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Retirement Plans (Deferred Compensation Plan) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009
Retirement Plans [Abstract] |  |  |
Maximum Employee Contribution Of Annual Salary To Deferred Compensation Plan Percentage | 50.00% |  |
Maximum Employee Contribution Of Annual Bonus To Deferred Compensation Plan Percentage | 90.00% |  |
Maximum Employer Non Elective Contribution To Deferred Compensation Plan Percentage | 4.00% |  |
Deferred Compensation Plan Contributions By Employer | $ 0.1 | $ 0.2 | $ 0.1

X
- Definition Contributions made by employer to the Deferred Compensation Plan + References No definition available. + Details
Name: | sigi_DeferredCompensationPlanContributionsByEmployer
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- 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

X
- 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

X
- Definition Maximum percentage of employer non-elective contribution of employee's eligible compensation to the extent a particpant could not receive the maximum enhanced contribution in the Retirement Savings Plan due to limitations of the Retirement Savings Plan and the Internal Revenue Code. + References No definition available. + Details
Name: | sigi_MaximumEmployerNonElectiveContributionToDeferredCompensationPlanPercentage
Namespace Prefix: | sigi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Details
Name: | us-gaap_CompensationAndRetirementDisclosureAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration