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Reserve for Losses and Loss Expenses (Exposure to Environmental Claims) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Liability For Environmental Claims Gross	$ 32,764	$ 35,575	$ 36,012	$ 43,609
Liability For Environmental Claims Net	25,167	27,769	27,916	35,766
Asbestos [Member]
Liability For Environmental Claims Gross	8,897	9,170	8,412	9,979
Liability For Environmental Claims Net	7,518	7,791	6,586	8,167
Landfill Sites Related Claims [Member]
Liability For Environmental Claims Gross	12,300
Liability For Environmental Claims Net	7,400
Leaking Underground Storage Tanks Claims [Member]
Liability For Environmental Claims Gross	11,600
Liability For Environmental Claims Net	$ 10,300

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