XML 144 R112.htm IDEA: XBRL DOCUMENT
v3.24.0.1

Retirement Plans (Discount Rate narrative) (Details) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2023 | Dec. 31, 2022
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Change in Defined Benefit Plan Discount Rate | 19 |
Retirement Income Plan [Member] |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate | 5.02% | 5.21%

X
- Definition Period change in weighted average rate for the present value of future retirement benefits cash flows, used to determine the benefit obligation of the defined benefit plan. + References No definition available. + Details
Name: | sigi_ChangeInDefinedBenefitPlanDiscountRate
Namespace Prefix: | sigi_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Weighted average rate for present value of future retirement benefits cash flows, used to determine benefit obligation of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (k)(1) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
Name: | us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingBenefitObligationDiscountRate
Namespace Prefix: | us-gaap_
Data Type: | dtr-types: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_DefinedBenefitPlansAndOtherPostretirementBenefitPlansTableTextBlockLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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