XML 37 R78.htm IDEA: XBRL DOCUMENT
v2.4.1.9

Fair Value Measurements (Quantitative Information of Our Financial Assets and Liabilities That Were Disclosed at Fair Value) (Details) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | $ 318,137 us-gaap_HeldToMaturitySecurities | $ 392,879 us-gaap_HeldToMaturitySecurities
Notes payable | 379,297 us-gaap_LongTermDebt | 392,414 us-gaap_LongTermDebt
Fair Value [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | 333,961 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember | 416,981 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Notes payable | 404,270 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember | 354,010 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 | 185,000 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member | 146,298 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 | 333,961 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 416,981 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member
Notes payable | 219,270 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member | 207,712 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 | 5,394 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_InvestmentTypeAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember | 5,591 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 | 5,394 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_InvestmentTypeAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember | 5,591 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 | 299,132 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 369,756 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 | 299,132 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 369,756 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 | 21,422 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember | 30,274 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 | 21,422 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember | 30,274 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 | 2,823 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember | 3,415 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 | 2,823 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember | 3,415 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 (CMBS) [Member] | Fair Value [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Held-to-maturity Securities | 5,190 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | 7,945 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Commercial Mortgage-backed Securities (CMBS) [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 (CMBS) [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 | 5,190 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember | 7,945 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember
Commercial Mortgage-backed Securities (CMBS) [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
Two Point Ninety 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 |  | 13,319 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_TwoPointNinetyPercentBorrowingFromFederalHomeLoanBankMember
Two Point Ninety 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_TwoPointNinetyPercentBorrowingFromFederalHomeLoanBankMember
Two Point Ninety 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 |  | 13,319 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_LongtermDebtTypeAxis = sigi_TwoPointNinetyPercentBorrowingFromFederalHomeLoanBankMember
Two Point Ninety 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_TwoPointNinetyPercentBorrowingFromFederalHomeLoanBankMember
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 | 45,244 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_OnePointTwentyFivePercentBorrowingFromFederalHomeLoanBankMember | 45,259 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 | 45,244 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_LongtermDebtTypeAxis = sigi_OnePointTwentyFivePercentBorrowingFromFederalHomeLoanBankMember | 45,259 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 | 59,181 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_SevenPointTwentyFivePercentSeniorNotesMember | 50,887 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 | 59,181 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_LongtermDebtTypeAxis = sigi_SevenPointTwentyFivePercentSeniorNotesMember | 50,887 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 | 114,845 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_SixPointSeventyPercentSeniorNotesMember | 98,247 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 | 114,845 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_LongtermDebtTypeAxis = sigi_SixPointSeventyPercentSeniorNotesMember | 98,247 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] |  |
Notes payable | 0 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_LongtermDebtTypeAxis = sigi_SixPointSeventyPercentSeniorNotesMember | 0 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_LongtermDebtTypeAxis = sigi_SixPointSeventyPercentSeniorNotesMember
Five Point Eight Hundred Seventy Five [Member] | Fair Value [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Notes payable | 185,000 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_FivePointEightHundredSeventyFiveMember | 146,298 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_FivePointEightHundredSeventyFiveMember
Five Point Eight Hundred Seventy Five [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 | 185,000 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_LongtermDebtTypeAxis = sigi_FivePointEightHundredSeventyFiveMember | 146,298 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_LongtermDebtTypeAxis = sigi_FivePointEightHundredSeventyFiveMember
Five Point Eight Hundred Seventy Five [Member] | Significant Other Observable Inputs (Level 2) [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_FairValueInputsLevel2Member / us-gaap_LongtermDebtTypeAxis = sigi_FivePointEightHundredSeventyFiveMember | 0 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_LongtermDebtTypeAxis = sigi_FivePointEightHundredSeventyFiveMember
Five Point Eight Hundred Seventy Five [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_FivePointEightHundredSeventyFiveMember | $ 0 us-gaap_LongTermDebt / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_LongtermDebtTypeAxis = sigi_FivePointEightHundredSeventyFiveMember

X
- Details
Name: | us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.1) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.6) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
Name: | us-gaap_HeldToMaturitySecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 7 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 9 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20, 22 -Article 5 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
Name: | us-gaap_LongTermDebt
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
Name: | us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_EstimateOfFairValueFairValueDisclosureMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InvestmentTypeAxis=us-gaap_AssetBackedSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InvestmentTypeAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InvestmentTypeAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InvestmentTypeAxis=us-gaap_ForeignGovernmentDebtSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InvestmentTypeAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=sigi_FivePointEightHundredSeventyFiveMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=sigi_OnePointTwentyFivePercentBorrowingFromFederalHomeLoanBankMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=sigi_SevenPointTwentyFivePercentSeniorNotesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=sigi_SixPointSeventyPercentSeniorNotesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_LongtermDebtTypeAxis=sigi_TwoPointNinetyPercentBorrowingFromFederalHomeLoanBankMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |