XML 149 R119.htm IDEA: XBRL DOCUMENT
v3.25.0.1

Retirement Plans - Schedule of Allocation of Plan Assets (Details) | Dec. 31, 2024 | Dec. 31, 2023
--- | --- | ---
Defined Benefit Plan Disclosure [Line Items] |  |
Actual Percentage | 100.00% | 100.00%
Percent hedge against projected benefit obligation | 91.00% |
Total return seeking assets |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Actual Percentage | 54.00% | 52.00%
Liability hedging assets |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Actual Percentage | 44.00% | 45.00%
Short-term investments |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Actual Percentage | 2.00% | 3.00%
Minimum | Total return seeking assets |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Target Percentage | 28.00% |
Minimum | Liability hedging assets |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Target Percentage | 42.00% |
Maximum | Total return seeking assets |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Target Percentage | 58.00% |
Maximum | Liability hedging assets |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Target Percentage | 72.00% |

X
- Definition Percent hedge provided by the liability hedging assets against the projected benefit obligation. + References No definition available. + Details
Name: | sigi_PercentHedgeAgainstProjectedBenefitObligation
Namespace Prefix: | sigi_
Data Type: | dtr-types1:percentItemType
Balance Type: | na
Period Type: | instant

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 Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan. + 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)(i) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 + Details
Name: | us-gaap_DefinedBenefitPlanPlanAssetsTargetAllocationPercentage
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Percentage of actual investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(i) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 + Details
Name: | us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | instant

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

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

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |