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Commitments and Contingencies Stuctured Settlement Annuity (Details) $ in Millions | Dec. 31, 2020 USD ($)
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Commitments and Contingencies Disclosure [Abstract] |
Purchase Amount Of Annuities | $ 29.1

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- Definition The amount of annuities purchased from life insurance companies to fulfill obligations under claim settlements that provide for periodic future payments to claimants. + References No definition available. + Details
Name: | sigi_PurchaseAmountOfAnnuities
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- References No definition available. + Details
Name: | us-gaap_CommitmentsAndContingenciesDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration