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

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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 + Details
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