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Fair Value Measurements (Carrying Value and Estimated Fair Value of Financial Instruments) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Held-to-maturity Securities	$ 318,137us-gaap_HeldToMaturitySecurities	$ 392,879us-gaap_HeldToMaturitySecurities
Available-for-sale Securities, Debt Securities	4,066,122us-gaap_AvailableForSaleSecuritiesDebtSecurities	3,715,536us-gaap_AvailableForSaleSecuritiesDebtSecurities
Equity securities, AFS	191,400us-gaap_AvailableForSaleSecuritiesEquitySecurities	192,771us-gaap_AvailableForSaleSecuritiesEquitySecurities
Short-term investments	131,972us-gaap_OtherShortTermInvestments	174,251us-gaap_OtherShortTermInvestments
Notes payable	379,297us-gaap_LongTermDebt	392,414us-gaap_LongTermDebt
Carrying Amount [Member]
Held-to-maturity Securities	318,137us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember	392,879us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
Available-for-sale Securities, Debt Securities	4,066,122us-gaap_AvailableForSaleSecuritiesDebtSecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember	3,715,536us-gaap_AvailableForSaleSecuritiesDebtSecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
Equity securities, AFS	191,400us-gaap_AvailableForSaleSecuritiesEquitySecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember	192,771us-gaap_AvailableForSaleSecuritiesEquitySecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
Short-term investments	131,972us-gaap_OtherShortTermInvestments / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember	174,251us-gaap_OtherShortTermInvestments / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
Notes payable	379,297us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember	392,414us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
Carrying Amount [Member] | Two Point Ninety Percent Borrowing From Federal Home Loan Bank [Member]
Notes payable	0us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_TwoPointNinetyPercentBorrowingFromFederalHomeLoanBankMember	13,000us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_TwoPointNinetyPercentBorrowingFromFederalHomeLoanBankMember
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	49,916us-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	99,401us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_SixPointSeventyPercentSeniorNotesMember	99,498us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_SixPointSeventyPercentSeniorNotesMember
Carrying Amount [Member] | Five Point Eight Hundred Seventy Five [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	333,961us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember	416,981us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Available-for-sale Securities, Debt Securities	4,066,122us-gaap_AvailableForSaleSecuritiesDebtSecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember	3,715,536us-gaap_AvailableForSaleSecuritiesDebtSecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Equity securities, AFS	191,400us-gaap_AvailableForSaleSecuritiesEquitySecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember	192,771us-gaap_AvailableForSaleSecuritiesEquitySecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Short-term investments	131,972us-gaap_OtherShortTermInvestments / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember	174,251us-gaap_OtherShortTermInvestments / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Notes payable	404,270us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember	354,010us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Fair Value [Member] | Two Point Ninety Percent Borrowing From Federal Home Loan Bank [Member]
Notes payable	0us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_TwoPointNinetyPercentBorrowingFromFederalHomeLoanBankMember	13,319us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_TwoPointNinetyPercentBorrowingFromFederalHomeLoanBankMember
Fair Value [Member] | One Point Twenty Five Percent Borrowing From Federal Home Loan Bank [Member]
Notes payable	45,244us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_OnePointTwentyFivePercentBorrowingFromFederalHomeLoanBankMember	45,259us-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	59,181us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_SevenPointTwentyFivePercentSeniorNotesMember	50,887us-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	114,845us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_SixPointSeventyPercentSeniorNotesMember	98,247us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_SixPointSeventyPercentSeniorNotesMember
Fair Value [Member] | Five Point Eight Hundred Seventy Five [Member]
Notes payable	$ 185,000us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_FivePointEightHundredSeventyFiveMember	$ 146,298us-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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Amount of equity securities categorized neither as held-to-maturity nor as trading.

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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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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