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Investments (OTTI by Asset Type) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010 securities	Jun. 30, 2011	Jun. 30, 2010 securities
OTTI Losses Gross	$ (163)	$ 6,162	$ 369	$ 12,235
Other-than-temporary impairments on fixed maturity securities recognized in other comprehensive income	(332)	(22)	(430)	(2,189)
OTTI Losses Recognized in Earnings	169	6,184	799	14,424
Number of impairments due to significant decline in share price		1		1
Equity Securities [Member]
OTTI Losses Gross		672		672
Other-than-temporary impairments on fixed maturity securities recognized in other comprehensive income		0		0
OTTI Losses Recognized in Earnings		672		672
Obligations of States and Political Subdivisions [Member] | Fixed Maturity Securities [Member]
OTTI Losses Gross			17
Other-than-temporary impairments on fixed maturity securities recognized in other comprehensive income			0
OTTI Losses Recognized in Earnings			17
Corporate Securities [Member] | Fixed Maturity Securities [Member]
OTTI Losses Gross			244
Other-than-temporary impairments on fixed maturity securities recognized in other comprehensive income			0
OTTI Losses Recognized in Earnings			244
Asset-backed Securities ("ABS") [Member] | Fixed Maturity Securities [Member]
OTTI Losses Gross				158
Other-than-temporary impairments on fixed maturity securities recognized in other comprehensive income				127
OTTI Losses Recognized in Earnings				31
Commercial Mortgage-backed Securities ("CMBS") [Member] | Fixed Maturity Securities [Member]
OTTI Losses Gross	(260)	3,405	(186)	3,445
Other-than-temporary impairments on fixed maturity securities recognized in other comprehensive income	(402)	569	(658)	(2,052)
OTTI Losses Recognized in Earnings	142	2,836	472	5,497
Average unrealized unrecognized loss as compared to amortized cost percentage		87.00%		87.00%
Residential Mortgage Backed Securities ("RMBS") [Member] | Fixed Maturity Securities [Member]
OTTI Losses Gross	97	2,085	294	7,960
Other-than-temporary impairments on fixed maturity securities recognized in other comprehensive income	70	(591)	228	(264)
OTTI Losses Recognized in Earnings	27	2,676	66	8,224
Number of Impairments due to intent to sell				2
Fixed Maturity Securities [Member]
OTTI Losses Gross		5,490		11,563
Other-than-temporary impairments on fixed maturity securities recognized in other comprehensive income		(22)		(2,189)
OTTI Losses Recognized in Earnings		$ 5,512		$ 13,752

X

- Definition

The average unrealized/unrecognized loss in comparison to the securities' amortized cost.

+ Details

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X

- Definition

This item represents the number of investments which made up the impairment losses for debt and equity securities recognized in earnings for the reporting periods due to the Company's intent to sell.

+ Details

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Namespace Prefix:	sigi
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X

- Definition

This item represents the number of investments which made up the impairment losses for debt and equity securities recognized in earnings for the reporting periods due to a significant decline in share price.

+ Details

Name:	sigi_NumberOfImpairmentsDueToSignificantDeclineInSharePrice
Namespace Prefix:	sigi
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Balance Type:	na
Period Type:	instant

X

- Definition

Total other than temporary impairment losses on investments.

+ Details

Name:	sigi_OtherThanTemporaryImpairments
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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Non-credit portion of other-than-temporary impairment losses on investments recognized in other comprehensive income.

+ Details

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Namespace Prefix:	sigi
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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X

- Definition

This element represents the amount of loss recognized for other than temporary impairments of investments in debt and equity securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 16

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