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Retirement Plans (Retirement Savings Plan) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |  |
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Apr. 05, 2013 | Dec. 31, 2011
Minimum employee contribution of defined compensation to retirement savings plan percentage | 2.00%sigi_MinimumEmployeeContributionOfDefinedCompensationToRetirementSavingsPlanPercentage |  |  |  |
Maximum employee contribution of defined compensation to retirement savings plan percentage | 50.00%us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent |  |  |  |
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 |  |
Non Elective Employer Contribution [Member] | Retirement Savings Plan [Member] |  |  |  |  |
Maximum employer match of employee contribution of defined compensation to retirement savings plan percentage |  |  |  | 4.00%sigi_MaximumEmployerMatchOfEmployeeContributionOfDefinedCompensationToRetirementSavingsPlanPercentage / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_RetirementSavingsPlanMember / sigi_ScheduleOfPlanTermsInEffectAndRevisionsOfRetirementSavingsPlanAxis = sigi_NonElectiveEmployerContributionMember | 4.00%sigi_MaximumEmployerMatchOfEmployeeContributionOfDefinedCompensationToRetirementSavingsPlanPercentage / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_RetirementSavingsPlanMember / sigi_ScheduleOfPlanTermsInEffectAndRevisionsOfRetirementSavingsPlanAxis = sigi_NonElectiveEmployerContributionMember
First Tier Of Employee Contributions [Member] | Employer Match [Member] | Retirement Savings Plan [Member] |  |  |  |  |
Maximum employer match of employee contribution of defined compensation to retirement savings plan percentage |  |  |  | 3.00%sigi_MaximumEmployerMatchOfEmployeeContributionOfDefinedCompensationToRetirementSavingsPlanPercentage / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_RetirementSavingsPlanMember / sigi_ScheduleOfPlanTermsInEffectAndRevisionsOfRetirementSavingsPlanAxis = sigi_EmployerMatchMember / sigi_ScheduleOfRetirementPlansAxis = sigi_FirstTierOfEmployeeContributionsMember | 3.00%sigi_MaximumEmployerMatchOfEmployeeContributionOfDefinedCompensationToRetirementSavingsPlanPercentage / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_RetirementSavingsPlanMember / sigi_ScheduleOfPlanTermsInEffectAndRevisionsOfRetirementSavingsPlanAxis = sigi_EmployerMatchMember / sigi_ScheduleOfRetirementPlansAxis = sigi_FirstTierOfEmployeeContributionsMember
Defined contribution plan, employer matching contribution, percentage |  |  |  | 100.00%sigi_DefinedContributionPlanEmployerMatchingContributionPercentage / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_RetirementSavingsPlanMember / sigi_ScheduleOfPlanTermsInEffectAndRevisionsOfRetirementSavingsPlanAxis = sigi_EmployerMatchMember / sigi_ScheduleOfRetirementPlansAxis = sigi_FirstTierOfEmployeeContributionsMember | 100.00%sigi_DefinedContributionPlanEmployerMatchingContributionPercentage / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_RetirementSavingsPlanMember / sigi_ScheduleOfPlanTermsInEffectAndRevisionsOfRetirementSavingsPlanAxis = sigi_EmployerMatchMember / sigi_ScheduleOfRetirementPlansAxis = sigi_FirstTierOfEmployeeContributionsMember
Second Tier Of Employee Contributions [Member] | Employer Match [Member] | Retirement Savings Plan [Member] |  |  |  |  |
Maximum employer match of employee contribution of defined compensation to retirement savings plan percentage |  |  |  | 3.00%sigi_MaximumEmployerMatchOfEmployeeContributionOfDefinedCompensationToRetirementSavingsPlanPercentage / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_RetirementSavingsPlanMember / sigi_ScheduleOfPlanTermsInEffectAndRevisionsOfRetirementSavingsPlanAxis = sigi_EmployerMatchMember / sigi_ScheduleOfRetirementPlansAxis = sigi_SecondTierOfEmployeeContributionsMember | 3.00%sigi_MaximumEmployerMatchOfEmployeeContributionOfDefinedCompensationToRetirementSavingsPlanPercentage / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_RetirementSavingsPlanMember / sigi_ScheduleOfPlanTermsInEffectAndRevisionsOfRetirementSavingsPlanAxis = sigi_EmployerMatchMember / sigi_ScheduleOfRetirementPlansAxis = sigi_SecondTierOfEmployeeContributionsMember
Defined contribution plan, employer matching contribution, percentage |  |  |  | 50.00%sigi_DefinedContributionPlanEmployerMatchingContributionPercentage / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_RetirementSavingsPlanMember / sigi_ScheduleOfPlanTermsInEffectAndRevisionsOfRetirementSavingsPlanAxis = sigi_EmployerMatchMember / sigi_ScheduleOfRetirementPlansAxis = sigi_SecondTierOfEmployeeContributionsMember | 50.00%sigi_DefinedContributionPlanEmployerMatchingContributionPercentage / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_RetirementSavingsPlanMember / sigi_ScheduleOfPlanTermsInEffectAndRevisionsOfRetirementSavingsPlanAxis = sigi_EmployerMatchMember / sigi_ScheduleOfRetirementPlansAxis = sigi_SecondTierOfEmployeeContributionsMember

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- Definition Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan. + References No definition available. + Details
Name: | sigi_DefinedContributionPlanEmployerMatchingContributionPercentage
Namespace Prefix: | sigi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Maximum percentage of an employee's contribution to the Retirement Savings Plan that the employer will match. + References No definition available. + Details
Name: | sigi_MaximumEmployerMatchOfEmployeeContributionOfDefinedCompensationToRetirementSavingsPlanPercentage
Namespace Prefix: | sigi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Minimum percentage of defined compensation that an employee can contribute to the Retirement Savings Plan. + References No definition available. + Details
Name: | sigi_MinimumEmployeeContributionOfDefinedCompensationToRetirementSavingsPlanPercentage
Namespace Prefix: | sigi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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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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- Definition Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | sigi_ScheduleOfPlanTermsInEffectAndRevisionsOfRetirementSavingsPlanAxis=sigi_NonElectiveEmployerContributionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | sigi_ScheduleOfRetirementPlansAxis=sigi_FirstTierOfEmployeeContributionsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | sigi_ScheduleOfRetirementPlansAxis=sigi_SecondTierOfEmployeeContributionsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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