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Investments (Net Investment Income) (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 6 Months Ended | 3 Months Ended | 6 Months Ended | 3 Months Ended | 6 Months Ended | 3 Months Ended | 6 Months Ended | 3 Months Ended | 6 Months Ended | 3 Months Ended | 6 Months Ended | 3 Months Ended | 6 Months Ended | 3 Months Ended | 6 Months Ended
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Jun. 30, 2013 | Jun. 30, 2012 | Jun. 30, 2013 | Jun. 30, 2012 | Jun. 30, 2013 Fixed Maturity Securities [Member] | Jun. 30, 2012 Fixed Maturity Securities [Member] | Jun. 30, 2013 Fixed Maturity Securities [Member] | Jun. 30, 2012 Fixed Maturity Securities [Member] | Jun. 30, 2013 Equity Securities [Member] | Jun. 30, 2012 Equity Securities [Member] | Jun. 30, 2013 Equity Securities [Member] | Jun. 30, 2012 Equity Securities [Member] | Jun. 30, 2013 Short-term Investments [Member] | Jun. 30, 2012 Short-term Investments [Member] | Jun. 30, 2013 Short-term Investments [Member] | Jun. 30, 2012 Short-term Investments [Member] | Jun. 30, 2013 Other Investments [Member] | Jun. 30, 2012 Other Investments [Member] | Jun. 30, 2013 Other Investments [Member] | Jun. 30, 2012 Other Investments [Member] | Jun. 30, 2013 Miscellaneous Investment Income [Member] | Jun. 30, 2012 Miscellaneous Investment Income [Member] | Jun. 30, 2013 Miscellaneous Investment Income [Member] | Jun. 30, 2012 Miscellaneous Investment Income [Member] | Jun. 30, 2013 Investment Expenses [Member] | Jun. 30, 2012 Investment Expenses [Member] | Jun. 30, 2013 Investment Expenses [Member] | Jun. 30, 2012 Investment Expenses [Member] | Mar. 31, 2013 Partnership Interest [Member] | Mar. 31, 2012 Partnership Interest [Member] | Mar. 31, 2013 Partnership Interest [Member] | Mar. 31, 2012 Partnership Interest [Member]
Net investment income earned | $ 34,003 | $ 34,006 | $ 66,873 | $ 66,634 | $ 30,298 | $ 31,759 | $ 60,387 | $ 63,109 | $ 1,874 | $ 1,280 | $ 3,081 | $ 2,517 | $ 29 | $ 29 | $ 81 | $ 67 | $ 3,869 | $ 2,963 | $ 7,471 | $ 4,963 | $ 0 | $ 25 | $ 0 | $ 64 | $ (2,067) | $ (2,050) | $ (4,147) | $ (4,086) | $ 3,900 | $ 3,000 | $ 7,500 | $ 5,000

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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