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Commitments and Contingencies (Narrative) (Details) (USD $) In Thousands | Jun. 30, 2011
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Commitments and Contingencies [Abstract] |
Remaining Commitment | $ 61,620

X
- Definition n/a + Details
Name: | sigi_CommitmentsAndContingenciesAbstract
Namespace Prefix: | sigi
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition This item represents contractual obligations to invest additional amounts on alternative investments. These additional investments may or may not be required. + Details
Name: | sigi_RemainingCommitment
Namespace Prefix: | sigi
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant