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Consolidation Statements of Comprehensive Income (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Net income	$ 39,708us-gaap_NetIncomeLoss	$ 17,974us-gaap_NetIncomeLoss
Unrealized holding gains arising during period	15,586us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax	21,426us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax
Amount reclassified into net income: Held-to-maturity securities	(170)sigi_AmortizationofNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleNetofTax	(296)sigi_AmortizationofNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleNetofTax
Amount reclassified into net income: Non-credit other-than-temporary impairments	232sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesNetOfTax	0sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesNetOfTax
Amount reclassified into net income: Realized gains on available for sale securities	(12,932)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax	(4,699)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax
Total unrealized gains on investment securities	2,716sigi_TotalUnrealizedGainsLossesonInvestmentSecurities	16,431sigi_TotalUnrealizedGainsLossesonInvestmentSecurities
Amount reclassified into net income: Net actuarial loss	1,111us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax	247us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansForNetGainLossNetOfTax
Total defined benefit pension and post-retirement plans	1,111us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax	247us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax
Other comprehensive income	3,827us-gaap_OtherComprehensiveIncomeLossNetOfTax	16,678us-gaap_OtherComprehensiveIncomeLossNetOfTax
Comprehensive income	$ 43,535us-gaap_ComprehensiveIncomeNetOfTax	$ 34,652us-gaap_ComprehensiveIncomeNetOfTax

X

- Definition

Net of tax amount of the reclassification adjustment related 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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Amount after tax of reclassification adjustment from accumulated other comprehensive income related to non-credit OTTI securities.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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