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Investments (OTTI by Asset Type) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
OTTI Losses Gross | $ 11,104 us-gaap_ImpairmentOfInvestments | $ 5,566 us-gaap_ImpairmentOfInvestments | $ 1,711 us-gaap_ImpairmentOfInvestments
OTTI included in OCI | 0 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest | (77)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest | (2,553)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest
Total OTTI charges recognized in earnings | 11,104 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet | 5,643 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet | 4,264 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
Fixed Income Securities [Member] | AFS Fixed Income Securities [Member] |  |  |
OTTI Losses Gross | 7 us-gaap_ImpairmentOfInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FixedMaturitiesMember / us-gaap_StatementScenarioAxis = sigi_AfsFixedMaturitySecuritiesMember | 16 us-gaap_ImpairmentOfInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FixedMaturitiesMember / us-gaap_StatementScenarioAxis = sigi_AfsFixedMaturitySecuritiesMember | (1,462)us-gaap_ImpairmentOfInvestments / us-gaap_InvestmentTypeAxis = us-gaap_FixedMaturitiesMember / us-gaap_StatementScenarioAxis = sigi_AfsFixedMaturitySecuritiesMember
OTTI included in OCI | 0 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_InvestmentTypeAxis = us-gaap_FixedMaturitiesMember / us-gaap_StatementScenarioAxis = sigi_AfsFixedMaturitySecuritiesMember | (30)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_InvestmentTypeAxis = us-gaap_FixedMaturitiesMember / us-gaap_StatementScenarioAxis = sigi_AfsFixedMaturitySecuritiesMember | (2,553)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_InvestmentTypeAxis = us-gaap_FixedMaturitiesMember / us-gaap_StatementScenarioAxis = sigi_AfsFixedMaturitySecuritiesMember
Total OTTI charges recognized in earnings | 7 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / us-gaap_InvestmentTypeAxis = us-gaap_FixedMaturitiesMember / us-gaap_StatementScenarioAxis = sigi_AfsFixedMaturitySecuritiesMember | 46 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / us-gaap_InvestmentTypeAxis = us-gaap_FixedMaturitiesMember / us-gaap_StatementScenarioAxis = sigi_AfsFixedMaturitySecuritiesMember | 1,091 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / us-gaap_InvestmentTypeAxis = us-gaap_FixedMaturitiesMember / us-gaap_StatementScenarioAxis = sigi_AfsFixedMaturitySecuritiesMember
Equity Securities [Member] |  |  |
OTTI Losses Gross | 10,517 us-gaap_ImpairmentOfInvestments / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember | 3,747 us-gaap_ImpairmentOfInvestments / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember | 3,173 us-gaap_ImpairmentOfInvestments / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember
OTTI included in OCI | 0 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember | 0 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember | 0 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember
Total OTTI charges recognized in earnings | 10,517 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember | 3,747 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember | 3,173 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember
Other Investments [Member] |  |  |
OTTI Losses Gross | 580 us-gaap_ImpairmentOfInvestments / us-gaap_InvestmentTypeAxis = us-gaap_OtherInvestmentsMember | 1,847 us-gaap_ImpairmentOfInvestments / us-gaap_InvestmentTypeAxis = us-gaap_OtherInvestmentsMember |
OTTI included in OCI | 0 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_InvestmentTypeAxis = us-gaap_OtherInvestmentsMember | 0 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_InvestmentTypeAxis = us-gaap_OtherInvestmentsMember |
Total OTTI charges recognized in earnings | 580 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / us-gaap_InvestmentTypeAxis = us-gaap_OtherInvestmentsMember | 1,847 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / us-gaap_InvestmentTypeAxis = us-gaap_OtherInvestmentsMember |
Available-for-sale Securities [Member] |  |  |
OTTI Losses Gross | 10,524 us-gaap_ImpairmentOfInvestments / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_AvailableforsaleSecuritiesMember | 3,763 us-gaap_ImpairmentOfInvestments / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_AvailableforsaleSecuritiesMember |
OTTI included in OCI | 0 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_AvailableforsaleSecuritiesMember | (30)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_AvailableforsaleSecuritiesMember |
Total OTTI charges recognized in earnings | 10,524 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_AvailableforsaleSecuritiesMember | 3,793 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_AvailableforsaleSecuritiesMember |
Fixed Income Securities [Member] | Asset-backed Securities [Member] | AFS Fixed Income Securities [Member] |  |  |
OTTI Losses Gross |  |  | 98 us-gaap_ImpairmentOfInvestments / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_FixedMaturitiesMember / us-gaap_StatementScenarioAxis = sigi_AfsFixedMaturitySecuritiesMember
OTTI included in OCI |  |  | 0 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_FixedMaturitiesMember / us-gaap_StatementScenarioAxis = sigi_AfsFixedMaturitySecuritiesMember
Total OTTI charges recognized in earnings |  |  | 98 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_FixedMaturitiesMember / us-gaap_StatementScenarioAxis = sigi_AfsFixedMaturitySecuritiesMember
Fixed Income Securities [Member] | Asset-backed Securities [Member] | Held To Maturity Fixed Maturity Securities [Member] |  |  |
OTTI Losses Gross |  | (44)us-gaap_ImpairmentOfInvestments / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_FixedMaturitiesMember / us-gaap_StatementScenarioAxis = sigi_HeldToMaturityFixedMaturitySecuritiesMember |
OTTI included in OCI |  | (47)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_FixedMaturitiesMember / us-gaap_StatementScenarioAxis = sigi_HeldToMaturityFixedMaturitySecuritiesMember |
Total OTTI charges recognized in earnings |  | 3 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_FixedMaturitiesMember / us-gaap_StatementScenarioAxis = sigi_HeldToMaturityFixedMaturitySecuritiesMember |
Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member] | AFS Fixed Income Securities [Member] |  |  |
OTTI Losses Gross |  |  | (1,525)us-gaap_ImpairmentOfInvestments / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_FixedMaturitiesMember / us-gaap_StatementScenarioAxis = sigi_AfsFixedMaturitySecuritiesMember
OTTI included in OCI |  |  | (2,335)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_FixedMaturitiesMember / us-gaap_StatementScenarioAxis = sigi_AfsFixedMaturitySecuritiesMember
Total OTTI charges recognized in earnings |  |  | 810 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_FixedMaturitiesMember / us-gaap_StatementScenarioAxis = sigi_AfsFixedMaturitySecuritiesMember
Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member] | AFS Fixed Income Securities [Member] |  |  |
OTTI Losses Gross | 7 us-gaap_ImpairmentOfInvestments / us-gaap_InvestmentTypeAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_FixedMaturitiesMember / us-gaap_StatementScenarioAxis = sigi_AfsFixedMaturitySecuritiesMember | 16 us-gaap_ImpairmentOfInvestments / us-gaap_InvestmentTypeAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_FixedMaturitiesMember / us-gaap_StatementScenarioAxis = sigi_AfsFixedMaturitySecuritiesMember | (35)us-gaap_ImpairmentOfInvestments / us-gaap_InvestmentTypeAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_FixedMaturitiesMember / us-gaap_StatementScenarioAxis = sigi_AfsFixedMaturitySecuritiesMember
OTTI included in OCI | 0 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_InvestmentTypeAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_FixedMaturitiesMember / us-gaap_StatementScenarioAxis = sigi_AfsFixedMaturitySecuritiesMember | (30)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_InvestmentTypeAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_FixedMaturitiesMember / us-gaap_StatementScenarioAxis = sigi_AfsFixedMaturitySecuritiesMember | (218)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossBeforeTaxIncludingPortionAttributableToNoncontrollingInterest / us-gaap_InvestmentTypeAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_FixedMaturitiesMember / us-gaap_StatementScenarioAxis = sigi_AfsFixedMaturitySecuritiesMember
Total OTTI charges recognized in earnings | 7 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / us-gaap_InvestmentTypeAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_FixedMaturitiesMember / us-gaap_StatementScenarioAxis = sigi_AfsFixedMaturitySecuritiesMember | 46 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / us-gaap_InvestmentTypeAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_FixedMaturitiesMember / us-gaap_StatementScenarioAxis = sigi_AfsFixedMaturitySecuritiesMember | 183 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / us-gaap_InvestmentTypeAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = us-gaap_FixedMaturitiesMember / us-gaap_StatementScenarioAxis = sigi_AfsFixedMaturitySecuritiesMember
Securities Which Will Not Recover In The Near Term [Member] | Equity Securities [Member] |  |  |
Total OTTI charges recognized in earnings |  | 2,000 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = sigi_SecuritiesWhichWillNotRecoverInNearTermMember | 1,000 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = sigi_SecuritiesWhichWillNotRecoverInNearTermMember
Securities In Loss Position With Intent to Sell [Member] | Equity Securities [Member] |  |  |
Total OTTI charges recognized in earnings | $ 9,000 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = sigi_SecuritiesInLossPositionWithIntentToSellMember | $ 1,700 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = sigi_SecuritiesInLossPositionWithIntentToSellMember | $ 2,200 us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember / sigi_ScheduleOfOttiByAssetTypeAxis = sigi_SecuritiesInLossPositionWithIntentToSellMember

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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=31042434&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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