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Accounting Pronouncements Adoption of Accounting Pronouncements (Details) $ in Thousands | Dec. 31, 2019 USD ($)
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Adoption of Accounting Pronouncements [Abstract] |
Cumulative effect of new accounting principle in period of adoption | $ 1,435

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- Definition Adoption of Accounting Pronouncements [Abstract] + References No definition available. + Details
Name: | sigi_AdoptionofAccountingPronouncementsAbstract
Namespace Prefix: | sigi_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption. + References No definition available. + Details
Name: | us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant