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Retirement Plans (Deferred Compensation Plan) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Deferred Compensation Plan Discretionary Contributions	$ 0sigi_DeferredCompensationArrangementWithIndividualEmployerDiscretionaryContributions	$ 0sigi_DeferredCompensationArrangementWithIndividualEmployerDiscretionaryContributions	$ 0sigi_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	0.2us-gaap_DeferredCompensationArrangementWithIndividualEmployerContribution	0.2us-gaap_DeferredCompensationArrangementWithIndividualEmployerContribution
Retirement Savings Plan [Member]
Defined Benefit Plan, Contributions by Employer	$ 13.4us-gaap_DefinedBenefitPlanContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_RetirementSavingsPlanMember	$ 12.2us-gaap_DefinedBenefitPlanContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_RetirementSavingsPlanMember	$ 8.2us-gaap_DefinedBenefitPlanContributionsByEmployer / us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis = sigi_RetirementSavingsPlanMember

X

- Definition

Amount of discretionary contributions made by the employer to a deferred compensation plan.

+ References

No definition available.

+ Details

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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

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

X

- 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

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Details

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