XML 42 R118.htm IDEA: XBRL DOCUMENT
v2.4.1.9

Schedule I - Summay of Investments - Other Than Investments in Related Parties (Details) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014
--- | ---
Amortized Cost or Cost | $ 4,683,151 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost
Carrying Amount | 4,806,834 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount
Held-to-maturity Securities [Member] |
Amortized Cost or Cost | 317,179 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_HeldtomaturitySecuritiesMember
Fair Value | 333,961 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_HeldtomaturitySecuritiesMember
Carrying Amount | 318,137 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_HeldtomaturitySecuritiesMember
Held-to-maturity Securities [Member] | Foreign Government Obligations [Member] |
Amortized Cost or Cost | 5,292 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_InvestmentTypeAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_HeldtomaturitySecuritiesMember
Fair Value | 5,394 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_InvestmentTypeAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_HeldtomaturitySecuritiesMember
Carrying Amount | 5,339 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_InvestmentTypeAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_HeldtomaturitySecuritiesMember
Held-to-maturity Securities [Member] | Obligations of States and Political Subdivisions [Member] |
Amortized Cost or Cost | 285,301 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_HeldtomaturitySecuritiesMember
Fair Value | 299,132 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_HeldtomaturitySecuritiesMember
Carrying Amount | 287,372 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_HeldtomaturitySecuritiesMember
Held-to-maturity Securities [Member] | Public utilities [Member] |
Amortized Cost or Cost | 11,019 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_InvestmentTypeAxis = us-gaap_PublicUtilityBondsMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_HeldtomaturitySecuritiesMember
Fair Value | 12,483 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_InvestmentTypeAxis = us-gaap_PublicUtilityBondsMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_HeldtomaturitySecuritiesMember
Carrying Amount | 11,000 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_InvestmentTypeAxis = us-gaap_PublicUtilityBondsMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_HeldtomaturitySecuritiesMember
Held-to-maturity Securities [Member] | All Other Corporate Securities [Member] |
Amortized Cost or Cost | 7,880 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_InvestmentTypeAxis = us-gaap_AllOtherCorporateBondsMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_HeldtomaturitySecuritiesMember
Fair Value | 8,939 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_InvestmentTypeAxis = us-gaap_AllOtherCorporateBondsMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_HeldtomaturitySecuritiesMember
Carrying Amount | 7,626 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_InvestmentTypeAxis = us-gaap_AllOtherCorporateBondsMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_HeldtomaturitySecuritiesMember
Held-to-maturity Securities [Member] | Asset-backed Securities [Member] |
Amortized Cost or Cost | 2,818 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_HeldtomaturitySecuritiesMember
Fair Value | 2,823 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_HeldtomaturitySecuritiesMember
Carrying Amount | 2,363 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_HeldtomaturitySecuritiesMember
Held-to-maturity Securities [Member] | Commercial Mortgage-backed Securities (CMBS) [Member] |
Amortized Cost or Cost | 4,869 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_HeldtomaturitySecuritiesMember
Fair Value | 5,190 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_HeldtomaturitySecuritiesMember
Carrying Amount | 4,437 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_HeldtomaturitySecuritiesMember
AFS Fixed Income Securities [Member] |
Amortized Cost or Cost | 3,975,786 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
Fair Value | 4,066,122 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
Carrying Amount | 4,066,122 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
AFS Fixed Income Securities [Member] | U.S. Government and Government Agencies [Member] |
Amortized Cost or Cost | 116,666 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_InvestmentTypeAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
Fair Value | 124,130 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_InvestmentTypeAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
Carrying Amount | 124,130 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_InvestmentTypeAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
AFS Fixed Income Securities [Member] | Foreign Government Obligations [Member] |
Amortized Cost or Cost | 27,035 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_InvestmentTypeAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
Fair Value | 27,831 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_InvestmentTypeAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
Carrying Amount | 27,831 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_InvestmentTypeAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
AFS Fixed Income Securities [Member] | Obligations of States and Political Subdivisions [Member] |
Amortized Cost or Cost | 1,208,776 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
Fair Value | 1,246,264 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
Carrying Amount | 1,246,264 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
AFS Fixed Income Securities [Member] | Public utilities [Member] |
Amortized Cost or Cost | 149,006 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_InvestmentTypeAxis = us-gaap_PublicUtilityBondsMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
Fair Value | 150,977 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_InvestmentTypeAxis = us-gaap_PublicUtilityBondsMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
Carrying Amount | 150,977 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_InvestmentTypeAxis = us-gaap_PublicUtilityBondsMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
AFS Fixed Income Securities [Member] | All Other Corporate Securities [Member] |
Amortized Cost or Cost | 1,614,421 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_InvestmentTypeAxis = us-gaap_AllOtherCorporateBondsMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
Fair Value | 1,648,829 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_InvestmentTypeAxis = us-gaap_AllOtherCorporateBondsMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
Carrying Amount | 1,648,829 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_InvestmentTypeAxis = us-gaap_AllOtherCorporateBondsMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
AFS Fixed Income Securities [Member] | Asset-backed Securities [Member] |
Amortized Cost or Cost | 176,837 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
Fair Value | 177,224 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
Carrying Amount | 177,224 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
AFS Fixed Income Securities [Member] | Commercial Mortgage-backed Securities (CMBS) [Member] |
Amortized Cost or Cost | 177,932 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
Fair Value | 179,593 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
Carrying Amount | 179,593 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities (RMBS) [Member] |
Amortized Cost or Cost | 505,113 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_InvestmentTypeAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
Fair Value | 511,274 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_InvestmentTypeAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
Carrying Amount | 511,274 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_InvestmentTypeAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = sigi_AfsFixedMaturitySecuritiesMember
Equity Securities [Member] |
Amortized Cost or Cost | 159,011 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_EquitySecuritiesMember
Fair Value | 191,400 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_EquitySecuritiesMember
Carrying Amount | 191,400 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_EquitySecuritiesMember
Equity Securities [Member] | Public Utility, Equities [Member] |
Amortized Cost or Cost | 8,815 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_InvestmentTypeAxis = us-gaap_PublicUtilityEquitiesMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_EquitySecuritiesMember
Fair Value | 10,284 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_InvestmentTypeAxis = us-gaap_PublicUtilityEquitiesMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_EquitySecuritiesMember
Carrying Amount | 10,284 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_InvestmentTypeAxis = us-gaap_PublicUtilityEquitiesMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_EquitySecuritiesMember
Equity Securities [Member] | Banks, Trust and Insurance Companies [Member] |
Amortized Cost or Cost | 30,187 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_InvestmentTypeAxis = us-gaap_BanksTrustAndInsuranceEquitiesMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_EquitySecuritiesMember
Fair Value | 35,348 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_InvestmentTypeAxis = us-gaap_BanksTrustAndInsuranceEquitiesMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_EquitySecuritiesMember
Carrying Amount | 35,348 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_InvestmentTypeAxis = us-gaap_BanksTrustAndInsuranceEquitiesMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_EquitySecuritiesMember
Equity Securities [Member] | Industrial, Miscellaneous, and All Other [Member] |
Amortized Cost or Cost | 120,009 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_InvestmentTypeAxis = us-gaap_IndustrialMiscellaneousAndAllOthersMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_EquitySecuritiesMember
Fair Value | 145,768 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_InvestmentTypeAxis = us-gaap_IndustrialMiscellaneousAndAllOthersMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_EquitySecuritiesMember
Carrying Amount | 145,768 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_InvestmentTypeAxis = us-gaap_IndustrialMiscellaneousAndAllOthersMember / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_EquitySecuritiesMember
Short-term Investments [Member] |
Amortized Cost or Cost | 131,972 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_ShortTermInvestmentsMember
Fair Value | 131,972 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_ShortTermInvestmentsMember
Carrying Amount | 131,972 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_ShortTermInvestmentsMember
Other Investments [Member] |
Amortized Cost or Cost | 99,203 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_OtherInvestmentsMember
Carrying Amount | $ 99,203 us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount / us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis = us-gaap_OtherInvestmentsMember

X
- Definition This element represents the carrying (reported) amount of the entity's investments as of the balance sheet date. This amount may represent the individual investment's or group of similar investments' original cost, fair value, or such other amount at which the investment is shown in the entity's balance sheet. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 15 -Article 12 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 7 -Subparagraph (SX 210.12-15) -URI http://asc.fasb.org/extlink&oid=34726532&loc=d3e611379-123010 + Details
Name: | us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition This item represents the original cost of the entity's investments including investments in equity securities and, as to fixed maturities, original cost reduced by repayments and adjusted for amortization of premiums or accrual of discounts. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 15 -Article 12 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 7 -Subparagraph (SX 210.12-15) -URI http://asc.fasb.org/extlink&oid=34726532&loc=d3e611379-123010 + Details
Name: | us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition This item represents the reported amount of the entity's investments measured at fair value. Fair value is the price that would be received to sell an investment in an orderly transaction between market participants at the balance sheet [measurement] date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 15 -Article 12 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 7 -Subparagraph (SX 210.12-15) -URI http://asc.fasb.org/extlink&oid=34726532&loc=d3e611379-123010 + Details
Name: | us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis=sigi_AfsFixedMaturitySecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis=us-gaap_HeldtomaturitySecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis=us-gaap_OtherInvestmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesByTypeAxis=us-gaap_ShortTermInvestmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |