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Invesments (Net Investment Income on Other Investments, by Strategy) (Details) (USD $) In Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2011 | Jun. 30, 2010 | Jun. 30, 2011 | Jun. 30, 2010
Net investment income earned | $ 39,345 | $ 36,545 | $ 82,818 | $ 71,251
Other Securities [Member] |  |  |  |
Net investment income earned | 22 | 16 | 47 | 53
Other Investments [Member] |  |  |  |
Net investment income earned | 7,922 | 4,884 | 19,588 | 8,816
Energy/Power Generation [Member] | Alternative Investments [Member] |  |  |  |
Net investment income earned | 1,284 | 965 | 5,839 | 3,031
Secondary Private Equity Member | Alternative Investments [Member] |  |  |  |
Net investment income earned | 3,067 | 2,665 | 4,716 | 3,684
Distressed Debt [Member] | Alternative Investments [Member] |  |  |  |
Net investment income earned | 421 | (41) | 1,394 | 723
Private Equity [Member] | Alternative Investments [Member] |  |  |  |
Net investment income earned | 1,181 | 99 | 3,758 | 807
Real Estate Strategy [Member] | Alternative Investments [Member] |  |  |  |
Net investment income earned | 681 | (494) | 1,450 | (2,359)
Mezzanine Financing [Member] | Alternative Investments [Member] |  |  |  |
Net investment income earned | 701 | 1,692 | 1,061 | 2,629
Venture Capital [Member] | Alternative Investments [Member] |  |  |  |
Net investment income earned | $ 565 | $ (18) | $ 1,323 | $ 248

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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). + Details
Name: | sigi_NetInvestmentIncomeEarned
Namespace Prefix: | sigi
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration