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Investments (Other Investment Portfolio by Strategy and the Remaining Commitment Amount Associated With Each Strategy) (Details) (USD $) In Thousands, unless otherwise specified | Sep. 30, 2014 | Dec. 31, 2013
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Carrying Value | $ 106,548 | $ 107,875
Other Investments [Member] |  |
Carrying Value | 106,548 | 107,875
Remaining Commitment | 50,309 |
Alternative Investments [Member] |  |
Carrying Value | 100,097 | 106,211
Remaining Commitment | 50,309 |
Secondary Private Equity Member | Alternative Investments [Member] |  |
Carrying Value | 23,064 | 25,618
Remaining Commitment | 6,978 |
Private Equity [Member] | Alternative Investments [Member] |  |
Carrying Value | 21,569 | 20,192
Remaining Commitment | 9,181 |
Energy Power Generation [Member] | Alternative Investments [Member] |  |
Carrying Value | 16,659 | 17,361
Remaining Commitment | 6,984 |
Mezzanine Financing [Member] | Alternative Investments [Member] |  |
Carrying Value | 11,273 | 12,738
Remaining Commitment | 13,774 |
Real estate [Member] | Alternative Investments [Member] |  |
Carrying Value | 11,212 | 11,698
Remaining Commitment | 10,061 |
Distressed debt [Member] | Alternative Investments [Member] |  |
Carrying Value | 9,205 | 11,579
Remaining Commitment | 2,981 |
Venture Capital [Member] | Alternative Investments [Member] |  |
Carrying Value | 7,115 | 7,025
Remaining Commitment | 350 |
Other Securities [Member] |  |
Carrying Value | 6,451 | 1,664
Remaining Commitment | $ 0 |

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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
Name: | sigi_RemainingCommitment
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