<?xml version='1.0' encoding='UTF-8'?>
<FilingSummary>
  <Version>3.3.0.814</Version>
  <ProcessingTime/>
  <ReportFormat>html</ReportFormat>
  <ContextCount>525</ContextCount>
  <ElementCount>307</ElementCount>
  <EntityCount>1</EntityCount>
  <FootnotesReported>true</FootnotesReported>
  <SegmentCount>117</SegmentCount>
  <ScenarioCount>0</ScenarioCount>
  <TuplesReported>false</TuplesReported>
  <UnitCount>5</UnitCount>
  <MyReports>
    <Report instance="sigi-20150930.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-20150930.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-20150930.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-20150930.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-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R5.htm</HtmlFileName>
      <LongName>1003000 - 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-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R6.htm</HtmlFileName>
      <LongName>1004000 - 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-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R7.htm</HtmlFileName>
      <LongName>1004501 - 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-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R8.htm</HtmlFileName>
      <LongName>1005000 - Statement - Consolidated Statements of Cash Flow</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ConsolidatedStatementsOfCashFlow</Role>
      <ShortName>Consolidated Statements of Cash Flow</ShortName>
      <MenuCategory>Statements</MenuCategory>
      <Position>8</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R9.htm</HtmlFileName>
      <LongName>2101100 - Disclosure - Basis of Presentation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/BasisOfPresentation</Role>
      <ShortName>Basis of Presentation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>9</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R10.htm</HtmlFileName>
      <LongName>2103100 - Disclosure - Accounting Pronouncements</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/AccountingPronouncements</Role>
      <ShortName>Accounting Pronouncements</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>10</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R11.htm</HtmlFileName>
      <LongName>2104100 - 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>11</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R12.htm</HtmlFileName>
      <LongName>2105100 - Disclosure - Investments</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/Investments</Role>
      <ShortName>Investments</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>12</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R13.htm</HtmlFileName>
      <LongName>2106100 - Disclosure - Indebtedness</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/Indebtedness</Role>
      <ShortName>Indebtedness</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>13</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R14.htm</HtmlFileName>
      <LongName>2107100 - 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>14</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R15.htm</HtmlFileName>
      <LongName>2108100 - Disclosure - Reinsurance</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/Reinsurance</Role>
      <ShortName>Reinsurance</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>15</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R16.htm</HtmlFileName>
      <LongName>2110100 - Disclosure - Segment Information</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/SegmentInformation</Role>
      <ShortName>Segment Information</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>16</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R17.htm</HtmlFileName>
      <LongName>2111100 - Disclosure - Retirement Plans</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/RetirementPlans</Role>
      <ShortName>Retirement Plans</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>17</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R18.htm</HtmlFileName>
      <LongName>2113100 - Disclosure - Comprehensive Income</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/ComprehensiveIncome</Role>
      <ShortName>Comprehensive Income</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>18</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R19.htm</HtmlFileName>
      <LongName>2114100 - Disclosure - Litigation</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/Litigation</Role>
      <ShortName>Litigation</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>19</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R20.htm</HtmlFileName>
      <LongName>2115100 - Disclosure - Subsequent Events (Narrative)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/SubsequentEventsNarrative</Role>
      <ShortName>Subsequent Events (Narrative)</ShortName>
      <MenuCategory>Notes</MenuCategory>
      <Position>20</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R21.htm</HtmlFileName>
      <LongName>2304301 - 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>21</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R22.htm</HtmlFileName>
      <LongName>2305301 - 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>22</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R23.htm</HtmlFileName>
      <LongName>2307301 - 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>23</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R24.htm</HtmlFileName>
      <LongName>2308301 - 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>24</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R25.htm</HtmlFileName>
      <LongName>2310301 - 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>25</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R26.htm</HtmlFileName>
      <LongName>2311301 - Disclosure - Retirement Plans (Table)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/RetirementPlansTable</Role>
      <ShortName>Retirement Plans (Table)</ShortName>
      <MenuCategory>Tables</MenuCategory>
      <ParentRole>http://www.selective.com/role/RetirementPlans</ParentRole>
      <Position>26</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R27.htm</HtmlFileName>
      <LongName>2313301 - 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>27</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R28.htm</HtmlFileName>
      <LongName>2404402 - 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>28</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R29.htm</HtmlFileName>
      <LongName>2405402 - 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>29</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R30.htm</HtmlFileName>
      <LongName>2405403 - 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>30</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R31.htm</HtmlFileName>
      <LongName>2405404 - Disclosure - Investments (Fair Value and Gross Pre-Tax Net Unrealized Unrecognized Loss of Securities By Length of Time) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/InvestmentsFairValueAndGrossPreTaxNetUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeDetails</Role>
      <ShortName>Investments (Fair Value and Gross Pre-Tax Net Unrealized Unrecognized Loss of Securities By Length of Time) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/InvestmentsTables</ParentRole>
      <Position>31</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R32.htm</HtmlFileName>
      <LongName>2405405 - 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>32</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R33.htm</HtmlFileName>
      <LongName>2405406 - Disclosure - Investments (HTM Fixed Income Securities by Contractual Maturity) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/InvestmentsHtmFixedIncomeSecuritiesByContractualMaturityDetails</Role>
      <ShortName>Investments (HTM Fixed Income Securities by Contractual Maturity) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/InvestmentsTables</ParentRole>
      <Position>33</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R34.htm</HtmlFileName>
      <LongName>2405407 - Disclosure - Investments (AFS Fixed Income securities by Contractual Maturity) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/InvestmentsAfsFixedIncomeSecuritiesByContractualMaturityDetails</Role>
      <ShortName>Investments (AFS Fixed Income securities by Contractual Maturity) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/InvestmentsTables</ParentRole>
      <Position>34</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R35.htm</HtmlFileName>
      <LongName>2405408 - 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>35</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R36.htm</HtmlFileName>
      <LongName>2405409 - Disclosure - Investments (Aggregated Income Statement Summarized Financial Information for Partnerhips in our Alternative Investment Portfolio) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/InvestmentsAggregatedIncomeStatementSummarizedFinancialInformationForPartnerhipsInOurAlternativeInvestmentPortfolioDetails</Role>
      <ShortName>Investments (Aggregated Income Statement Summarized Financial Information for Partnerhips in our Alternative Investment Portfolio) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/InvestmentsTables</ParentRole>
      <Position>36</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R37.htm</HtmlFileName>
      <LongName>2405410 - 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>37</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R38.htm</HtmlFileName>
      <LongName>2405411 - 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>38</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R39.htm</HtmlFileName>
      <LongName>2405412 - 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>39</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R40.htm</HtmlFileName>
      <LongName>2405413 - 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>40</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R41.htm</HtmlFileName>
      <LongName>2405414 - Disclosure - Investments (Gross Credit Loss Impairments of Fixed Maturity Securities for Which a Portion of the OTTI Charge was Recognized in OCI) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/InvestmentsGrossCreditLossImpairmentsOfFixedMaturitySecuritiesForWhichPortionOfOttiChargeWasRecognizedInOciDetails</Role>
      <ShortName>Investments (Gross Credit Loss Impairments of Fixed Maturity Securities for Which a Portion of the OTTI Charge was Recognized in OCI) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/InvestmentsTables</ParentRole>
      <Position>41</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R42.htm</HtmlFileName>
      <LongName>2405415 - 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>42</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R43.htm</HtmlFileName>
      <LongName>2406401 - Disclosure - Indebtedness (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/IndebtednessDetails</Role>
      <ShortName>Indebtedness (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/Indebtedness</ParentRole>
      <Position>43</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R44.htm</HtmlFileName>
      <LongName>2407402 - 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>44</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R45.htm</HtmlFileName>
      <LongName>2407403 - Disclosure - Fair Value Measurements (Quantitative Disclosures of Fair Value Assets) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/FairValueMeasurementsQuantitativeDisclosuresOfFairValueAssetsDetails</Role>
      <ShortName>Fair Value Measurements (Quantitative Disclosures of Fair Value Assets) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/FairValueMeasurementsTables</ParentRole>
      <Position>45</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R46.htm</HtmlFileName>
      <LongName>2407404 - 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>46</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R47.htm</HtmlFileName>
      <LongName>2407405 - 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>47</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R48.htm</HtmlFileName>
      <LongName>2408402 - 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>48</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R49.htm</HtmlFileName>
      <LongName>2408403 - 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>49</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R50.htm</HtmlFileName>
      <LongName>2410402 - 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>
      <Position>50</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R51.htm</HtmlFileName>
      <LongName>2410403 - Disclosure - Segment Information (Revenue by Segment) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/SegmentInformationRevenueBySegmentDetails</Role>
      <ShortName>Segment Information (Revenue by Segment) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/SegmentInformationTables</ParentRole>
      <Position>51</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R52.htm</HtmlFileName>
      <LongName>2410404 - Disclosure - Segment Information (Income Before Federal Income Tax) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/SegmentInformationIncomeBeforeFederalIncomeTaxDetails</Role>
      <ShortName>Segment Information (Income Before Federal Income Tax) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/SegmentInformationTables</ParentRole>
      <Position>52</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R53.htm</HtmlFileName>
      <LongName>2410405 - Disclosure - Segment Information (Reconciliation of Segment Results to Income Before Federal Income Tax (Details))</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/SegmentInformationReconciliationOfSegmentResultsToIncomeBeforeFederalIncomeTaxDetails</Role>
      <ShortName>Segment Information (Reconciliation of Segment Results to Income Before Federal Income Tax (Details))</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/SegmentInformationTables</ParentRole>
      <Position>53</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R54.htm</HtmlFileName>
      <LongName>2411402 - Disclosure - Retirement Plans (Components of Net Periodic Benefit Cost) (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/RetirementPlansComponentsOfNetPeriodicBenefitCostDetails</Role>
      <ShortName>Retirement Plans (Components of Net Periodic Benefit Cost) (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <ParentRole>http://www.selective.com/role/RetirementPlansTable</ParentRole>
      <Position>54</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R55.htm</HtmlFileName>
      <LongName>2411403 - 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/RetirementPlansTable</ParentRole>
      <Position>55</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R56.htm</HtmlFileName>
      <LongName>2413402 - 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>56</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R57.htm</HtmlFileName>
      <LongName>2413403 - 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>57</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R58.htm</HtmlFileName>
      <LongName>2413404 - 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>58</Position>
    </Report>
    <Report instance="sigi-20150930.xml">
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <HtmlFileName>R59.htm</HtmlFileName>
      <LongName>2415401 - Disclosure - Subsequent Events Narrative (Details)</LongName>
      <ReportType>Sheet</ReportType>
      <Role>http://www.selective.com/role/SubsequentEventsNarrativeDetails</Role>
      <ShortName>Subsequent Events Narrative (Details)</ShortName>
      <MenuCategory>Details</MenuCategory>
      <Position>59</Position>
    </Report>
    <Report>
      <IsDefault>false</IsDefault>
      <HasEmbeddedReports>false</HasEmbeddedReports>
      <LongName>All Reports</LongName>
      <ReportType>Book</ReportType>
      <ShortName>All Reports</ShortName>
    </Report>
  </MyReports>
  <Logs>
    <Log type="Info">In ''Consolidated Balance Sheets'', column(s) 3, 4 are contained in other reports, so were removed by flow through suppression.</Log>
    <Log type="Info">In ''Consolidated Balance Sheets (Parenthetical)'', column(s) 7 are contained in other reports, so were removed by flow through suppression.</Log>
    <Log type="Info">In ''Consolidated Statements of Income'', column(s) 1 are contained in other reports, so were removed by flow through suppression.</Log>
    <Log type="Info">In ''Consolidated Statements of Cash Flow'', column(s) 1, 2 are contained in other reports, so were removed by flow through suppression.</Log>
  </Logs>
  <InputFiles>
    <File>sigi-20150930.xml</File>
    <File>sigi-20150930_cal.xml</File>
    <File>sigi-20150930_def.xml</File>
    <File>sigi-20150930_lab.xml</File>
    <File>sigi-20150930_pre.xml</File>
    <File>sigi-20150930.xsd</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies/>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>

