XML 147 R115.htm IDEA: XBRL DOCUMENT
v3.24.0.1

Retirement Plans (Plan's Allocated Target Ranges and Actual Weighted Average Asset Allocation by Investment Categories) (Details) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2023 | Dec. 31, 2022
Actual percentage |  | 100.00% | 100.00%
Percent Hedge Against Projected Benefit Obligation |  | 90.00% |
Return Seeking Assets [Member] |  |  |
Actual percentage | [1] | 52.00% | 71.00%
Liability Hedging Assets [Member] |  |  |
Actual percentage |  | 45.00% | 27.00%
Short-term Investments [Member] |  |  |
Actual percentage |  | 3.00% | 2.00%
Minimum [Member] | Return Seeking Assets [Member] |  |  |
Defined Benefit Plan, Plan Assets, Investment Policy and Strategy, Description |  | 28 |
Minimum [Member] | Liability Hedging Assets [Member] |  |  |
Defined Benefit Plan, Plan Assets, Investment Policy and Strategy, Description |  | 42 |
Minimum [Member] | Short-term Investments [Member] |  |  |
Defined Benefit Plan, Plan Assets, Investment Policy and Strategy, Description |  | - |
Maximum [Member] | Return Seeking Assets [Member] |  |  |
Defined Benefit Plan, Plan Assets, Investment Policy and Strategy, Description |  | 58 |
Maximum [Member] | Liability Hedging Assets [Member] |  |  |
Defined Benefit Plan, Plan Assets, Investment Policy and Strategy, Description |  | 72 |
Maximum [Member] | Short-term Investments [Member] |  |  |
Defined Benefit Plan, Plan Assets, Investment Policy and Strategy, Description |  | - |
[1] | Includes limited partnerships.

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 Description of investment policy and strategy for defined benefit plan asset. Includes, but is not limited to, investment allocation decision and factor pertinent to understanding investment policy and strategy for plan asset. Factor includes, but is not limited to, investment goal, risk management practice, permitted and prohibited investment including use of derivative, diversification, and relationship between plan asset and obligation. + 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_DefinedBenefitPlanInvestmentPoliciesAndStrategiesNarrativeDescription
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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: |