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Invesments (Net Investment Income on Other Investments, by Strategy) (Details) (USD $) In Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2011 | Sep. 30, 2010 | Sep. 30, 2011 | Sep. 30, 2010
Net investment income earned | $ 35,786 | $ 32,986 | $ 118,604 | $ 104,237
Energy/Power Generation [Member] | Alternative Investments [Member] |  |  |  |
Net investment income earned | 1,760 | 409 | 7,599 | 3,440
Private Equity [Member] | Alternative Investments [Member] |  |  |  |
Net investment income earned | 1,640 | 543 | 5,398 | 1,350
Secondary Private Equity Member | Alternative Investments [Member] |  |  |  |
Net investment income earned | 1,179 | 240 | 5,895 | 3,924
Distressed Debt [Member] | Alternative Investments [Member] |  |  |  |
Net investment income earned | (153) | 472 | 1,241 | 1,195
Real Estate Strategy [Member] | Alternative Investments [Member] |  |  |  |
Net investment income earned | 35 | 303 | 1,485 | (2,056)
Venture Capital [Member] | Alternative Investments [Member] |  |  |  |
Net investment income earned | 125 | 15 | 1,448 | 263
Mezzanine Financing [Member] | Alternative Investments [Member] |  |  |  |
Net investment income earned | (133) | 377 | 928 | 3,006
Other Securities [Member] |  |  |  |
Net investment income earned | 41 | 41 | 88 | 94
Other Investments [Member] |  |  |  |
Net investment income earned | 4,494 | 2,400 | 24,082 | 11,216
Alternative Investments [Member] |  |  |  |
Net investment income earned | $ 4,500 | $ 2,300 | $ 24,000 | $ 11,100

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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