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v2.4.1.9

Summary of Significant Accounting Policies (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 9 Months Ended | 12 Months Ended
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Sep. 30, 2014 | Sep. 30, 2013 | Sep. 30, 2012 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Depreciation |  |  |  | $ 12.6 us-gaap_Depreciation | $ 10.2 us-gaap_Depreciation | $ 9.2 us-gaap_Depreciation
Percentage of fixed maturity portfolio utilizing market approach |  |  |  | 99.00%sigi_PercentageOfFixedMaturityPortfolioUtilizingMarketApproach |  |
Investment yields before tax assumed in premium deficiency assessment | 3.00%sigi_InvestmentYieldsAssumedInPremiumDeficiencyAssessment | 3.00%sigi_InvestmentYieldsAssumedInPremiumDeficiencyAssessment | 3.10%sigi_InvestmentYieldsAssumedInPremiumDeficiencyAssessment |  |  |
Deferred policy acquisition costs amortized to expense |  |  |  | $ 364.3 us-gaap_DeferredPolicyAcquisitionCostAmortizationExpense | $ 331.8 us-gaap_DeferredPolicyAcquisitionCostAmortizationExpense | $ 298.5 us-gaap_DeferredPolicyAcquisitionCostAmortizationExpense

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- Definition This element represents the investment yields assumed in the premium deficiency assessment, which are based on the entity's actual average investment yield before tax as of the calculation date. + References No definition available. + Details
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- Definition This element represents the percentage of our fixed maturity portfolio utilizing a market approach, primarily using matrix pricing models prepared by external pricing services. + References No definition available. + Details
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Namespace Prefix: | sigi_
Data Type: | num:percentItemType
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Period Type: | instant

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- Definition Amount of expense related to deferred policy acquisition costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 16 -Article 12 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.7(a)) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 7 -Article 7 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 18 -Article 12 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=35755530&loc=d3e11264-158415 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 30 -Section 35 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=28247317&loc=d3e10157-158409 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 45 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=6480726&loc=d3e6691-158385 + Details
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- Definition The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
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