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Reinsurance Recoverable, NFIP (Details) $ in Thousands | 12 Months Ended
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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)

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- 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

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- 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

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

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