Litigation (Narrative) (Details)	Sep. 30, 2012
Loss Contingency, Information about Litigation Matters [Abstract]
Number of Insurance Subsidiaries	10

The number of insurance subsidiaries of the entity.

No definition available.

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