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Comprehensive Income (Components of Accumulated Other Comprehensive Income) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance,December 31, 2014	$ 19,788us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
Amounts reclassified into Net Income: Non-credit other-than-temporary impairments	232sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesNetOfTax	0sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesNetOfTax
OCI before reclassifications, Defined Benefit Pension and Post Retirement Plans	0us-gaap_OtherComprehensiveIncomeDefinedBenefitPlansNetUnamortizedGainLossArisingDuringPeriodNetOfTax
Amounts reclassified from AOCI, HTM Related	(170)sigi_AmortizationofNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleNetofTax	(296)sigi_AmortizationofNetUnrealizedGainsLossesOnHeldToMaturityTransferredFromAvailableForSaleNetofTax
Amounts reclassified from AOCI, All Other	(12,932)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax	(4,699)us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForSaleOfSecuritiesNetOfTax
Other comprehensive income	3,827us-gaap_OtherComprehensiveIncomeLossNetOfTax	16,678us-gaap_OtherComprehensiveIncomeLossNetOfTax
Other Comprehensive Income, Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax	1,111us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax	247us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax
Ending Balance, March, 2015	23,615us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax
Available-for-sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance,December 31, 2014	80,284us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember
OCI before reclassifications	15,586us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember
Other comprehensive income	2,654us-gaap_OtherComprehensiveIncomeLossNetOfTax / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember
Ending Balance, March, 2015	82,938us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember
Investments [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance,December 31, 2014	80,393us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_InvestmentsMember
OCI before reclassifications	15,586us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_InvestmentsMember
Amounts reclassified from AOCI	(12,870)us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_InvestmentsMember
Other comprehensive income	2,716us-gaap_OtherComprehensiveIncomeLossNetOfTax / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_InvestmentsMember
Ending Balance, March, 2015	83,109us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_InvestmentsMember
Accumulated Other-than-Temporary Impairment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance,December 31, 2014	(514)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherThanTemporaryImpairmentMember
OCI before reclassifications	0us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherThanTemporaryImpairmentMember
Other comprehensive income	232us-gaap_OtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherThanTemporaryImpairmentMember
Ending Balance, March, 2015	(282)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherThanTemporaryImpairmentMember
Accumulated Net Unrealized Investment Gain (Loss) [Member] | Held-to-maturity Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance,December 31, 2014	623us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_HeldtomaturitySecuritiesMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
OCI before reclassifications	0us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_HeldtomaturitySecuritiesMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
Other comprehensive income	(170)us-gaap_OtherComprehensiveIncomeLossNetOfTax / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_HeldtomaturitySecuritiesMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
Ending Balance, March, 2015	453us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_HeldtomaturitySecuritiesMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
Accumulated Defined Benefit Plans Adjustment [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance,December 31, 2014	(60,605)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
Other Comprehensive Income, Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax	1,111us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
Ending Balance, March, 2015	(59,494)us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedDefinedBenefitPlansAdjustmentMember
Accumulated other comprehensive income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance,December 31, 2014	19,788us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember
OCI before reclassifications	15,586us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember
Amounts reclassified from AOCI	(11,759)us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember
Other comprehensive income	3,827us-gaap_OtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember	16,678us-gaap_OtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember
Ending Balance, March, 2015	$ 23,615us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember

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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Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

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Amount after tax of (increase) decrease in the value of the projected benefit obligation related to a change in an actuarial assumption and increase (decrease) in the value of the plan assets resulting from experience different from that assumed, that has not been recognized in net periodic benefit (cost) credit.

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Amount after tax, before reclassification adjustments of other comprehensive income (loss).

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Amount after tax and reclassification adjustments of other comprehensive income (loss).

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Amount after tax of reclassification adjustment from accumulated other comprehensive income for unrealized gain (loss) realized upon the sale of available-for-sale securities.

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