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Adoption of Accounting Pronouncements(Other Information) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2011
Underwriting loss | $ (103,584)
Combined Ratio | 107.20%
As Originally Reported [Member] |
Underwriting loss | $ (106,919)
Combined Ratio | 107.40%

X
- Definition Net premiums earned and other insurance income less net loss and loss expenses incurred, loss adjustment expenses, policy acquisition costs and other underwriting expenses. + References No definition available. + Details
Name: | sigi_Underwritinglossprofitgross
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Ratio of non-life losses and loss expenses, non-life acquisition expenses and non-life general and administrative expenses, to non-life net premiums earned. + References No definition available. + Details
Name: | us-gaap_CombinedRatio
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration