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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

X

- Details

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