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Fair Value Measurements (Carrying Value and Estimated Fair Value of Financial Instruments) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Held-to-maturity Securities	$ 296,557us-gaap_HeldToMaturitySecurities	$ 318,137us-gaap_HeldToMaturitySecurities
Available-for-sale Securities	4,174,242us-gaap_AvailableForSaleSecuritiesDebtSecurities	4,066,122us-gaap_AvailableForSaleSecuritiesDebtSecurities
Equity securities, AFS	211,571us-gaap_AvailableForSaleSecuritiesEquitySecurities	191,400us-gaap_AvailableForSaleSecuritiesEquitySecurities
Short-term investments	124,376us-gaap_OtherShortTermInvestments	131,972us-gaap_OtherShortTermInvestments
Notes payable	394,301us-gaap_LongTermDebt	379,297us-gaap_LongTermDebt
Carrying Amount [Member]
Held-to-maturity Securities	296,557us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember	318,137us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
Available-for-sale Securities	4,174,242us-gaap_AvailableForSaleSecuritiesDebtSecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember	4,066,122us-gaap_AvailableForSaleSecuritiesDebtSecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
Equity securities, AFS	211,571us-gaap_AvailableForSaleSecuritiesEquitySecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember	191,400us-gaap_AvailableForSaleSecuritiesEquitySecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
Short-term investments	124,376us-gaap_OtherShortTermInvestments / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember	131,972us-gaap_OtherShortTermInvestments / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
Notes payable	394,301us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember	379,297us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
Carrying Amount [Member] | Point sixty three Percent Borrowing From Federal Home Loan Bank [Member]
Notes payable	15,000us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_PointsixtythreePercentBorrowingFromFederalHomeLoanBankMember	0us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_PointsixtythreePercentBorrowingFromFederalHomeLoanBankMember
Carrying Amount [Member] | One Point Twenty Five Percent Borrowing From Federal Home Loan Bank [Member]
Notes payable	45,000us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_OnePointTwentyFivePercentBorrowingFromFederalHomeLoanBankMember	45,000us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_OnePointTwentyFivePercentBorrowingFromFederalHomeLoanBankMember
Carrying Amount [Member] | Seven Point Twenty Five Percent Senior Notes [Member]
Notes payable	49,896us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_SevenPointTwentyFivePercentSeniorNotesMember	49,896us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_SevenPointTwentyFivePercentSeniorNotesMember
Carrying Amount [Member] | Six Point Seventy Percent Senior Notes [Member]
Notes payable	99,405us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_SixPointSeventyPercentSeniorNotesMember	99,401us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_SixPointSeventyPercentSeniorNotesMember
Carrying Amount [Member] | Five Point Eight Hundred Seventy Five Senior Notes [Member]
Notes payable	185,000us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_FivePointEightHundredSeventyFiveMember	185,000us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_FivePointEightHundredSeventyFiveMember
Fair Value [Member]
Held-to-maturity Securities	311,093us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember	333,961us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Available-for-sale Securities	4,174,242us-gaap_AvailableForSaleSecuritiesDebtSecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember	4,066,122us-gaap_AvailableForSaleSecuritiesDebtSecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Equity securities, AFS	211,571us-gaap_AvailableForSaleSecuritiesEquitySecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember	191,400us-gaap_AvailableForSaleSecuritiesEquitySecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Short-term investments	124,376us-gaap_OtherShortTermInvestments / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember	131,972us-gaap_OtherShortTermInvestments / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Notes payable	427,568us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember	404,270us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Fair Value [Member] | Point sixty three Percent Borrowing From Federal Home Loan Bank [Member]
Notes payable	14,994us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_PointsixtythreePercentBorrowingFromFederalHomeLoanBankMember	0us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_PointsixtythreePercentBorrowingFromFederalHomeLoanBankMember
Fair Value [Member] | One Point Twenty Five Percent Borrowing From Federal Home Loan Bank [Member]
Notes payable	45,338us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_OnePointTwentyFivePercentBorrowingFromFederalHomeLoanBankMember	45,244us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_OnePointTwentyFivePercentBorrowingFromFederalHomeLoanBankMember
Fair Value [Member] | Seven Point Twenty Five Percent Senior Notes [Member]
Notes payable	60,407us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_SevenPointTwentyFivePercentSeniorNotesMember	59,181us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_SevenPointTwentyFivePercentSeniorNotesMember
Fair Value [Member] | Six Point Seventy Percent Senior Notes [Member]
Notes payable	117,389us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_SixPointSeventyPercentSeniorNotesMember	114,845us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_SixPointSeventyPercentSeniorNotesMember
Fair Value [Member] | Five Point Eight Hundred Seventy Five Senior Notes [Member]
Notes payable	$ 189,440us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_FivePointEightHundredSeventyFiveMember	$ 185,000us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_FivePointEightHundredSeventyFiveMember

X

- Definition

Amount of debt securities categorized neither as held-to-maturity nor trading.

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Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Other investments not otherwise specified in the taxonomy that will be realized in a short period of time, usually less than one year or the normal operating cycle, whichever is longer.

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