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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, 2013 | Dec. 31, 2012
--- | --- | ---
Carrying Value | $ 107,875 | $ 114,076
Alternative Investments [Member] |  |
Carrying Value | 106,211 | 109,664
Remaining Commitment | 56,538 |
Alternative Investments [Member] | Private Equity [Member] |  |
Carrying Value | 20,192 | 18,344
Remaining Commitment | 9,998 |
Alternative Investments [Member] | Real Estate Strategy [Member] |  |
Carrying Value | 11,698 | 11,751
Remaining Commitment | 10,203 |
Retirement Income Plan [Member] | Alternative Investments [Member] |  |
Carrying Value | 12,159 | 12,631
Remaining Commitment | 3,851 |
Retirement Income Plan [Member] | Alternative Investments [Member] | Equity Long Short Hedge [Member] |  |
Carrying Value | 41 | 41
Remaining Commitment | 0 |
Retirement Income Plan [Member] | Alternative Investments [Member] | Private Equity [Member] |  |
Carrying Value | 9,899 | 10,385
Remaining Commitment | 3,293 |
Retirement Income Plan [Member] | Alternative Investments [Member] | Real Estate Strategy [Member] |  |
Carrying Value | 2,219 | 2,205
Remaining Commitment | $ 558 |

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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 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 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 12 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph g -Article 7 Reference 4: 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 + Details
Name: | us-gaap_OtherInvestments
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant