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: