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Discontinued Operations (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2013 | Sep. 30, 2013 | Dec. 31, 2009
Percentage Of Interest Sold In Subsidiary |  |  | 100.00%
Receivable amount related to sale of subsidiary |  | $ 1.0 |
Discontinued gain (loss) from disposal of discontinued operation, before federal income tax | $ (1.5) |  |

X
- Definition This item represents the percentage of the company's interest in its subsidiary that was sold. + References No definition available. + Details
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Namespace Prefix: | sigi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Receivble amount related to sale of subsidiary + References No definition available. + Details
Name: | sigi_ReceivableRelatedToSaleOfSubsidiary
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of gain (loss), before tax expense or benefit and not previously recognized, resulting from the sale of a business component. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6892542&loc=d3e957-107759 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6360339&loc=d3e1361-107760 + Details
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Namespace Prefix: | us-gaap_
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Period Type: | duration