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Discontinued Operations (Narrative) (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011 securities years	Jun. 30, 2010	Jun. 30, 2011 securities years	Jun. 30, 2010	Dec. 31, 2009
Discontinued Operations [Abstract]
Percentage of interest sold in Selective HR					100.00%
Period over which proceeds are scheduled to be received	10		10
Estimated sales price of Selective HR	$ 6,900,000		$ 6,900,000
Sales price received on subsidiary	2,400,000		2,400,000
After-tax charge	$ 0	$ (1,325,000)	$ 0	$ (2,115,000)

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- Definition

n/a

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Namespace Prefix:	sigi
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- Definition

This item represents the estimated proceeds associated with the sale of a subsidiary.

+ Details

Name:	sigi_EstimatedSalesPriceOfSubsidiary
Namespace Prefix:	sigi
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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- Definition

This item represents the percentage of the company's interest in its subsidiary that was sold.

+ Details

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

X

- Definition

The number of years over which the proceeds from sale of subsidiary are scheduled to be received.

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Name:	sigi_PeriodOverWhichProceedsAreScheduledToBeReceived
Namespace Prefix:	sigi
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

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- Definition

Amount of sales price for sale of subsidiary that has been received as of the balance sheet date.

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Namespace Prefix:	sigi
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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- Definition

Gain (loss) after tax expense (benefit), not previously recognized and resulting from the sale of a business component, which is recognized at the date of sale. A gain (loss) reflects the amount by which the consideration received exceeds (is exceeded by) the net carrying amount (reflecting previous provisions for loss on disposal, if any) of the disposal group.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 144
-Paragraph 43

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 144
-Paragraph 47
-Subparagraph b

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