XML 55 R43.htm IDEA: XBRL DOCUMENT
v3.7.0.1

Indebtedness Indebtedness (Details) - Zero Point Seventy Five Percent Borrowing From Federal Home Loan Bank Due March 21, 2017 [Member] $ in Millions | Jun. 30, 2017 USD ($)
--- | ---
Short-term Debt [Line Items] |
Federal Home Loan Bank Advances | $ 64
Federal Home Loan Bank, Advances, Interest Rate | 0.75%

X
- 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

X
- 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

X
- 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

X
- Details
Name: | us-gaap_ShortTermDebtTypeAxis=sigi_ZeroPointSeventyFivePercentBorrowingFromFederalHomeLoanBankDueMarch212017Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |