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Indebtedness (Details) (Point sixty three Percent Borrowing From Federal Home Loan Bank [Member], USD $) In Millions, unless otherwise specified | Mar. 31, 2015
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Point sixty three Percent Borrowing From Federal Home Loan Bank [Member] |
Debt Instrument [Line Items] |
FHLBI Borrowing | $ 15 us-gaap_DebtInstrumentFaceAmount / us-gaap_DebtInstrumentAxis = sigi_PointsixtythreePercentBorrowingFromFederalHomeLoanBankMember

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- Definition Face (par) amount of debt instrument at time of issuance. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 55 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=34725769&loc=d3e28878-108400 + Details
Name: | us-gaap_DebtInstrumentFaceAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | us-gaap_DebtInstrumentLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration