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Share-Based Payments (Narrative) (Details) (USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value | $ 0.8 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue | $ 1.3 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue | $ 0.8 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue
Stock Issued During Period Shares | 769,389 us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation | 862,662 us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation | 857,403 us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation
Number of options granted | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross |  |
Stock Issued During Period Shares Dividend Reinvestment Plan | 58,309 us-gaap_StockIssuedDuringPeriodSharesDividendReinvestmentPlan | 63,349 us-gaap_StockIssuedDuringPeriodSharesDividendReinvestmentPlan | 90,110 us-gaap_StockIssuedDuringPeriodSharesDividendReinvestmentPlan
Expense charged | 18.6 us-gaap_AllocatedShareBasedCompensationExpense | 19.9 us-gaap_AllocatedShareBasedCompensationExpense | 13.8 us-gaap_AllocatedShareBasedCompensationExpense
Corresponding income tax benefit | 6.2 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense | 6.8 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense | 4.8 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense
Omnibus Stock Plan 2014 [Member] |  |  |
Maximum exercise period for an option grant | 10 years |  |
Number of authorized shares | 3,500,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_OmnibusStockPlan2014Member |  |
Number of shares available in reserve | 3,491,894 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_OmnibusStockPlan2014Member |  |
DEUs accrued | 43 sigi_DividendEquivalentUnitsShareBasedCompensationAccrued / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_OmnibusStockPlan2014Member |  |
DEU issuances | 0 sigi_DividendEquivalentUnitsShareBasedCompensationIssued / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_OmnibusStockPlan2014Member |  |
Number of options to purchase common stock | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesPurchasedForAward / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_OmnibusStockPlan2014Member |  |
Forfeitures during the period | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_OmnibusStockPlan2014Member |  |
Cash Incentive Plan [Member] |  |  |
Cash units granted, net of forfeiture | 60,305 sigi_CashUnitsGrantedDuringPeriodShareBasedCompensation / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_CashIncentivePlanMember | 55,365 sigi_CashUnitsGrantedDuringPeriodShareBasedCompensation / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_CashIncentivePlanMember | 46,961 sigi_CashUnitsGrantedDuringPeriodShareBasedCompensation / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_CashIncentivePlanMember
Stock Option III [Member] |  |  |
Maximum exercise period for an option grant | 10 years |  |
Number of shares available in reserve | 223,440 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_StockOptionIiiMember |  |
Stock Option Plan For Directors [Member] |  |  |
Number of shares available in reserve | 36,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_StockOptionPlanForDirectorsMember |  |
Stock Compensation Plan For Non Employee Directors [Member] |  |  |
Number of shares available in reserve | 94,290 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_StockCompensationPlanForNonEmployeeDirectorsMember |  |
Issuances under plan | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_StockCompensationPlanForNonEmployeeDirectorsMember | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_StockCompensationPlanForNonEmployeeDirectorsMember | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesIssuedInPeriod / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_StockCompensationPlanForNonEmployeeDirectorsMember
Employee Stock Purchase Plan [Member] |  |  |
Stock Issued During Period Shares | 106,832 us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = us-gaap_EmployeeStockMember | 122,951 us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = us-gaap_EmployeeStockMember | 129,081 us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = us-gaap_EmployeeStockMember
Number of authorized shares | 1,500,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = us-gaap_EmployeeStockMember |  |
Number of shares available in reserve | 764,098 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = us-gaap_EmployeeStockMember |  |
Discount from market price, offering date | 85.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountFromMarketPriceOfferingDate / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = us-gaap_EmployeeStockMember |  |
Discount from market price, purchase date | 85.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountFromMarketPricePurchaseDate / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = us-gaap_EmployeeStockMember |  |
Agent Stock Purchase Plan [Member] |  |  |
Stock Issued During Period Shares | 78,724 us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_AgentStockPurchasePlanMember | 86,388 us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_AgentStockPurchasePlanMember | 89,723 us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_AgentStockPurchasePlanMember
Number of authorized shares | 3,000,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_AgentStockPurchasePlanMember |  |
Number of shares available in reserve | 2,019,296 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_AgentStockPurchasePlanMember |  |
Discount from market price, purchase date | 10.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountFromMarketPricePurchaseDate / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_AgentStockPurchasePlanMember |  |
Expense charged | 0.2 us-gaap_AllocatedShareBasedCompensationExpense / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_AgentStockPurchasePlanMember | 0.2 us-gaap_AllocatedShareBasedCompensationExpense / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_AgentStockPurchasePlanMember | 0.2 us-gaap_AllocatedShareBasedCompensationExpense / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_AgentStockPurchasePlanMember
Corresponding income tax benefit | $ 0.1 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_AgentStockPurchasePlanMember | $ 0.1 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_AgentStockPurchasePlanMember | $ 0.1 us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_AgentStockPurchasePlanMember
Omnibus Stock Plan [Member] |  |  |
Maximum exercise period for an option grant | 10 years |  |
Number of shares available in reserve | 3,035,652 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_OmnibusStockPlanMember |  |
DEUs accrued | 24,010 sigi_DividendEquivalentUnitsShareBasedCompensationAccrued / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_OmnibusStockPlanMember | 23,505 sigi_DividendEquivalentUnitsShareBasedCompensationAccrued / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_OmnibusStockPlanMember | 32,558 sigi_DividendEquivalentUnitsShareBasedCompensationAccrued / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_OmnibusStockPlanMember
DEU issuances | 30,991 sigi_DividendEquivalentUnitsShareBasedCompensationIssued / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_OmnibusStockPlanMember | 39,296 sigi_DividendEquivalentUnitsShareBasedCompensationIssued / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_OmnibusStockPlanMember | 48,224 sigi_DividendEquivalentUnitsShareBasedCompensationIssued / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_OmnibusStockPlanMember
Number of options to purchase common stock | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesPurchasedForAward / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_OmnibusStockPlanMember | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesPurchasedForAward / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_OmnibusStockPlanMember | 0 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesPurchasedForAward / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_OmnibusStockPlanMember
Restricted Stock Units (RSUs) [Member] | Omnibus Stock Plan 2014 [Member] |  |  |
Number of restricted stock units granted net of forfeitures | 4,023 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGranted / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_OmnibusStockPlan2014Member |  |
Restricted Stock Units (RSUs) [Member] | Omnibus Stock Plan [Member] |  |  |
Number of restricted stock units granted net of forfeitures | 354,357 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGranted / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_OmnibusStockPlanMember | 376,163 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGranted / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_OmnibusStockPlanMember | 326,213 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGranted / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockUnitsRSUMember / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_OmnibusStockPlanMember
Stock Option [Member] | Stock Option Plan For Directors [Member] |  |  |
Number of options granted | 6,000 us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross / us-gaap_AwardTypeAxis = sigi_StockOptionShareBasedCompensationMember / sigi_ShareBasedCompensationArrangementsAwardTypeAxis = sigi_StockOptionPlanForDirectorsMember |  |

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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 Discount rate from fair value on offering date that participants pay for shares. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Net number of non-option equity instruments granted to participants. + 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)(1)(iv)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). + 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 (a)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares. + 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)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option 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)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Gross number of share options (or share units) granted during the period. + References No definition available. + Details
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- Definition Number of share instruments newly issued under a share-based compensation 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 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of shares purchased on the open market during the period for issuance to employees under the 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 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 + Details
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- Definition Number of shares issued during the period from a dividend reinvestment plan (DRIP). A dividend reinvestment plan allows the shareholders to reinvest dividends paid to them by the entity on new issues of stock by the entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6928386&loc=d3e21463-112644 + Details
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