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Investments (Net Investment Income) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Statement [Line Items]
Net investment income earned	$ 32,108us-gaap_NetInvestmentIncome	$ 34,292us-gaap_NetInvestmentIncome	$ 36,774us-gaap_NetInvestmentIncome	$ 35,534us-gaap_NetInvestmentIncome	$ 35,313us-gaap_NetInvestmentIncome	$ 32,457us-gaap_NetInvestmentIncome	$ 34,003us-gaap_NetInvestmentIncome	$ 32,870us-gaap_NetInvestmentIncome	$ 138,708us-gaap_NetInvestmentIncome	$ 134,643us-gaap_NetInvestmentIncome	$ 131,877us-gaap_NetInvestmentIncome
Fixed Income Securities [Member]
Statement [Line Items]
Net investment income earned									126,489us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_FixedMaturitiesMember	121,582us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_FixedMaturitiesMember	124,687us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_FixedMaturitiesMember
Equity Securities, Dividend Income [Member]
Statement [Line Items]
Net investment income earned									7,449us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember	6,140us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember	6,215us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember
Short-term Investments [Member]
Statement [Line Items]
Net investment income earned									66us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember	117us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember	151us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember
Other Investment Portfolio [Member]
Statement [Line Items]
Net investment income earned									13,580us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = sigi_OtherInvestmentPortfolioMember	15,208us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = sigi_OtherInvestmentPortfolioMember	8,996us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = sigi_OtherInvestmentPortfolioMember
Investment Expenses [Member]
Statement [Line Items]
Net investment income earned									$ (8,876)us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = sigi_InvestmentExpensesMember	$ (8,404)us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = sigi_InvestmentExpensesMember	$ (8,172)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



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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 2

-Article 7

+ Details

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