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Basis of Presentation Commercial mortgage loans (Details) | 3 Months Ended
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Mar. 31, 2020
Commercial mortgage loans [Abstract] |
Percent of commercial mortgage loans of our invested assets | 1.00%

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- Definition Commercial mortgage loans [Abstract] + References No definition available. + Details
Name: | sigi_CommercialmortgageloansAbstract
Namespace Prefix: | sigi_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Percent of our investment portfolio that are classified as commercial mortgage loans. + References No definition available. + Details
Name: | sigi_PercentofCommercialMortgageLoans
Namespace Prefix: | sigi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration