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Retirement Plans (Schedule of Alternative Investment Portfolio by Strategy and Remaining Commitment) (Details) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2012 | Dec. 31, 2011
--- | --- | ---
Carrying Value | $ 114,076 | $ 128,301
Alternative Investments [Member] |  |
Carrying Value | 109,664 | 124,548
Remaining Commitment | 55,908 |
Alternative Investments [Member] | Private Equity [Member] |  |
Carrying Value | 18,344 | 21,736
Remaining Commitment | 4,594 |
Retirement Income Plan [Member] | Alternative Investments [Member] |  |
Carrying Value | 12,631 | 17,016
Remaining Commitment |  | 4,291
Retirement Income Plan [Member] | Alternative Investments [Member] | Equity Long Short Hedge [Member] |  |
Carrying Value | 41 | 1,836
Remaining Commitment | 0 |
Retirement Income Plan [Member] | Alternative Investments [Member] | Private Equity [Member] |  |
Carrying Value | 10,385 | 12,586
Remaining Commitment | 3,703 |
Retirement Income Plan [Member] | Alternative Investments [Member] | Real Estate Strategy [Member] |  |
Carrying Value | 2,205 | 2,594
Remaining Commitment | $ 588 |

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- Definition This item represents contractual obligations to invest additional amounts on our other investment portfolio. These additional investments may or may not be required. + References No definition available. + Details
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Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Other investments not otherwise specified in the taxonomy. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.1(f,g)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.12) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 12 -Article 5 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph g -Article 7 + Details
Name: | us-gaap_OtherInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant