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Invesments (Net Investment Income on Other Investments, by Strategy) (Details) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Net investment income earned	$ 35,786	$ 32,986	$ 118,604	$ 104,237
Energy/Power Generation [Member] | Alternative Investments [Member]
Net investment income earned	1,760	409	7,599	3,440
Private Equity [Member] | Alternative Investments [Member]
Net investment income earned	1,640	543	5,398	1,350
Secondary Private Equity Member | Alternative Investments [Member]
Net investment income earned	1,179	240	5,895	3,924
Distressed Debt [Member] | Alternative Investments [Member]
Net investment income earned	(153)	472	1,241	1,195
Real Estate Strategy [Member] | Alternative Investments [Member]
Net investment income earned	35	303	1,485	(2,056)
Venture Capital [Member] | Alternative Investments [Member]
Net investment income earned	125	15	1,448	263
Mezzanine Financing [Member] | Alternative Investments [Member]
Net investment income earned	(133)	377	928	3,006
Other Securities [Member]
Net investment income earned	41	41	88	94
Other Investments [Member]
Net investment income earned	4,494	2,400	24,082	11,216
Alternative Investments [Member]
Net investment income earned	$ 4,500	$ 2,300	$ 24,000	$ 11,100

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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