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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 | $ 19,788 us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax |
Amounts reclassified into Net Income: Non-credit other-than-temporary impairments | 232 sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesNetOfTax | 0 sigi_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIForNonCreditOtherThanTemporaryImpairedSecuritiesNetOfTax
OCI before reclassifications, Defined Benefit Pension and Post Retirement Plans | 0 us-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 | 3,827 us-gaap_OtherComprehensiveIncomeLossNetOfTax | 16,678 us-gaap_OtherComprehensiveIncomeLossNetOfTax
Other Comprehensive Income, Pension and Other Postretirement Benefit Plans, Adjustment, Net of Tax | 1,111 us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax | 247 us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax
Ending Balance, March, 2015 | 23,615 us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax |
Available-for-sale Securities [Member] |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |
Beginning Balance,December 31, 2014 | 80,284 us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember |
OCI before reclassifications | 15,586 us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember |
Other comprehensive income | 2,654 us-gaap_OtherComprehensiveIncomeLossNetOfTax / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember |
Ending Balance, March, 2015 | 82,938 us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_AvailableforsaleSecuritiesMember |
Investments [Member] |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |
Beginning Balance,December 31, 2014 | 80,393 us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_InvestmentsMember |
OCI before reclassifications | 15,586 us-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 | 2,716 us-gaap_OtherComprehensiveIncomeLossNetOfTax / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_InvestmentsMember |
Ending Balance, March, 2015 | 83,109 us-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 | 0 us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherThanTemporaryImpairmentMember |
Other comprehensive income | 232 us-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 | 623 us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_GainLossOnInvestmentsByCategoryAxis = us-gaap_HeldtomaturitySecuritiesMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember |
OCI before reclassifications | 0 us-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 | 453 us-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 | 1,111 us-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 | 19,788 us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember |
OCI before reclassifications | 15,586 us-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 | 3,827 us-gaap_OtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember | 16,678 us-gaap_OtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember
Ending Balance, March, 2015 | $ 23,615 us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedOtherComprehensiveIncomeMember |

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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669686-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14 -URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e681-108580 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e637-108580 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 31 -Article 5 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 + Details
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