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Investments (Net Investment Income) (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2012 | Sep. 30, 2012 | Jun. 30, 2012 | Mar. 31, 2012 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
Statement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net investment income earned | $ 35,313 | $ 32,457 | $ 34,003 | $ 32,870 | $ 34,593 | $ 30,650 | $ 34,006 | $ 32,628 | $ 134,643 | $ 131,877 | $ 147,443
Fixed Maturity Securities [Member] |  |  |  |  |  |  |  |  |  |  |
Statement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net investment income earned |  |  |  |  |  |  |  |  | 121,582 | 124,687 | 129,710
Equity Securities, Dividend Income [Member] |  |  |  |  |  |  |  |  |  |  |
Statement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net investment income earned |  |  |  |  |  |  |  |  | 6,140 | 6,215 | 4,535
Short-term Investments [Member] |  |  |  |  |  |  |  |  |  |  |
Statement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net investment income earned |  |  |  |  |  |  |  |  | 117 | 151 | 160
Other Investment Portfolio [Member] |  |  |  |  |  |  |  |  |  |  |
Statement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net investment income earned |  |  |  |  |  |  |  |  | 15,208 | 8,996 | 20,539
Miscellaneous Investment Income [Member] |  |  |  |  |  |  |  |  |  |  |
Statement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net investment income earned |  |  |  |  |  |  |  |  | 0 | 0 | 133
Investment Expenses [Member] |  |  |  |  |  |  |  |  |  |  |
Statement [Line Items] |  |  |  |  |  |  |  |  |  |  |
Net investment income earned |  |  |  |  |  |  |  |  | $ (8,404) | $ (8,172) | $ (7,634)

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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
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Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration