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Reserve for Losses and Loss Expenses (Exposure to Environmental Claims) (Details) (USD $) In Thousands, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
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Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Loss and Loss expense reserves, gross | $ 30,653 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross | $ 32,764 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross | $ 35,575 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross | $ 36,012 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross
Liability for Asbestos and Environmental Claims, Net | 22,994 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet | 25,167 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet | 27,769 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet | 27,916 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet
Asbestos [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Loss and Loss expense reserves, gross | 8,751 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember | 8,897 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember | 9,170 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember | 8,412 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember
Liability for Asbestos and Environmental Claims, Net | 7,314 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember | 7,518 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember | 7,791 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember | 6,586 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember
Landfill Sites Related Claims [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Loss and Loss expense reserves, gross | 11,500 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_LandfillSitesRelatedClaimsMember |  |  |
Liability for Asbestos and Environmental Claims, Net | 6,556 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_LandfillSitesRelatedClaimsMember |  |  |
Leaking Underground Storage Tanks Claims [Member] |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Loss and Loss expense reserves, gross | 10,400 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_LeakingUndergroundStorageTanksMember |  |  |
Liability for Asbestos and Environmental Claims, Net | $ 9,124 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_LeakingUndergroundStorageTanksMember |  |  |

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- Definition Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to asbestos and environmental claims, before estimated recoveries from reinsurers. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
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- Definition Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to asbestos and environmental claims, net of estimated reinsurance recoveries. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
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