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Fair Value Measurements (Quantitative Information of Our Financial Assets and Liabilities That Were Disclosed at Fair Value) (Details) (USD $) In Thousands, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | $ 296,557 us-gaap_HeldToMaturitySecurities | $ 318,137 us-gaap_HeldToMaturitySecurities
Notes payable | 394,301 us-gaap_LongTermDebt | 379,297 us-gaap_LongTermDebt
Fair Value [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | 311,093 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember | 333,961 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Notes payable | 427,568 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember | 404,270 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | 0 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 0 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Notes payable | 189,440 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 185,000 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member
Significant Other Observable Inputs (Level 2) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | 311,093 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 333,961 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Notes payable | 238,128 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 219,270 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Significant Unobservable Inputs (Level 3) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | 0 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | 0 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Notes payable | 0 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | 0 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Foreign Government [Member] | Fair Value [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | 5,338 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_InvestmentTypeAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember | 5,394 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_InvestmentTypeAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember
Foreign Government [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | 0 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_InvestmentTypeAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember | 0 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_InvestmentTypeAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember
Foreign Government [Member] | Significant Other Observable Inputs (Level 2) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | 5,338 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_InvestmentTypeAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember | 5,394 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_InvestmentTypeAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember
Foreign Government [Member] | Significant Unobservable Inputs (Level 3) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | 0 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_InvestmentTypeAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember | 0 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_InvestmentTypeAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember
Obligations of States and Political Subdivisions [Member] | Fair Value [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | 277,030 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 299,132 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Obligations of States and Political Subdivisions [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | 0 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 0 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Obligations of States and Political Subdivisions [Member] | Significant Other Observable Inputs (Level 2) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | 277,030 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 299,132 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Obligations of States and Political Subdivisions [Member] | Significant Unobservable Inputs (Level 3) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | 0 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 0 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Corporate Securities [Member] | Fair Value [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | 21,199 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember | 21,422 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember
Corporate Securities [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | 0 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember | 0 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember
Corporate Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | 21,199 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember | 21,422 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember
Corporate Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | 0 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember | 0 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember
Asset-Backed Securities [Member] | Fair Value [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | 2,480 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember | 2,823 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember
Asset-Backed Securities [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | 0 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember | 0 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember
Asset-Backed Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | 2,480 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember | 2,823 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember
Asset-Backed Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | 0 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember | 0 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember
Commercial Mortgage Backed Securities [Member] | Fair Value [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | 5,046 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | 5,190 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Commercial Mortgage Backed Securities [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | 0 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | 0 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Commercial Mortgage Backed Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | 5,046 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | 5,190 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Commercial Mortgage Backed Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | 0 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | 0 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Point sixty three Percent Borrowing From Federal Home Loan Bank [Member] | Fair Value [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Notes payable | 14,994 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_PointsixtythreePercentBorrowingFromFederalHomeLoanBankMember |
Point sixty three Percent Borrowing From Federal Home Loan Bank [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Notes payable | 0 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_LongtermDebtTypeAxis = sigi_PointsixtythreePercentBorrowingFromFederalHomeLoanBankMember |
Point sixty three Percent Borrowing From Federal Home Loan Bank [Member] | Significant Other Observable Inputs (Level 2) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Notes payable | 14,994 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_LongtermDebtTypeAxis = sigi_PointsixtythreePercentBorrowingFromFederalHomeLoanBankMember |
Point sixty three Percent Borrowing From Federal Home Loan Bank [Member] | Significant Unobservable Inputs (Level 3) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Notes payable | 0 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_LongtermDebtTypeAxis = sigi_PointsixtythreePercentBorrowingFromFederalHomeLoanBankMember |
One Point Twenty Five Percent Borrowing From Federal Home Loan Bank [Member] | Fair Value [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Notes payable | 45,338 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_OnePointTwentyFivePercentBorrowingFromFederalHomeLoanBankMember | 45,244 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_OnePointTwentyFivePercentBorrowingFromFederalHomeLoanBankMember
One Point Twenty Five Percent Borrowing From Federal Home Loan Bank [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Notes payable | 0 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_LongtermDebtTypeAxis = sigi_OnePointTwentyFivePercentBorrowingFromFederalHomeLoanBankMember | 0 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_LongtermDebtTypeAxis = sigi_OnePointTwentyFivePercentBorrowingFromFederalHomeLoanBankMember
One Point Twenty Five Percent Borrowing From Federal Home Loan Bank [Member] | Significant Other Observable Inputs (Level 2) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Notes payable | 45,338 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_LongtermDebtTypeAxis = sigi_OnePointTwentyFivePercentBorrowingFromFederalHomeLoanBankMember | 45,244 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_LongtermDebtTypeAxis = sigi_OnePointTwentyFivePercentBorrowingFromFederalHomeLoanBankMember
One Point Twenty Five Percent Borrowing From Federal Home Loan Bank [Member] | Significant Unobservable Inputs (Level 3) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Notes payable | 0 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_LongtermDebtTypeAxis = sigi_OnePointTwentyFivePercentBorrowingFromFederalHomeLoanBankMember | 0 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_LongtermDebtTypeAxis = sigi_OnePointTwentyFivePercentBorrowingFromFederalHomeLoanBankMember
Seven Point Twenty Five Percent Senior Notes [Member] | Fair Value [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Notes payable | 60,407 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_SevenPointTwentyFivePercentSeniorNotesMember | 59,181 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_SevenPointTwentyFivePercentSeniorNotesMember
Seven Point Twenty Five Percent Senior Notes [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Notes payable | 0 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_LongtermDebtTypeAxis = sigi_SevenPointTwentyFivePercentSeniorNotesMember | 0 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_LongtermDebtTypeAxis = sigi_SevenPointTwentyFivePercentSeniorNotesMember
Seven Point Twenty Five Percent Senior Notes [Member] | Significant Other Observable Inputs (Level 2) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Notes payable | 60,407 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_LongtermDebtTypeAxis = sigi_SevenPointTwentyFivePercentSeniorNotesMember | 59,181 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_LongtermDebtTypeAxis = sigi_SevenPointTwentyFivePercentSeniorNotesMember
Seven Point Twenty Five Percent Senior Notes [Member] | Significant Unobservable Inputs (Level 3) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Notes payable | 0 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_LongtermDebtTypeAxis = sigi_SevenPointTwentyFivePercentSeniorNotesMember | 0 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_LongtermDebtTypeAxis = sigi_SevenPointTwentyFivePercentSeniorNotesMember
Six Point Seventy Percent Senior Notes [Member] | Fair Value [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Notes payable | 117,389 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_SixPointSeventyPercentSeniorNotesMember | 114,845 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_SixPointSeventyPercentSeniorNotesMember
Six Point Seventy Percent Senior Notes [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Notes payable | 0 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_LongtermDebtTypeAxis = sigi_SixPointSeventyPercentSeniorNotesMember | 0 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_LongtermDebtTypeAxis = sigi_SixPointSeventyPercentSeniorNotesMember
Six Point Seventy Percent Senior Notes [Member] | Significant Other Observable Inputs (Level 2) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Notes payable | 117,389 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_LongtermDebtTypeAxis = sigi_SixPointSeventyPercentSeniorNotesMember | 114,845 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_LongtermDebtTypeAxis = sigi_SixPointSeventyPercentSeniorNotesMember
Six Point Seventy Percent Senior Notes [Member] | Significant Unobservable Inputs (Level 3) [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
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