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Investments (Net Investment Income) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Net investment income earned	$ 32,870	$ 32,628
Fixed Maturity Securities [Member]
Net investment income earned	30,089	31,350
Equity Securities [Member]
Net investment income earned	1,207	1,237
Short-term Investments [Member]
Net investment income earned	52	38
Other Investments [Member]
Net investment income earned	3,602	2,000
Miscellaneous Investment Income [Member]
Net investment income earned	0	39
Investment Expenses [Member]
Net investment income earned	$ (2,080)	$ (2,036)

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