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

X
- Definition n/a + Details
Name: | sigi_OrganizationAbstract
Namespace Prefix: | sigi
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- 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