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Reserve for Losses and Loss Expenses (Exposure to Environmental Claims) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Liability For Environmental Claims Gross	$ 46,475		$ 47,291	$ 50,474	$ 51,515
Liability For Environmental Claims Net	38,380		39,449	41,602	44,093
Asbestos Related Claims [Member]
Liability For Environmental Claims Gross	8,412		9,979	11,056	14,269
Liability For Environmental Claims Net	6,586		8,167	9,244	12,969
Landfill Sites Related Claims [Member]
Liability For Environmental Claims Gross	16,400
Liability For Environmental Claims Net	11,500
Other Environmental Claims [Member]
Liability For Environmental Claims Gross	21,700	[1]
Liability For Environmental Claims Net	$ 20,300	[1]
[1] 1 Consists of leaking underground storage tanks and other latent environmental exposures.

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