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Indebtedness (Short Term Debt Activity) (Details) - Two Point Sixty Four Percent Borrowing from Federal Home Loan Bank Due March 28, 2019 [Member] $ in Millions | Mar. 31, 2019 USD ($)
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Short-term Debt [Line Items] |
Federal Home Loan Bank Advances | $ 50
Federal Home Loan Bank, Advances, Interest Rate | 2.64%

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- Definition Amount of Federal Home Loan Bank (FHLBank) advances to member financial institutions. + References No definition available. + Details
Name: | us-gaap_FederalHomeLoanBankAdvances
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Interest rate of advances made and reported by Federal Home Loan Bank (FHLBank). + References No definition available. + Details
Name: | us-gaap_FederalHomeLoanBankAdvancesInterestRate
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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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_ShortTermDebtLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_ShortTermDebtTypeAxis=sigi_TwoPointSixtyFourPercentBorrowingfromFederalHomeLoanBankDueMarch282019Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |