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v2.4.1.9

Retirement Plans (Deferred Compensation Plan) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Deferred Compensation Plan Discretionary Contributions | $ 0 sigi_DeferredCompensationArrangementWithIndividualEmployerDiscretionaryContributions | $ 0 sigi_DeferredCompensationArrangementWithIndividualEmployerDiscretionaryContributions | $ 0 sigi_DeferredCompensationArrangementWithIndividualEmployerDiscretionaryContributions
Maximum Employee Contribution Of Annual Salary To Deferred Compensation Plan Percentage | 50.00%sigi_MaximumEmployeeContributionOfAnnualSalaryToDeferredCompensationPlanPercentage |  |
Maximum Employee Contribution Of Annual Bonus To Deferred Compensation Plan Percentage | 100.00%sigi_MaximumEmployeeContributionOfAnnualBonusToDeferredCompensationPlanPercentage |  |
Deferred Compensation Plan Contributions By Employer | 0.2 us-gaap_DeferredCompensationArrangementWithIndividualEmployerContribution | 0.2 us-gaap_DeferredCompensationArrangementWithIndividualEmployerContribution |
Retirement Savings Plan [Member] |  |  |
Defined Benefit Plan, Contributions by Employer | $ 13.4 us-gaap_DefinedBenefitPlanContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_RetirementSavingsPlanMember | $ 12.2 us-gaap_DefinedBenefitPlanContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_RetirementSavingsPlanMember | $ 8.2 us-gaap_DefinedBenefitPlanContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_RetirementSavingsPlanMember

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- Definition Amount of discretionary contributions made by the employer to a deferred compensation plan. + References No definition available. + Details
Name: | sigi_DeferredCompensationArrangementWithIndividualEmployerDiscretionaryContributions
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Maximum percentage of defined compensation, related to annual bonus, that an employee can contribute to the nonqualified deferred compensation plan. + References No definition available. + Details
Name: | sigi_MaximumEmployeeContributionOfAnnualBonusToDeferredCompensationPlanPercentage
Namespace Prefix: | sigi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Maximum percentage of defined compensation, related to annual base salary, that an employee can contribute to the nonqualified deferred compensation plan. + References No definition available. + Details
Name: | sigi_MaximumEmployeeContributionOfAnnualSalaryToDeferredCompensationPlanPercentage
Namespace Prefix: | sigi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition The amount of payment into the fund established for purposes of making future disbursements to the individual in accordance with the terms and understanding of the deferred compensation arrangement. + References No definition available. + Details
Name: | us-gaap_DeferredCompensationArrangementWithIndividualEmployerContribution
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition The increase in the fair value of plan assets from contributions made by the employer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=35743120&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanContributionsByEmployer
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis=sigi_RetirementSavingsPlanMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |