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Investments (Fair Value and Gross Pre-Tax Net Unrealized Unrecognized Loss of Securities By Length of Time) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
AFS Fair Value, Less than 12 months	$ 466,387us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue		$ 1,411,803us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
AFS Unrealized Losses, Less than 12 months	(2,408)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss	[1]	(49,106)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss	[1]
AFS Fair Value, 12 months or longer	442,058us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue		40,731us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
AFS Unrealized Loss, 12 Months or Longer	(8,170)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss	[1]	(2,865)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss	[1]
HTM Fair Value, Less than 12 months	196us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue		65us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
HTM Unrealized Losses, Less than 12 months	(3)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss	[1]	(5)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss	[1]
HTM Unrecognized Gains, Less than 12 months	1sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionLessThanTwelveMonths		5sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionLessThanTwelveMonths
HTM Fair Value, 12 months or longer	2,235us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue		2,931us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
HTM Unrealized Losses, 12 Months or Longer	(455)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss	[1]	(675)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss	[1]
HTM Unrecognized Gains, 12 months or longer	439sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionTwelveMonthsorLonger		635sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionTwelveMonthsorLonger
Available For Sale and Held To Maturity Securities Continuous Unrealized Loss Position Less Than Twelve Months Fair Value	466,583sigi_AvailableForSaleandHeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue		1,411,868sigi_AvailableForSaleandHeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Available For Sale And Held To Maturity Securities Continuous Unrealized Loss Position Less Than Twelve Months Aggregate Losses	2,411sigi_AvailableForSaleAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsAggregateLosses	[1]	49,111sigi_AvailableForSaleAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsAggregateLosses	[1]
Held To Maturity And Available For Sale Securities Unrecognized Gains Losses Aggregate Continuous Unrealized Loss Position Less Than Twelve Months	1sigi_HeldToMaturityAndAvailableForSaleSecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionLessThanTwelveMonths		5sigi_HeldToMaturityAndAvailableForSaleSecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionLessThanTwelveMonths
Available For Sale and Held To Maturity Securities Continuous Unrealized Loss Position Twelve Months Or Longer Fair Value	444,293sigi_AvailableForSaleandHeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue		43,662sigi_AvailableForSaleandHeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Available For Sale And Held To Maturity Securities Continuous Unrealized Loss Position Twelve Months Or Longer Aggregate Losses	8,625sigi_AvailableForSaleAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerAggregateLosses	[1]	3,540sigi_AvailableForSaleAndHeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerAggregateLosses	[1]
Held To Maturity And Available For Sale Securities Unrecognized Gains Losses Aggregate Continuous Unrealized Loss Position For Twelve Months Or Longer	439sigi_HeldToMaturityAndAvailableForSaleSecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionForTwelveMonthsOrLonger		635sigi_HeldToMaturityAndAvailableForSaleSecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionForTwelveMonthsOrLonger
Obligations of States and Political Subdivisions [Member]
HTM Fair Value, Less than 12 months	196us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember		65us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
HTM Unrealized Losses, Less than 12 months	(3)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember	[1]	(5)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember	[1]
HTM Unrecognized Gains, Less than 12 months	1sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionLessThanTwelveMonths / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember		5sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionLessThanTwelveMonths / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
HTM Fair Value, 12 months or longer	0us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember		441us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
HTM Unrealized Losses, 12 Months or Longer	0us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember	[1]	(20)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember	[1]
HTM Unrecognized Gains, 12 months or longer	0sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionTwelveMonthsorLonger / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember		14sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionTwelveMonthsorLonger / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Asset-backed Securities [Member]
HTM Fair Value, Less than 12 months	0us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember		0us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember
HTM Unrealized Losses, Less than 12 months	0us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember	[1]	0us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember	[1]
HTM Unrecognized Gains, Less than 12 months	0sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionLessThanTwelveMonths / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember		0sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionLessThanTwelveMonths / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember
HTM Fair Value, 12 months or longer	2,235us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember		2,490us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember
HTM Unrealized Losses, 12 Months or Longer	(455)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember	[1]	(655)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember	[1]
HTM Unrecognized Gains, 12 months or longer	439sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionTwelveMonthsorLonger / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember		621sigi_HeldToMaturitySecuritiesUnrecognizedGainsLossesAggregateContinuousUnrealizedLossPositionTwelveMonthsorLonger / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember
AFS Fixed Income Securities [Member]
AFS Fair Value, Less than 12 months	461,125us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember		1,410,679us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember
AFS Unrealized Losses, Less than 12 months	(2,072)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]	(49,010)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]
AFS Fair Value, 12 months or longer	442,058us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember		40,731us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember
AFS Unrealized Loss, 12 Months or Longer	(8,170)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]	(2,865)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]
AFS Fixed Income Securities [Member] | U.S. Government and Government Agencies [Member]
AFS Fair Value, Less than 12 months	7,567us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_USTreasuryAndGovernmentMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember		16,955us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_USTreasuryAndGovernmentMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember
AFS Unrealized Losses, Less than 12 months	(13)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_USTreasuryAndGovernmentMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]	(500)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_USTreasuryAndGovernmentMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]
AFS Fair Value, 12 months or longer	10,866us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_USTreasuryAndGovernmentMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember		507us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_USTreasuryAndGovernmentMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember
AFS Unrealized Loss, 12 Months or Longer	(115)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_USTreasuryAndGovernmentMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]	(4)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_USTreasuryAndGovernmentMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]
AFS Fixed Income Securities [Member] | Foreign Government [Member]
AFS Fair Value, Less than 12 months			2,029us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember
AFS Unrealized Losses, Less than 12 months			(30)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]
AFS Fair Value, 12 months or longer			2,955us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember
AFS Unrealized Loss, 12 Months or Longer			(42)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_ForeignGovernmentDebtSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]
AFS Fixed Income Securities [Member] | Obligations of States and Political Subdivisions [Member]
AFS Fair Value, Less than 12 months	47,510us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember		442,531us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember
AFS Unrealized Losses, Less than 12 months	(105)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]	(19,120)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]
AFS Fair Value, 12 months or longer	64,018us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember		13,530us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember
AFS Unrealized Loss, 12 Months or Longer	(624)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]	(905)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]
AFS Fixed Income Securities [Member] | Corporate Securities [Member]
AFS Fair Value, Less than 12 months	276,648us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember		511,100us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember
AFS Unrealized Losses, Less than 12 months	(1,734)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]	(15,911)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]
AFS Fair Value, 12 months or longer	153,613us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember		14,771us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember
AFS Unrealized Loss, 12 Months or Longer	(4,075)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]	(1,138)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_CorporateDebtSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]
AFS Fixed Income Securities [Member] | Asset-backed Securities [Member]
AFS Fair Value, Less than 12 months	113,202us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember		68,725us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember
AFS Unrealized Losses, Less than 12 months	(178)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]	(468)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]
AFS Fair Value, 12 months or longer	15,618us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember		0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember
AFS Unrealized Loss, 12 Months or Longer	(195)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]	0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_AssetBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]
AFS Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member]
AFS Fair Value, Less than 12 months	12,799us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember		100,396us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember
AFS Unrealized Losses, Less than 12 months	(34)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]	(2,950)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]
AFS Fair Value, 12 months or longer	59,219us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember		6,298us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember
AFS Unrealized Loss, 12 Months or Longer	(743)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]	(516)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member]
AFS Fair Value, Less than 12 months	3,399us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember		268,943us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_InvestmentTypeAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember
AFS Unrealized Losses, Less than 12 months	(8)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]	(10,031)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]
AFS Fair Value, 12 months or longer	138,724us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember		2,670us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_InvestmentTypeAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember
AFS Unrealized Loss, 12 Months or Longer	(2,418)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]	(260)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_InvestmentTypeAxis = us-gaap_ResidentialMortgageBackedSecuritiesMember / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = sigi_AfsFixedMaturitySecuritiesMember	[1]
Equity Securities [Member]
AFS Fair Value, Less than 12 months	5,262us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = us-gaap_EquitySecuritiesMember		1,124us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = us-gaap_EquitySecuritiesMember
AFS Unrealized Losses, Less than 12 months	(336)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = us-gaap_EquitySecuritiesMember	[1]	(96)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = us-gaap_EquitySecuritiesMember	[1]
AFS Fair Value, 12 months or longer	0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = us-gaap_EquitySecuritiesMember		0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = us-gaap_EquitySecuritiesMember
AFS Unrealized Loss, 12 Months or Longer	$ 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = us-gaap_EquitySecuritiesMember	[1]	$ 0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / sigi_ScheduleOfFairValueAndGrossPreTaxUnrealizedUnrecognizedLossOfSecuritiesByLengthOfTimeAxis = us-gaap_EquitySecuritiesMember	[1]
[1] Gross unrealized losses include non-OTTI unrealized amounts and OTTI losses recognized in AOCI. In addition, this column includes remaining unrealized gain or loss amounts on securities that were transferred to an HTM designation in the first quarter of 2009 for those securities that are in a net unrealized/unrecognized loss position.

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