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Investments (Number of Securities in an Unrealized Unrecognized Loss Position) (Details) (USD $) In Thousands, unless otherwise specified | Jun. 30, 2011 securities years | Dec. 31, 2010 securities years
--- | --- | ---
Number of securities in an unrealized/unrecognized loss position | 225 | 199
Unrealized Unrecognized Loss | $ 12,935 | $ 21,557
Eighty Percent to Ninety-Nine Percent [Member] |  |
Number of securities in an unrealized/unrecognized loss position | 221 | 193
Unrealized Unrecognized Loss | 9,331 | 16,310
Sixty Percent to Seventy-Nine Percent [Member] |  |
Number of securities in an unrealized/unrecognized loss position | 1 | 2
Unrealized Unrecognized Loss | 23 | 1,125
Forty Percent to Fifty-Nine Percent [Member] |  |
Number of securities in an unrealized/unrecognized loss position | 2 | 2
Unrealized Unrecognized Loss | 2,529 | 2,160
Twenty Percent to Thirty-Nine Percent [Member] |  |
Number of securities in an unrealized/unrecognized loss position | 1 | 1
Unrealized Unrecognized Loss | 1,052 | 986
Zero Percent to Nineteen Percent [Member] |  |
Number of securities in an unrealized/unrecognized loss position | 0 | 1
Unrealized Unrecognized Loss | $ 0 | $ 976

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- Definition This item contains disclosure of the number of available-for-sale and held-to-maturity securities determined to be in an unrealized/unrecognized loss position. + Details
Name: | sigi_NumberOfSecuritiesInUnrealizedUnrecognizedLossPosition
Namespace Prefix: | sigi
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Definition This item represents the sum of (i) the excess of amortized cost over fair value for available-for-sale securities that are in a loss position and (ii) the market fluctuations on held-to-maturity securities from the later of (a) the date a security is designated as held-to-maturity; or (b) the date that an other-than-temporary impairment charge is recognized on a held-to-maturity security for securities that are in an unrecognized loss position. + Details
Name: | sigi_UnrealizedUnrecognizedLoss
Namespace Prefix: | sigi
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant