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Consolidation Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Net income	$ 141,827us-gaap_NetIncomeLoss	$ 106,418us-gaap_NetIncomeLoss	$ 37,963us-gaap_NetIncomeLoss
Unrealized holding gains (losses) arising during period	47,411us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax	(54,557)us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax	30,937us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax
Non-credit portion of other-than-temporary impairments recognized in other comprehensive income	0sigi_NonCreditPortionOfOtherThanTemporaryImpairmentsRecognizedInOtherComprehensiveIncome	50sigi_NonCreditPortionOfOtherThanTemporaryImpairmentsRecognizedInOtherComprehensiveIncome	1,660sigi_NonCreditPortionOfOtherThanTemporaryImpairmentsRecognizedInOtherComprehensiveIncome
Amount reclassified into net income: Held-to-maturity securities	(844)sigi_AmortizationofNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleNetOfTax	(1,025)sigi_AmortizationofNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleNetOfTax	(1,581)sigi_AmortizationofNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleNetOfTax
Non-credit other-than-temporary impairment	1,085sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesNetOfTax	9sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesNetOfTax	182sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesNetOfTax
Realized gains on available for sale securities	(18,762)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax	(15,301)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax	(6,118)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax
Total unrealized gains (losses) on investment securities	28,890sigi_TotalUnrealizedGainsLossesonInvestmentSecurities	(70,824)sigi_TotalUnrealizedGainsLossesonInvestmentSecurities	25,080sigi_TotalUnrealizedGainsLossesonInvestmentSecurities
Net actuarial (loss) gain	(35,189)us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax	38,775us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax	(17,268)us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax
Amount reclassified into net income: Net actuarial loss	1,236us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax	2,843us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax	3,837us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax
Amount reclassified into net income: Prior service cost	0us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditNetOfTax	6us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditNetOfTax	97us-gaap_OtherComprehensiveIncomeLossAmortizationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetPriorServiceCostCreditNetOfTax
Amount reclassified into net income: Curtailment expense	0sigi_OtherComprehensiveIncomeLossAmortizationPensionAndOtherPostRetirementBenefitPlansCurtailmentExpenseRecognizedInNetPeriodicBenefitCostNetOfTax	11sigi_OtherComprehensiveIncomeLossAmortizationPensionAndOtherPostRetirementBenefitPlansCurtailmentExpenseRecognizedInNetPeriodicBenefitCostNetOfTax	0sigi_OtherComprehensiveIncomeLossAmortizationPensionAndOtherPostRetirementBenefitPlansCurtailmentExpenseRecognizedInNetPeriodicBenefitCostNetOfTax
Total defined benefit pension and post-retirement plans	(33,953)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax	41,635us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax	(13,334)us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax
Other comprehensive (loss) income	(5,063)us-gaap_OtherComprehensiveIncomeLossNetOfTax	(29,189)us-gaap_OtherComprehensiveIncomeLossNetOfTax	11,746us-gaap_OtherComprehensiveIncomeLossNetOfTax
Comprehensive income	$ 136,764us-gaap_ComprehensiveIncomeNetOfTax	$ 77,229us-gaap_ComprehensiveIncomeNetOfTax	$ 49,709us-gaap_ComprehensiveIncomeNetOfTax

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Net of tax amount of the reclassification adjustment relaated to held-to-maturity securities transferred from the available-for-sale category. This amount includes amortization and amounts realized from the disposal of these securities.

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