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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