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v3.25.1

Reinsurance - Narrative (Details) $ in Millions | 3 Months Ended
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Mar. 31, 2025 USD ($)
Effects of Reinsurance [Line Items] |
Percentage of NFIP | 100.00%
Federal and state pools |
Effects of Reinsurance [Line Items] |
Change in reinsurance recoverable gross | $ (134.2)

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- Definition Change In Reinsurance Recoverable, Gross + References No definition available. + Details
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Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Percentage Of Unpaid NFIP Claims Ceded By Federal Government + References No definition available. + Details
Name: | sigi_PercentageOfUnpaidNFIPClaimsCededByFederalGovernment
Namespace Prefix: | sigi_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

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- 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_EffectsOfReinsuranceLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | sigi_NonRatedReinsurerAxis=sigi_TotalfederalandstatepoolsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |