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v3.23.2

Reinsurance NFIP (Details) $ in Thousands | 6 Months Ended
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Jun. 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 | $ (100,200)

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-types: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: |