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Commitments and Contingencies (Narrative) (Details) (USD $) | 12 Months Ended
--- | ---
Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009
Purchase amount of annuities | $ 8,400,000 |  |
Rental expense for operating leases for office space and equipment | 11,600,000 | 11,400,000 | 11,500,000
Total future minimum rental commitments under non-cancelable leases | 25,400,000 |  |
Other Investments [Member] |  |  |
Remaining Commitment | $ 56,865,000 |  |

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- Definition The amount of annuities purchased from life insurance companies to fulfill obligations under claim settlements that provide for periodic future payments to claimants. + References No definition available. + Details
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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
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X
- Definition Rental expense incurred for leased assets including furniture and equipment which has not been recognized in costs and expenses applicable to sales and revenues; for example, cost of goods sold or other operating costs and expenses. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition The total of future contractually required payments on leases defined as operating. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 13 -Paragraph 122 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 13 -Paragraph 16 -Subparagraph b(i) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 10 -Section 55 -Paragraph 40 -Subparagraph (Note 3) -URI http://asc.fasb.org/extlink&oid=6584154&loc=d3e38371-112697 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41502-112717 + Details
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