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v2.4.1.9

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 | $ 296,557 us-gaap_HeldToMaturitySecurities | $ 318,137 us-gaap_HeldToMaturitySecurities
Available-for-sale Securities | 4,174,242 us-gaap_AvailableForSaleSecuritiesDebtSecurities | 4,066,122 us-gaap_AvailableForSaleSecuritiesDebtSecurities
Equity securities, AFS | 211,571 us-gaap_AvailableForSaleSecuritiesEquitySecurities | 191,400 us-gaap_AvailableForSaleSecuritiesEquitySecurities
Short-term investments | 124,376 us-gaap_OtherShortTermInvestments | 131,972 us-gaap_OtherShortTermInvestments
Notes payable | 394,301 us-gaap_LongTermDebt | 379,297 us-gaap_LongTermDebt
Carrying Amount [Member] |  |
Held-to-maturity Securities | 296,557 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember | 318,137 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
Available-for-sale Securities | 4,174,242 us-gaap_AvailableForSaleSecuritiesDebtSecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember | 4,066,122 us-gaap_AvailableForSaleSecuritiesDebtSecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
Equity securities, AFS | 211,571 us-gaap_AvailableForSaleSecuritiesEquitySecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember | 191,400 us-gaap_AvailableForSaleSecuritiesEquitySecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
Short-term investments | 124,376 us-gaap_OtherShortTermInvestments / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember | 131,972 us-gaap_OtherShortTermInvestments / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
Notes payable | 394,301 us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember | 379,297 us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember
Carrying Amount [Member] | Point sixty three Percent Borrowing From Federal Home Loan Bank [Member] |  |
Notes payable | 15,000 us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_PointsixtythreePercentBorrowingFromFederalHomeLoanBankMember | 0 us-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 | 45,000 us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_OnePointTwentyFivePercentBorrowingFromFederalHomeLoanBankMember | 45,000 us-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 | 49,896 us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_SevenPointTwentyFivePercentSeniorNotesMember | 49,896 us-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 | 99,405 us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_SixPointSeventyPercentSeniorNotesMember | 99,401 us-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 | 185,000 us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_FivePointEightHundredSeventyFiveMember | 185,000 us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_FivePointEightHundredSeventyFiveMember
Fair Value [Member] |  |
Held-to-maturity Securities | 311,093 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember | 333,961 us-gaap_HeldToMaturitySecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Available-for-sale Securities | 4,174,242 us-gaap_AvailableForSaleSecuritiesDebtSecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember | 4,066,122 us-gaap_AvailableForSaleSecuritiesDebtSecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Equity securities, AFS | 211,571 us-gaap_AvailableForSaleSecuritiesEquitySecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember | 191,400 us-gaap_AvailableForSaleSecuritiesEquitySecurities / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Short-term investments | 124,376 us-gaap_OtherShortTermInvestments / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember | 131,972 us-gaap_OtherShortTermInvestments / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Notes payable | 427,568 us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember | 404,270 us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember
Fair Value [Member] | Point sixty three Percent Borrowing From Federal Home Loan Bank [Member] |  |
Notes payable | 14,994 us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_PointsixtythreePercentBorrowingFromFederalHomeLoanBankMember | 0 us-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 | 45,338 us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_OnePointTwentyFivePercentBorrowingFromFederalHomeLoanBankMember | 45,244 us-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 | 60,407 us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_SevenPointTwentyFivePercentSeniorNotesMember | 59,181 us-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 | 117,389 us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_SixPointSeventyPercentSeniorNotesMember | 114,845 us-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 | $ 189,440 us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_FivePointEightHundredSeventyFiveMember | $ 185,000 us-gaap_LongTermDebt / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_LongtermDebtTypeAxis = sigi_FivePointEightHundredSeventyFiveMember

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- Definition Amount of debt securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=51813488&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49167202&loc=d3e26610-111562 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 2 -Subparagraph (b)-(g) -URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62557-112803 + Details
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- Definition Amount of equity securities categorized neither as held-to-maturity nor as trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=51813488&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49167202&loc=d3e26610-111562 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62557-112803 + Details
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- Definition Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=51813488&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.1) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.6) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20, 22 -Article 5 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.8) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.5) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.1(g)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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