XML 115 R122.htm IDEA: XBRL DOCUMENT
v3.19.3.a.u2

Commitments and Contingencies Stuctured Settlement Annuity (Details) $ in Millions | Dec. 31, 2019 USD ($)
--- | ---
Commitments and Contingencies Disclosure [Abstract] |
Purchase Amount Of Annuities | $ 25.5

X
- Definition The amount of annuities purchased from life insurance companies to fulfill obligations under claim settlements that provide for periodic future payments to claimants. + References No definition available. + Details
Name: | sigi_PurchaseAmountOfAnnuities
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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