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Investments (Net Investment Income) (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2013 | Sep. 30, 2012 | Sep. 30, 2013 | Sep. 30, 2012
Net investment income earned | $ 32,457 | $ 30,650 | $ 99,330 | $ 97,284
Fixed Maturity Securities [Member] |  |  |  |
Net investment income earned | 30,569 | 30,839 | 90,956 | 93,948
Equity Securities [Member] |  |  |  |
Net investment income earned | 1,341 | 1,268 | 4,422 | 3,785
Short-term Investments [Member] |  |  |  |
Net investment income earned | 21 | 36 | 102 | 103
Other Investment Portfolio [Member] |  |  |  |
Net investment income earned | 2,639 | 497 | 10,110 | 5,460
Miscellaneous Investment Income [Member] |  |  |  |
Net investment income earned | 0 | 41 | 0 | 105
Investment Expenses [Member] |  |  |  |
Net investment income earned | $ (2,113) | $ (2,031) | $ (6,260) | $ (6,117)

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- Definition The income earned from investments in securities and other investments including dividends, interest, and returns on other investments. This item does not include realized gains (losses). + References No definition available. + Details
Name: | sigi_NetInvestmentIncomeEarned
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration