Name:	sigi_Series20231ClassACatastropheBonds
Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

The annual cap limiting the amount of aggregate subject losses for participating insurers under the Terrorism Risk Insurance Program.

No definition available.

Name:	sigi_TerrorismRiskInsuranceProgramAnnualCap
Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

The Company's estimated deductible for the year following the date of this report under the Terrorism Risk Insurance Program.

No definition available.

Name:	sigi_TerrorismRiskInsuranceProgramCompanysEstimatedDeductible
Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

The percentage that the insured has to pay for losses under the Terrorism Risk Insurance Program, beginning the next calendar year.

No definition available.

Name:	sigi_TerrorismRiskInsuranceProgramInsurerRetainer
Namespace Prefix:	sigi_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	instant

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.

No definition available.