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Organization (Narrative) (Details)	Dec. 31, 2011
Organization [Abstract]
Number Of States Which Excess And Surplus Subsidiary Acquired Operates	50

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- Definition

The number of states in which our E&S subsidiary is authorized to write business in as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	sigi_NumberOfStatesWhichExcessAndSurplusSubsidiaryAcquiredOperates
Namespace Prefix:	sigi_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

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- Details

Name:	sigi_OrganizationAbstract
Namespace Prefix:	sigi_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration