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Reserve for Losses and Loss Expenses (Exposure to Environmental Claims) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Liability For Environmental Claims Gross	$ 35,575	$ 36,012	$ 43,609	$ 46,920
Liability For Environmental Claims Net	27,769	27,916	35,766	38,047
Asbestos [Member]
Liability For Environmental Claims Gross	9,170	8,412	9,979	11,056
Liability For Environmental Claims Net	7,791	6,586	8,167	9,244
Landfill Sites Related Claims [Member]
Liability For Environmental Claims Gross	13,000
Liability For Environmental Claims Net	8,100
Leaking Underground Storage Tanks Claims [Member]
Liability For Environmental Claims Gross	13,400
Liability For Environmental Claims Net	$ 11,900

X

- Definition

Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to environmental claims, before estimated recoveries from reinsurers.

+ References

No definition available.

+ Details

Name:	sigi_LiabilityForEnvironmentalClaimsGross
Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to environmental claims, after estimated recoveries from reinsurers.

+ References

No definition available.

+ Details

Name:	sigi_LiabilityForEnvironmentalClaimsNet
Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant