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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

X

- 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