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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
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and Loss expense reserves, gross	$ 30,653us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross	$ 32,764us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross	$ 35,575us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross	$ 36,012us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross
Liability for Asbestos and Environmental Claims, Net	22,994us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet	25,167us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet	27,769us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet	27,916us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet
Asbestos [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and Loss expense reserves, gross	8,751us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember	8,897us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember	9,170us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember	8,412us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember
Liability for Asbestos and Environmental Claims, Net	7,314us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember	7,518us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember	7,791us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember	6,586us-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	11,500us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_LandfillSitesRelatedClaimsMember
Liability for Asbestos and Environmental Claims, Net	6,556us-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	10,400us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_LeakingUndergroundStorageTanksMember
Liability for Asbestos and Environmental Claims, Net	$ 9,124us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_LeakingUndergroundStorageTanksMember

X

- 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

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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)

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