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Segment Information (Narrative) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended |
--- | --- | ---
Dec. 31, 2013 | Dec. 31, 2014
Segment Reporting [Abstract] |  |
Goodwill balance for our Standard Commercial Lines reporting unit | $ 7,849 us-gaap_Goodwill | $ 7,849 us-gaap_Goodwill
Percentage of net premiums written related to insurance policies written in New Jersey | 23.00%sigi_GeographicConcentrationOfNetPremiumsWrittenPercentage |

X
- Definition This item represents the geographic concentration of net premiusm written, reflected as a percentage of New Jersey net written premiums compared to our total net premiums written. + References No definition available. + Details
Name: | sigi_GeographicConcentrationOfNetPremiumsWrittenPercentage
Namespace Prefix: | sigi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266 + Details
Name: | us-gaap_Goodwill
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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