName>2428403 - Disclosure - Share-Based Payments (Summary of Shares Available for Issuance) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ShareBasedPaymentsSummaryOfSharesAvailableForIssuanceDetails</Role>
      <ShortName>Share-Based Payments (Summary of Shares Available for Issuance) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/ShareBasedPaymentsTables</ParentRole>
      <Position>105</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R106.htm</HtmlFileName>
      <LongName>2428404 - Disclosure - Shared-Based Payments (Summary of Retired Plans) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/SharedBasedPaymentsSummaryOfRetiredPlansDetails</Role>
      <ShortName>Shared-Based Payments (Summary of Retired Plans) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>106</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R107.htm</HtmlFileName>
      <LongName>2428405 - Disclosure - Share-Based Payments (Summary of the Restricted Stock Unit Transactions under Share-Based Payment Plans) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ShareBasedPaymentsSummaryOfRestrictedStockUnitTransactionsUnderShareBasedPaymentPlansDetails</Role>
      <ShortName>Share-Based Payments (Summary of the Restricted Stock Unit Transactions under Share-Based Payment Plans) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/ShareBasedPaymentsTables</ParentRole>
      <Position>107</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R108.htm</HtmlFileName>
      <LongName>2428406 - Disclosure - Share-Based Payments (Summary of Stock Option Transactions under Share-Based Payment Plans) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ShareBasedPaymentsSummaryOfStockOptionTransactionsUnderShareBasedPaymentPlansDetails</Role>
      <ShortName>Share-Based Payments (Summary of Stock Option Transactions under Share-Based Payment Plans) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/ShareBasedPaymentsTables</ParentRole>
      <Position>108</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R109.htm</HtmlFileName>
      <LongName>2428407 - Disclosure - Shared-Based Payments (Summary of ESPP and ASPP Shares Issued) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/SharedBasedPaymentsSummaryOfEsppAndAsppSharesIssuedDetails</Role>
      <ShortName>Shared-Based Payments (Summary of ESPP and ASPP Shares Issued) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>109</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R110.htm</HtmlFileName>
      <LongName>2428408 - Disclosure - Share-Based Payments (Weighted Average Assumptions for Employee Stock Purchase Plan and Other Options) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ShareBasedPaymentsWeightedAverageAssumptionsForEmployeeStockPurchasePlanAndOtherOptionsDetails</Role>
      <ShortName>Share-Based Payments (Weighted Average Assumptions for Employee Stock Purchase Plan and Other Options) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/ShareBasedPaymentsTables</ParentRole>
      <Position>110</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R111.htm</HtmlFileName>
      <LongName>2428409 - Disclosure - Share-Based Payments (Weighted-Average Fair Value of Options and Stock Per Share) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ShareBasedPaymentsWeightedAverageFairValueOfOptionsAndStockPerShareDetails</Role>
      <ShortName>Share-Based Payments (Weighted-Average Fair Value of Options and Stock Per Share) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/ShareBasedPaymentsTables</ParentRole>
      <Position>111</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R112.htm</HtmlFileName>
      <LongName>2428410 - Disclosure - Shared-Based Payments (Summary of Share-Based Compensation Expense) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/SharedBasedPaymentsSummaryOfShareBasedCompensationExpenseDetails</Role>
      <ShortName>Shared-Based Payments (Summary of Share-Based Compensation Expense) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>112</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R113.htm</HtmlFileName>
      <LongName>2429401 - Disclosure - Related Party Transactions (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/RelatedPartyTransactionsNarrativeDetails</Role>
      <ShortName>Related Party Transactions (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/RelatedPartyTransactions</ParentRole>
      <Position>113</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R114.htm</HtmlFileName>
      <LongName>2430402 - Disclosure - Commitments and Contingencies (Narrative) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/CommitmentsAndContingenciesNarrativeDetails</Role>
      <ShortName>Commitments and Contingencies (Narrative) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/CommitmentsAndContingenciesTables</ParentRole>
      <Position>114</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R115.htm</HtmlFileName>
      <LongName>2430403 - Disclosure - Commitments and Contingencies (Future Minimum Rental Commitments) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/CommitmentsAndContingenciesFutureMinimumRentalCommitmentsDetails</Role>
      <ShortName>Commitments and Contingencies (Future Minimum Rental Commitments) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/CommitmentsAndContingenciesTables</ParentRole>
      <Position>115</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R116.htm</HtmlFileName>
      <LongName>2433402 - Disclosure - Statutory Financial Information, Capital Requirements and Restrictions on Dividends and Transfers of Funds (Statutory Financial Information) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/StatutoryFinancialInformationCapitalRequirementsAndRestrictionsOnDividendsAndTransfersOfFundsStatutoryFinancialInformationDetails</Role>
      <ShortName>Statutory Financial Information, Capital Requirements and Restrictions on Dividends and Transfers of Funds (Statutory Financial Information) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>116</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R117.htm</HtmlFileName>
      <LongName>2433403 - Disclosure - Statutory Financial Information, Capital Requirements, and Restrictions on Dividends and Transfers of Funds (Dividends) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/StatutoryFinancialInformationCapitalRequirementsAndRestrictionsOnDividendsAndTransfersOfFundsDividendsDetails</Role>
      <ShortName>Statutory Financial Information, Capital Requirements, and Restrictions on Dividends and Transfers of Funds (Dividends) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/StatutoryFinancialInformationCapitalRequirementsAndRestrictionsOnDividendsAndTransfersOfFundsTables</ParentRole>
      <Position>117</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R118.htm</HtmlFileName>
      <LongName>2436402 - Disclosure - Quarterly Financial Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/QuarterlyFinancialInformationDetails</Role>
      <ShortName>Quarterly Financial Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/QuarterlyFinancialInformationTables</ParentRole>
      <Position>118</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R119.htm</HtmlFileName>
      <LongName>2437402 - Disclosure - Subsequent Events (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/SubsequentEventsDetails</Role>
      <ShortName>Subsequent Events (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/SubsequentEventsNotes</ParentRole>
      <Position>119</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R120.htm</HtmlFileName>
      <LongName>2438401 - Disclosure - Schedule I - Summay of Investments - Other Than Investments in Related Parties (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ScheduleISummayOfInvestmentsOtherThanInvestmentsInRelatedPartiesDetails</Role>
      <ShortName>Schedule I - Summay of Investments - Other Than Investments in Related Parties (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/ScheduleISummayOfInvestmentsOtherThanInvestmentsInRelatedParties</ParentRole>
      <Position>120</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R121.htm</HtmlFileName>
      <LongName>2439401 - Schedule - Parent Corporation Balance Sheet (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ParentCorporationBalanceSheetDetails</Role>
      <ShortName>Parent Corporation Balance Sheet (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>121</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R122.htm</HtmlFileName>
      <LongName>2439402 - Disclosure - Parent Corporation Balance Sheets (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ParentCorporationBalanceSheetsDetails</Role>
      <ShortName>Parent Corporation Balance Sheets (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>122</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R123.htm</HtmlFileName>
      <LongName>2439403 - Schedule - Parent Corporation Statements of Income (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ParentCorporationStatementsOfIncomeDetails</Role>
      <ShortName>Parent Corporation Statements of Income (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>123</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R124.htm</HtmlFileName>
      <LongName>2439404 - Schedule - Parent Corporation Statements of Cash Flows (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ParentCorporationStatementsOfCashFlowsDetails</Role>
      <ShortName>Parent Corporation Statements of Cash Flows (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>124</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R125.htm</HtmlFileName>
      <LongName>2440401 - Disclosure - Schedule III - Supplementary Insurance Information (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ScheduleIiiSupplementaryInsuranceInformationDetails</Role>
      <ShortName>Schedule III - Supplementary Insurance Information (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/ScheduleIiiSupplementaryInsuranceInformation</ParentRole>
      <Position>125</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R126.htm</HtmlFileName>
      <LongName>2441401 - Disclosure - Schedule IV - Reinsurance (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ScheduleIvReinsuranceDetails</Role>
      <ShortName>Schedule IV - Reinsurance (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/ScheduleIvReinsurance</ParentRole>
      <Position>126</Position>
    </Report>
    <Report instance="sigi-20171231.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R127.htm</HtmlFileName>
      <LongName>2442401 - Disclosure - Schedule V -  Allowance for Uncollectible Premiums and Other Receivables (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ScheduleVAllowanceForUncollectiblePremiumsAndOtherReceivablesDetails</Role>
      <ShortName>Schedule V -  Allowance for Uncollectible Premiums and Other Receivables (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>127</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <InputFiles>
    <File>sigi-20171231.xml</File>
    <File>sigi-20171231.xsd</File>
    <File>sigi-20171231_cal.xml</File>
    <File>sigi-20171231_def.xml</File>
    <File>sigi-20171231_lab.xml</File>
    <File>sigi-20171231_pre.xml</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies>
    <BaseTaxonomy>http://xbrl.sec.gov/dei/2014-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://fasb.org/us-gaap/2017-01-31</BaseTaxonomy>
    <BaseTaxonomy>http://xbrl.sec.gov/invest/2013-01-31</BaseTaxonomy>
  </BaseTaxonomies>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>
