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Reinsurance Recoverable, NFIP (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($)
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Percentage of NFIP claims ceded to the federal government	100.00%
Total federal and state pools [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Change in Reinsurance Recoverable, Gross	$ (100,300)

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 flood premium, losses and loss expense claims ceded by federal government.

+ References

No definition available.

+ Details

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

X

- 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

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-5

+ Details

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

X

- Details

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