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Reinsurance - NFIP (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2024 USD ($)
Effects of Reinsurance [Line Items] |
Percentage of NFIP | 100.00%
Total rated reinsurers |
Effects of Reinsurance [Line Items] |
Change in reinsurance recoverable gross | $ 164.3
Federal and state pools |
Effects of Reinsurance [Line Items] |
Change in reinsurance recoverable gross | $ 238.3

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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: | duration

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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 No definition available. + Details
Name: | us-gaap_EffectsOfReinsuranceLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | srt_CreditRatingAMBestAxis=sigi_AMBestRatedReinsurerMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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