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Invesments (Net Investment Income on Other Investments, by Strategy) (Details) (USD $) In Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Net investment income earned	$ 39,345	$ 36,545	$ 82,818	$ 71,251
Other Securities [Member]
Net investment income earned	22	16	47	53
Other Investments [Member]
Net investment income earned	7,922	4,884	19,588	8,816
Energy/Power Generation [Member] | Alternative Investments [Member]
Net investment income earned	1,284	965	5,839	3,031
Secondary Private Equity Member | Alternative Investments [Member]
Net investment income earned	3,067	2,665	4,716	3,684
Distressed Debt [Member] | Alternative Investments [Member]
Net investment income earned	421	(41)	1,394	723
Private Equity [Member] | Alternative Investments [Member]
Net investment income earned	1,181	99	3,758	807
Real Estate Strategy [Member] | Alternative Investments [Member]
Net investment income earned	681	(494)	1,450	(2,359)
Mezzanine Financing [Member] | Alternative Investments [Member]
Net investment income earned	701	1,692	1,061	2,629
Venture Capital [Member] | Alternative Investments [Member]
Net investment income earned	$ 565	$ (18)	$ 1,323	$ 248

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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).

+ Details

Name:	sigi_NetInvestmentIncomeEarned
Namespace Prefix:	sigi
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration