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Investments (Net Investment Income) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Net investment income earned	$ 32,457	$ 30,650	$ 99,330	$ 97,284
Fixed Maturity Securities [Member]
Net investment income earned	30,569	30,839	90,956	93,948
Equity Securities [Member]
Net investment income earned	1,341	1,268	4,422	3,785
Short-term Investments [Member]
Net investment income earned	21	36	102	103
Other Investment Portfolio [Member]
Net investment income earned	2,639	497	10,110	5,460
Miscellaneous Investment Income [Member]
Net investment income earned	0	41	0	105
Investment Expenses [Member]
Net investment income earned	$ (2,113)	$ (2,031)	$ (6,260)	$ (6,117)

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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).

+ References

No definition available.

+ Details

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