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v2.4.1.9

Retirement Plans (Plan's Allocated Target and Ranges and Actual Weighted Average Asset Allocation by Investment Categories) (Details) | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Actual percentage | 100.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations | 100.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations |
Maximum percentage of equity security value of total plans portfolio value that is allowed | 8.00%sigi_MaximumPercentageOfAnySecurityValueOfTotalPlansPortfolioVaueThatIsAllowed |  |
Maximum percentage of any corporation outstanding shares that is allowed for retirement income plan equity portfolio | 5.00%sigi_MaximumPercentageOfAnyCorporationOutstandingSharesThatIsAllowedForRetirementIncomePlan |  |
Global Asset Allocation [Member] |  |  |
Actual percentage | 11.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_GlobalAssetAllocationMember |  | 12.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_GlobalAssetAllocationMember
Range minimum | 0.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_GlobalAssetAllocationMember |  |
Range maximum | 0.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_GlobalAssetAllocationMember |  |
Private Equity Funds [Member] |  |  |
Actual percentage | 4.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PrivateEquityFundsMember | 5.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PrivateEquityFundsMember |
Range minimum | 0.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PrivateEquityFundsMember |  |
Range maximum | 0.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = us-gaap_PrivateEquityFundsMember |  |
Cash And Short Term Investments [Member] |  |  |
Actual percentage | 1.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_CashAndShortTermInvestmentsMember | 1.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_CashAndShortTermInvestmentsMember |
Range minimum | 0.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_CashAndShortTermInvestmentsMember |  |
Range maximum | 0.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_CashAndShortTermInvestmentsMember |  |
Extended Duration Fixed Maturity [Member] |  |  |
Actual percentage | 59.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_TotalLongDurationFixedIncomeMember | 55.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_TotalLongDurationFixedIncomeMember |
Range minimum | 55.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_TotalLongDurationFixedIncomeMember |  |
Range maximum | 100.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_TotalLongDurationFixedIncomeMember |  |
Global Equity [Member] |  |  |
Actual percentage | 25.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_GlobalEquityMember | 27.00%us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_GlobalEquityMember |
Range minimum | 0.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMinimum / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_GlobalEquityMember |  |
Range maximum | 45.00%us-gaap_DefinedBenefitPlanTargetPlanAssetAllocationsRangeMaximum / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_GlobalEquityMember |  |

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- Definition The maximum percentage of outstanding shares of any one corporation that is allowed for the Retirement Income Plan. + References No definition available. + Details
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- Definition The maximum percentage of any individual security value of the total Retirement Income Plan's portfolio value that is allowed. + References No definition available. + Details
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- Definition Target allocation maximum percentage of investments of this type to total plan assets presented on a weighted average basis. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(i) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c)(i) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2446-114920 + Details
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- Definition Target allocation minimum percentage of investments of this type to total plan assets presented on a weighted average basis. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(i) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c)(i) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2446-114920 + Details
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- Definition Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e2446-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
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