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Adoption of Accounting Pronouncements (Narrative) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011
Adoption of Accounting Pronouncements [Abstract]
Potential after-tax impact on stockholders' equity	$ 51

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- Definition

Potential after-tax impact on the entity's stockholders' equity as a result of the adoption of the accounting pronouncement.

+ References

No definition available.

+ Details

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

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- Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration