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Retirement Plans (Deferred Compensation Plan) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2011
Maximum Employee Contribution Of Annual Salary To Deferred Compensation Plan Percentage	50.00%
Maximum Employee Contribution Of Annual Bonus To Deferred Compensation Plan Percentage	100.00%
Deferred Compensation Plan Contributions By Employer	$ 0.2	$ 0.1

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