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Investments (Net Investment Income) (Details) (USD $) | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2012 | Sep. 30, 2011 | Sep. 30, 2012 | Sep. 30, 2011
Net investment income earned | $ 30,650,000 | $ 35,786,000 | $ 97,284,000 | $ 118,604,000
Decrease In Net Investment Income Before Tax | 5,100,000 | 21,300,000 |  |
Fixed Maturity Securities [Member] |  |  |  |
Net investment income earned | 30,839,000 | 31,960,000 | 93,948,000 | 97,835,000
Decrease In Net Investment Income Before Tax | 1,100,000 |  | 3,900,000 |
Equity Securities [Member] |  |  |  |
Net investment income earned | 1,268,000 | 1,197,000 | 3,785,000 | 2,299,000
Short-term Investments [Member] |  |  |  |
Net investment income earned | 36,000 | 28,000 | 103,000 | 123,000
Other Investments [Member] |  |  |  |
Net investment income earned | 497,000 | 4,453,000 | 5,460,000 | 23,994,000
Miscellaneous Investment Income [Member] |  |  |  |
Net investment income earned | 41,000 | 41,000 | 105,000 | 88,000
Investment Expenses [Member] |  |  |  |
Net investment income earned | (2,031,000) | (1,893,000) | (6,117,000) | (5,735,000)
Alternative Investments [Member] |  |  |  |
Decrease In Net Investment Income Before Tax | $ 17,600,000 | $ 3,500,000 |  |

X
- Definition This item represents the increase (decrease) in income earned from investments in securities and other investments including dividends, interest, and returns on other investments in the current reporting period as compared to the comparable prior year period. This item does not include realized gains (losses). + References No definition available. + Details
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Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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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