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Reinsurance Recoverable, Narrative (Details) $ in Thousands | 12 Months Ended
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Dec. 31, 2023 USD ($)
Series 2023-1, Class A - Catastrophe bonds | $ 325,000
Terrorism Risk Insurance Program Companys Estimated Deductible | $ 543,000
Terrorism Risk Insurance Program Reimbursed Percentage Maximum | 80.00%
Terrorism Risk Insurance Program Annual Cap | $ 100,000,000
Terrorism Risk Insurance Program Insurer Retainer | 20.00%

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- Definition Class-A Principal-at-Risk Variable Rates Notes secured with High-Point Re Ltd., an independent Bermuda special purpose insurer. This catastrophe bond meets the criteria to be accounted for as reinsurance. + References No definition available. + Details
Name: | sigi_Series20231ClassACatastropheBonds
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The annual cap limiting the amount of aggregate subject losses for participating insurers under the Terrorism Risk Insurance Program. + References No definition available. + Details
Name: | sigi_TerrorismRiskInsuranceProgramAnnualCap
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition The Company's estimated deductible for the year following the date of this report under the Terrorism Risk Insurance Program. + References No definition available. + Details
Name: | sigi_TerrorismRiskInsuranceProgramCompanysEstimatedDeductible
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition The percentage that the insured has to pay for losses under the Terrorism Risk Insurance Program, beginning the next calendar year. + References No definition available. + Details
Name: | sigi_TerrorismRiskInsuranceProgramInsurerRetainer
Namespace Prefix: | sigi_
Data Type: | dtr-types1:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition The percentage of subject losses, after an insurer deductible, subject to an annual cap, reimbursed by the Federal Government under the Terrorism Risk Insurance Program, beginning the next calendar year. + References No definition available. + Details
Name: | sigi_TerrorismRiskInsuranceProgramReimbursedPercentageMaximum
Namespace Prefix: | sigi_
Data Type: | dtr-types1:percentItemType
Balance Type: | na
Period Type: | duration