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Investments (OTTI by Asset Type) (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
OTTI Losses Gross | $ 2,094 us-gaap_ImpairmentOfInvestments | $ 963 us-gaap_ImpairmentOfInvestments
Other-than-temporary impairments on fixed income securities recognized in other comprehensive income | 0 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest | 0 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest
OTTI Losses Recognized in Earnings | 2,094 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet | 963 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
Residential Mortgage Backed Securities [Member] | AFS Fixed Income Securities [Member] |  |
OTTI Losses Gross | 1 us-gaap_ImpairmentOfInvestments / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |
Other-than-temporary impairments on fixed income securities recognized in other comprehensive income | 0 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |
OTTI Losses Recognized in Earnings | 1 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / us-gaap_InvestmentTypeAxis = sigi_AFSFixedIncomeSecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember |
AFS Fixed Income Securities [Member] |  |
OTTI Losses Gross | 1,010 us-gaap_ImpairmentOfInvestments / sigi_ScheduleOfOttiByAssetTypeAxis = sigi_AFSFixedIncomeSecuritiesMember |
Other-than-temporary impairments on fixed income securities recognized in other comprehensive income | 0 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest / sigi_ScheduleOfOttiByAssetTypeAxis = sigi_AFSFixedIncomeSecuritiesMember |
OTTI Losses Recognized in Earnings | 1,010 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / sigi_ScheduleOfOttiByAssetTypeAxis = sigi_AFSFixedIncomeSecuritiesMember |
AFS Fixed Income Securities [Member] | Corporate Securities [Member] |  |
OTTI Losses Gross | 1,009 us-gaap_ImpairmentOfInvestments / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = sigi_AFSFixedIncomeSecuritiesMember |
Other-than-temporary impairments on fixed income securities recognized in other comprehensive income | 0 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = sigi_AFSFixedIncomeSecuritiesMember |
OTTI Losses Recognized in Earnings | 1,009 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = sigi_AFSFixedIncomeSecuritiesMember |
Total AFS Securities [Member] | Equity Securities [Member] |  |
OTTI Losses Gross | 1,084 us-gaap_ImpairmentOfInvestments / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_AvailableforsaleSecuritiesMember | 963 us-gaap_ImpairmentOfInvestments / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_AvailableforsaleSecuritiesMember
Other-than-temporary impairments on fixed income securities recognized in other comprehensive income | 0 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_AvailableforsaleSecuritiesMember | 0 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_AvailableforsaleSecuritiesMember
OTTI Losses Recognized in Earnings | $ 1,084 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_AvailableforsaleSecuritiesMember | $ 963 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_AvailableforsaleSecuritiesMember

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- Definition The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income. + 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=56944662&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 34 -URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e25351-111560 + Details
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- Definition Amount before tax of other than temporary impairment (OTTI) loss on investments in available-for-sale and held-to-maturity debt securities, recognized in other comprehensive loss. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 10A -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=36458714&loc=SL7669646-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 35 -Paragraph 35 -URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e25362-111560 + Details
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- Definition The amount of other than temporary impairment (OTTI) losses on equity securities, OTTI related to credit losses on debt securities, and OTTI losses on debt securities when the entity intends to sell the securities or it is more likely than not that the entity will be required to sell the securities before recovery of its amortized cost basis. Additionally, this item includes OTTI losses recognized during the period on investments accounted for under the cost method of accounting. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8B -URI http://asc.fasb.org/extlink&oid=27724398&loc=SL6284393-111563 + Details
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