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Investments (Net Investment Income) (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Net investment income earned	$ 34,006,000	$ 39,345,000	$ 66,634,000	$ 82,818,000
Decrease In Net Investment Income Before Tax	(5,300,000)	(16,200,000)
Fixed Maturity Securities [Member]
Net investment income earned	31,759,000	32,752,000	63,109,000	65,875,000
Equity Securities [Member]
Net investment income earned	1,280,000	785,000	2,517,000	1,102,000
Short-term Investments [Member]
Net investment income earned	29,000	33,000	67,000	95,000
Other Investments [Member]
Net investment income earned	2,963,000	7,900,000	4,963,000	19,541,000
Miscellaneous Investment Income [Member]
Net investment income earned	25,000	22,000	64,000	47,000
Investment Expenses [Member]
Net investment income earned	(2,050,000)	(2,147,000)	(4,086,000)	(3,842,000)
Alternative Investments [Member]
Decrease In Net Investment Income Before Tax	$ 4,700,000		$ 14,100,000

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