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Segment Information Narrative (Details) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Mar. 31, 2014 USD ($) | Sep. 30, 2015 USD ($) | Sep. 30, 2014 USD ($)
Number of Operating Segments |  | 4 |
Gross premiums of self-insured pooled group renewal book of business sold | $ 37,000 |  |
Proceeds from sale of renewal rights |  | $ 0 | $ 8,000

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- Definition This item represents the gross premiums written related to the self-insured group book of pooled public entity business. + References No definition available. + Details
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Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues. + References No definition available. + Details
Name: | us-gaap_NumberOfOperatingSegments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition The cash inflow associated with the amount received from the sale of a portion of the company's business, for example a segment, division, branch or other business, during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 12 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=56944662&loc=d3e3179-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Investing Activities -URI http://asc.fasb.org/extlink&oid=6516133 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration