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Significant Accounting Policies (Property and Equipment) (Details) | 12 Months Ended
--- | ---
Dec. 31, 2011 years
Computer hardware [Member] |
Estimated useful life | 3
Computer software [Member] |
Esimated useful life, mimimum | 3
Estimated useful life, maximum | 5
Internally developed software [Member] |
Estimated useful life | 5
Furniture and fixtures [Member] |
Estimated useful life | 10
Building and improvements [Member] |
Esimated useful life, mimimum | 5
Estimated useful life, maximum | 40

X
- Definition The estimated useful life of long-lived, physical assets used in the normal conduct of business and not intended for resale. Examples include land, buildings, machinery and equipment, and other types of furniture and equipment including, but not limited to, office equipment, furniture and fixtures, and computer equipment and software. + References No definition available. + Details
Name: | sigi_PropertyPlantAndEquipmentUsefulLifeEstimated
Namespace Prefix: | sigi_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | duration

X
- Definition The maximum useful life of long-lived, physical assets used in the normal conduct of business and not intended for resale. Examples include land, buildings, machinery and equipment, and other types of furniture and equipment including, but not limited to, office equipment, furniture and fixtures, and computer equipment and software. + References No definition available. + Details
Name: | us-gaap_PropertyPlantAndEquipmentUsefulLifeMaximum
Namespace Prefix: | us-gaap_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | duration

X
- Definition The minimum useful life of long lived, physical assets used in the normal conduct of business and not intended for resale. Examples include land, buildings, machinery and equipment, and other types of furniture and equipment including, but not limited to, office equipment, furniture and fixtures, and computer equipment and software. + References No definition available. + Details
Name: | us-gaap_PropertyPlantAndEquipmentUsefulLifeMinimum
Namespace Prefix: | us-gaap_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | duration