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Investments (Net Investment Income) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Net Investment Income [Line Items]
Net investment income earned	$ 26,917us-gaap_NetInvestmentIncome	$ 35,534us-gaap_NetInvestmentIncome
Fixed Income Securities [Member]
Net Investment Income [Line Items]
Net investment income earned	30,967us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_FixedMaturitiesMember	31,028us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_FixedMaturitiesMember
Equity Securities [Member]
Net Investment Income [Line Items]
Net investment income earned	1,792us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember	1,449us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember
Short-Term Investments [Member]
Net Investment Income [Line Items]
Net investment income earned	25us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember	19us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember
Other Investments [Member]
Net Investment Income [Line Items]
Net investment income earned	(3,540)us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_OtherInvestmentsMember	5,218us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_OtherInvestmentsMember
Investment Expenses [Member]
Net Investment Income [Line Items]
Net investment income earned	$ (2,327)us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = sigi_InvestmentExpensesMember	$ (2,180)us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = sigi_InvestmentExpensesMember

X

- Definition

Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 16

-Article 12



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.2)

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

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