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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