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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 | $ 32,108 us-gaap_NetInvestmentIncome | $ 34,292 us-gaap_NetInvestmentIncome | $ 36,774 us-gaap_NetInvestmentIncome | $ 35,534 us-gaap_NetInvestmentIncome | $ 35,313 us-gaap_NetInvestmentIncome | $ 32,457 us-gaap_NetInvestmentIncome | $ 34,003 us-gaap_NetInvestmentIncome | $ 32,870 us-gaap_NetInvestmentIncome | $ 138,708 us-gaap_NetInvestmentIncome | $ 134,643 us-gaap_NetInvestmentIncome | $ 131,877 us-gaap_NetInvestmentIncome
Fixed Income Securities [Member] |  |  |  |  |  |  |  |  |  |  |
Statement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net investment income earned |  |  |  |  |  |  |  |  | 126,489 us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_FixedMaturitiesMember | 121,582 us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_FixedMaturitiesMember | 124,687 us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_FixedMaturitiesMember
Equity Securities, Dividend Income [Member] |  |  |  |  |  |  |  |  |  |  |
Statement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net investment income earned |  |  |  |  |  |  |  |  | 7,449 us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember | 6,140 us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember | 6,215 us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_EquitySecuritiesMember
Short-term Investments [Member] |  |  |  |  |  |  |  |  |  |  |
Statement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net investment income earned |  |  |  |  |  |  |  |  | 66 us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember | 117 us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember | 151 us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = us-gaap_ShortTermInvestmentsMember
Other Investment Portfolio [Member] |  |  |  |  |  |  |  |  |  |  |
Statement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net investment income earned |  |  |  |  |  |  |  |  | 13,580 us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = sigi_OtherInvestmentPortfolioMember | 15,208 us-gaap_NetInvestmentIncome / us-gaap_InvestmentTypeAxis = sigi_OtherInvestmentPortfolioMember | 8,996 us-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

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- 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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