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Investments (Net Investment Income) (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Net investment income earned	$ 30,650,000	$ 35,786,000	$ 97,284,000	$ 118,604,000
Decrease In Net Investment Income Before Tax	5,100,000	21,300,000
Fixed Maturity Securities [Member]
Net investment income earned	30,839,000	31,960,000	93,948,000	97,835,000
Decrease In Net Investment Income Before Tax	1,100,000		3,900,000
Equity Securities [Member]
Net investment income earned	1,268,000	1,197,000	3,785,000	2,299,000
Short-term Investments [Member]
Net investment income earned	36,000	28,000	103,000	123,000
Other Investments [Member]
Net investment income earned	497,000	4,453,000	5,460,000	23,994,000
Miscellaneous Investment Income [Member]
Net investment income earned	41,000	41,000	105,000	88,000
Investment Expenses [Member]
Net investment income earned	(2,031,000)	(1,893,000)	(6,117,000)	(5,735,000)
Alternative Investments [Member]
Decrease In Net Investment Income Before Tax	$ 17,600,000	$ 3,500,000

X

- Definition

This item represents the increase (decrease) in income earned from investments in securities and other investments including dividends, interest, and returns on other investments in the current reporting period as compared to the comparable prior year period. This item does not include realized gains (losses).

+ References

No definition available.

+ Details

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Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
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Period Type:	duration

X

- 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