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Federal Income Taxes (Reconciliation of Federal Income Tax on Income at the Corporate Rate to the Effective Tax Rate) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Tax at statutory rate of 35%	$ 68,996us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	$ 50,331us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate	$ 13,172us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate
Tax-advantaged interest	(12,926)sigi_IncomeTaxReconciliationTaxAdvantagedInterestIncome	(12,718)sigi_IncomeTaxReconciliationTaxAdvantagedInterestIncome	(13,285)sigi_IncomeTaxReconciliationTaxAdvantagedInterestIncome
Dividends received deduction	(1,121)us-gaap_IncomeTaxReconciliationDeductionsDividends	(1,174)us-gaap_IncomeTaxReconciliationDeductionsDividends	(1,260)us-gaap_IncomeTaxReconciliationDeductionsDividends
Other	355us-gaap_IncomeTaxReconciliationOtherAdjustments	(52)us-gaap_IncomeTaxReconciliationOtherAdjustments	1,045us-gaap_IncomeTaxReconciliationOtherAdjustments
Total federal income tax expense (benefit)	$ 55,304us-gaap_IncomeTaxExpenseBenefit	$ 36,387us-gaap_IncomeTaxExpenseBenefit	$ (328)us-gaap_IncomeTaxExpenseBenefit

X

- Definition

The portion of the difference between total income tax expense or benefit as reported in the Income Statement for the period and the expected income tax expense or benefit computed by applying the domestic federal statutory income tax rates to pretax income from continuing operations attributable to investment income that is tax advantaged. Tax advantaged investment securities generally are, by statute, tax-reduced, tax-deferred, or tax-free.

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(h))

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-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 08

-Paragraph h

-Article 4



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Income Tax Expense (or Benefit)

-URI http://asc.fasb.org/extlink&oid=6515339



Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 9

-Subparagraph (a),(b)

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to deduction for dividend.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

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The amount of income tax expense or benefit for the period computed by applying the domestic federal statutory tax rates to pretax income from continuing operations.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to other adjustments.

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