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Investments (Net Investment Income) (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended
--- | ---
Mar. 31, 2014 | Mar. 31, 2013
Net Investment Income [Line Items] |  |
Net investment income earned | $ 35,534 | $ 32,870
Fixed Income Securities [Member] |  |
Net Investment Income [Line Items] |  |
Net investment income earned | 31,028 | 30,089
Equity Securities [Member] |  |
Net Investment Income [Line Items] |  |
Net investment income earned | 1,449 | 1,207
Short-term Investments [Member] |  |
Net Investment Income [Line Items] |  |
Net investment income earned | 19 | 52
Other Investments [Member] |  |
Net Investment Income [Line Items] |  |
Net investment income earned | 5,218 | 3,602
Investment Expenses [Member] |  |
Net Investment Income [Line Items] |  |
Net investment income earned | $ (2,180) | $ (2,080)

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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

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