Name:	sigi_DefinedContributionPlanEmployerMatchingContributionPercentage
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Maximum percentage of an employee's contribution to the Retirement Savings Plan that the employer will match.

No definition available.

Name:	sigi_MaximumEmployerMatchOfEmployeeContributionOfDefinedCompensationToRetirementSavingsPlanPercentage
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Minimum percentage of defined compensation that an employee can contribute to the Retirement Savings Plan.

No definition available.

Name:	sigi_MinimumEmployeeContributionOfDefinedCompensationToRetirementSavingsPlanPercentage
Namespace Prefix:	sigi_
Data Type:	num:percentItemType
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The increase in the fair value of plan assets from contributions made by the employer.

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

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Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan.

No definition available.

Name:	us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent
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