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Organization (Narrative) (Details)	Sep. 30, 2011
Organization [Abstract]
Insurance Operations, number of states in which entity operates	22

X

- Definition

n/a

+ Details

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

X

- Definition

The number of states the entity operates in as of the balance sheet date.

+ Details

Name:	us-gaap_NumberOfStatesInWhichEntityOperates
Namespace Prefix:	us-gaap
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant