XML 150 R120.htm IDEA: XBRL DOCUMENT
v3.25.0.1

Retirement Plans - Schedule of Quantitative Disclosures of our Pension Assets (Details) - Defined Benefit Pension Plan - USD ($) $ in Thousands | Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
--- | --- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | $ 332,755 | $ 345,132 | $ 342,974
Fair Value, Recurring |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 332,547 | 347,940 |
Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets/ Liabilities (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 329,764 | 345,211 |
Fair Value, Recurring | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 2,473 | 2,391 |
Fair Value, Recurring | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Global equity |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 40,643 | 39,038 |
Fair Value, Recurring | Global equity | Quoted Prices in Active Markets for Identical Assets/ Liabilities (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 40,643 | 39,038 |
Fair Value, Recurring | Global equity | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Global equity | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Diversified credit |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 47,158 | 42,630 |
Fair Value, Recurring | Diversified credit | Quoted Prices in Active Markets for Identical Assets/ Liabilities (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 47,158 | 42,630 |
Fair Value, Recurring | Diversified credit | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Diversified credit | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Real assets |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 58,427 | 59,603 |
Fair Value, Recurring | Real assets | Quoted Prices in Active Markets for Identical Assets/ Liabilities (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 58,427 | 59,603 |
Fair Value, Recurring | Real assets | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Real assets | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Liquid diversifiers |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 33,075 | 40,095 |
Fair Value, Recurring | Liquid diversifiers | Quoted Prices in Active Markets for Identical Assets/ Liabilities (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 33,075 | 40,095 |
Fair Value, Recurring | Liquid diversifiers | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Liquid diversifiers | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Real assets |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 28 | 27 |
Fair Value, Recurring | Real assets | Quoted Prices in Active Markets for Identical Assets/ Liabilities (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Real assets | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Real assets | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Private equity |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 282 | 311 |
Fair Value, Recurring | Private equity | Quoted Prices in Active Markets for Identical Assets/ Liabilities (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Private equity | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Private equity | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Fixed income |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 98,068 | 79,632 |
Fair Value, Recurring | Fixed income | Quoted Prices in Active Markets for Identical Assets/ Liabilities (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 98,068 | 79,632 |
Fair Value, Recurring | Fixed income | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Fixed income | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | U.S. Treasury overlay |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 47,302 | 75,198 |
Fair Value, Recurring | U.S. Treasury overlay | Quoted Prices in Active Markets for Identical Assets/ Liabilities (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 47,302 | 75,198 |
Fair Value, Recurring | U.S. Treasury overlay | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | U.S. Treasury overlay | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Short-term securities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 5,091 | 9,015 |
Fair Value, Recurring | Short-term securities | Quoted Prices in Active Markets for Identical Assets/ Liabilities (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 5,091 | 9,015 |
Fair Value, Recurring | Short-term securities | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Short-term securities | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Deposit administration contracts |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 2,473 | 2,391 |
Fair Value, Recurring | Deposit administration contracts | Quoted Prices in Active Markets for Identical Assets/ Liabilities (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Deposit administration contracts | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 2,473 | 2,391 |
Fair Value, Recurring | Deposit administration contracts | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Total equities |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 179,303 | 181,366 |
Fair Value, Recurring | Total equities | Quoted Prices in Active Markets for Identical Assets/ Liabilities (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 179,303 | 181,366 |
Fair Value, Recurring | Total equities | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Total equities | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Total limited partnerships |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 310 | 338 |
Fair Value, Recurring | Total limited partnerships | Quoted Prices in Active Markets for Identical Assets/ Liabilities (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Total limited partnerships | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Total limited partnerships | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Total return seeking assets |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 179,613 | 181,704 |
Fair Value, Recurring | Total return seeking assets | Quoted Prices in Active Markets for Identical Assets/ Liabilities (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 179,303 | 181,366 |
Fair Value, Recurring | Total return seeking assets | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Total return seeking assets | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Liability hedging assets |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 145,370 | 154,830 |
Fair Value, Recurring | Liability hedging assets | Quoted Prices in Active Markets for Identical Assets/ Liabilities (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 145,370 | 154,830 |
Fair Value, Recurring | Liability hedging assets | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Liability hedging assets | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 0 | 0 |
Fair Value, Recurring | Total cash and short-term investments |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 7,564 | 11,406 |
Fair Value, Recurring | Total cash and short-term investments | Quoted Prices in Active Markets for Identical Assets/ Liabilities (Level 1) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 5,091 | 9,015 |
Fair Value, Recurring | Total cash and short-term investments | Significant Other Observable Inputs (Level 2) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | 2,473 | 2,391 |
Fair Value, Recurring | Total cash and short-term investments | Significant Unobservable Inputs (Level 3) |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Total invested assets | $ 0 | $ 0 |

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 55 -Paragraph 17 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480482/715-20-55-17 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(02) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 Reference 6: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 Reference 7: http://www.xbrl.org/2003/role/exampleRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 + Details
Name: | us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
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_DefinedBenefitPlanByPlanAssetCategoriesAxis=sigi_GlobalEquityMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=sigi_DiversifiedCreditMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=sigi_RealAssetsEquityMemberMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=sigi_LiquidDiversifiersMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=sigi_RealAssetsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_PrivateEquityFundsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_FixedIncomeSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=sigi_USTreasuryFundMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

X
- Details
Name: | us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=sigi_DepositAdministrationContractsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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

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

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

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