XML 160 R128.htm IDEA: XBRL DOCUMENT
v3.20.4

Schedule I - Summary of Investments - Other Than Investments in Related Parties (Details) $ in Thousands | Dec. 31, 2020 USD ($)
--- | ---
Amortized Cost or Cost | $ 7,114,400
Carrying Amount | 7,505,599
Short-term Investments [Member] |
Amortized Cost or Cost | 409,865
Carrying Amount | 409,852
Other Investments [Member] |
Amortized Cost or Cost | 266,322
Carrying Amount | 266,322
Held-to-maturity Securities [Member] | Obligations of States and Political Subdivisions [Member] |
Amortized Cost or Cost | 4,505
Fair Value | 4,795
Carrying Amount | 4,507
Held-to-maturity Securities [Member] | Public Utilities |
Amortized Cost or Cost | 2,436
Fair Value | 2,579
Carrying Amount | 2,464
Held-to-maturity Securities [Member] | All other corporate securities |
Amortized Cost or Cost | 9,898
Fair Value | 10,627
Carrying Amount | 9,853
Held-to-maturity Securities [Member] | Fixed Income Securities [Member] |
Amortized Cost or Cost | 16,839
Fair Value | 18,001
Carrying Amount | 16,824
AFS Fixed Income Securities [Member] | US Government Agencies Debt Securities [Member] |
Amortized Cost or Cost | 110,038
Fair Value | 116,140
Carrying Amount | 116,140
AFS Fixed Income Securities [Member] | Foreign Government |
Amortized Cost or Cost | 16,801
Fair Value | 18,366
Carrying Amount | 18,366
AFS Fixed Income Securities [Member] | Obligations of States and Political Subdivisions [Member] |
Amortized Cost or Cost | 1,159,588
Fair Value | 1,247,137
Carrying Amount | 1,247,137
AFS Fixed Income Securities [Member] | Public Utilities |
Amortized Cost or Cost | 68,269
Fair Value | 73,821
Carrying Amount | 73,821
AFS Fixed Income Securities [Member] | All other corporate securities |
Amortized Cost or Cost | 2,083,934
Fair Value | 2,254,231
Carrying Amount | 2,254,231
AFS Fixed Income Securities [Member] | CLO and Other ABS |
Amortized Cost or Cost | 1,014,820
Fair Value | 1,026,551
Carrying Amount | 1,026,551
AFS Fixed Income Securities [Member] | Commercial Mortgage-backed Securities (CMBS) [Member] |
Amortized Cost or Cost | 999,485
Fair Value | 1,051,788
Carrying Amount | 1,051,788
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member] |
Amortized Cost or Cost | 620,582
Fair Value | 667,894
Carrying Amount | 667,894
AFS Fixed Income Securities [Member] | Fixed Income Securities [Member] |
Amortized Cost or Cost | 6,073,517
Fair Value | 6,455,928
Carrying Amount | 6,455,928
Equity Securities [Member] | Banks, trusts, and insurance companies |
Amortized Cost or Cost | 18,366
Fair Value | 17,474
Carrying Amount | 17,474
Equity Securities [Member] | Industrial, miscellaneous and all other |
Amortized Cost or Cost | 281,619
Fair Value | 291,158
Carrying Amount | 291,158
Equity Securities [Member] | Preferred Stock [Member] |
Amortized Cost or Cost | 1,566
Fair Value | 1,735
Carrying Amount | 1,735
Equity Securities [Member] | Equity Securities [Member] |
Amortized Cost or Cost | 301,551
Fair Value | 310,367
Carrying Amount | 310,367
Commercial Mortgage Loans [Member] |
Amortized Cost or Cost | 46,306
Carrying Amount | $ 46,306

X
- Definition Amount of investment held by insurance company, excluding investment in related party. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 7 -Subparagraph (SX 210.12-15(Column D)) -URI http://asc.fasb.org/extlink&oid=120401592&loc=d3e611379-123010 + Details
Name: | us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCarryingAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Cost of investment held by insurance company, excluding investment in related party. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 7 -Subparagraph (SX 210.12-15(Column B)) -URI http://asc.fasb.org/extlink&oid=120401592&loc=d3e611379-123010 + Details
Name: | us-gaap_SummaryOfInvestmentsOtherThanInvestmentsInRelatedPartiesCost
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Fair value of investment held by insurance company, excluding investment in related party. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 946 -SubTopic 320 -Section S99 -Paragraph 7 -Subparagraph (SX 210.12-15(Column C)) -URI http://asc.fasb.org/extlink&oid=120401592&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_InvestmentTypeAxis=us-gaap_ShortTermInvestmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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

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

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

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=sigi_AFSFixedIncomeSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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

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

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

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

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

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

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

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=sigi_CommercialMortgageLoansSIGIMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |