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Commitments and Contingencies (Narrative) (Details) (USD $) In Thousands	Jun. 30, 2011
Commitments and Contingencies [Abstract]
Remaining Commitment	$ 61,620

X

- Definition

n/a

+ Details

Name:	sigi_CommitmentsAndContingenciesAbstract
Namespace Prefix:	sigi
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

This item represents contractual obligations to invest additional amounts on alternative investments. These additional investments may or may not be required.

+ Details

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