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Retirement Plans (Retirement Savings Plan) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Deferred Compensation Arrangement With Individual, Employer Discretionary Contributions	$ 0.0	$ 0.0	$ 0.0
Retirement Savings Plan [Member]
Defined Benefit Plan, Contributions by Employer	$ 14.1	$ 13.4	$ 12.2

X

- 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

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=58740216&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanContributionsByEmployer
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: