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Share-Based Payments (Weighted-Average Fair Value of Options and Stock Per Share) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Share-based compensation expense charged against net income before tax | $ 18.6 us-gaap_AllocatedShareBasedCompensationExpense | $ 19.9 us-gaap_AllocatedShareBasedCompensationExpense | $ 13.8 us-gaap_AllocatedShareBasedCompensationExpense
Share-based compensation expense income tax benefit | $ 6.2 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense | $ 6.8 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense | $ 4.8 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
Restricted Stock Units (RSUs) [Member] |  |  |
Stock, weighted average grant date fair value | $ 21.58 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember | $ 21.03 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember | $ 17.62 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember
Agent Stock Purchase Plan [Member] |  |  |
Stock, weighted average grant date fair value | $ 2.42 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = sigi_AgentStockPurchasePlanMember | $ 2.40 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = sigi_AgentStockPurchasePlanMember | $ 1.76 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = sigi_AgentStockPurchasePlanMember
Employee Stock Purchase Plan [Member] |  |  |
Options, weighted average grant date fair value | $ 1.24 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockMember | $ 0.97 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockMember | $ 1.05 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockMember
15% of grant date market value | $ 3.87 sigi_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsEmployeeStockPurchasePlanGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockMember | $ 3.24 sigi_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsEmployeeStockPurchasePlanGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockMember | $ 2.70 sigi_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsEmployeeStockPurchasePlanGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockMember
Total ESPP | $ 5.11 sigi_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsEmployeeStockPurchasePlanGrantsInPeriodWeightedAverageGrantDateFairValueTotal / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockMember | $ 4.21 sigi_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsEmployeeStockPurchasePlanGrantsInPeriodWeightedAverageGrantDateFairValueTotal / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockMember | $ 3.75 sigi_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsEmployeeStockPurchasePlanGrantsInPeriodWeightedAverageGrantDateFairValueTotal / us-gaap_AwardTypeAxis = us-gaap_EmployeeStockMember

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- Definition The weighted average fair value discount at grant date, for employee stock purchase plan shares issued during the period. + References No definition available. + Details
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- Definition The total weighted average fair value at grant date for employee stock purchase plan shares issued during the period. + References No definition available. + Details
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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section F + Details
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- Definition The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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