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Reinsurance NFIP (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2023 USD ($)
National Flood Insurance Program [Member]
Reinsurance NFIP [Line Items]
Percentage of unpaid NFIP Claims ceded to federal government	100.00%
Total federal and state pools [Member]
Reinsurance NFIP [Line Items]
Change in Reinsurance Recoverable, Gross	$ (92,000)

X

- Definition

The net change in the amount before valuation allowance recoverable under reinsurance contracts.

+ References

No definition available.

+ Details

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

X

- Definition

Percentage of unpaid NFIP Claims ceded to federal government

+ References

No definition available.

+ Details

Name:	sigi_PercentageOfUnpaidNFIPClaimsCededToFederalGovernment
Namespace Prefix:	sigi_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Reinsurance NFIP

+ References

No definition available.

+ Details

Name:	sigi_ReinsuranceNFIPLineItems
Namespace Prefix:	sigi_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	sigi_FloodCededPremiumsAndLossesAxis=sigi_NationalFloodInsuranceProgramMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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