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Investments (Net Investment Income) (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended
--- | ---
Mar. 31, 2013 | Mar. 31, 2012
Net investment income earned | $ 32,870 | $ 32,628
Fixed Maturity Securities [Member] |  |
Net investment income earned | 30,089 | 31,350
Equity Securities [Member] |  |
Net investment income earned | 1,207 | 1,237
Short-term Investments [Member] |  |
Net investment income earned | 52 | 38
Other Investments [Member] |  |
Net investment income earned | 3,602 | 2,000
Miscellaneous Investment Income [Member] |  |
Net investment income earned | 0 | 39
Investment Expenses [Member] |  |
Net investment income earned | $ (2,080) | $ (2,036)

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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