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Commitments and Contingencies (Narrative) (Details) (Other Investments [Member], USD $) In Thousands, unless otherwise specified | Mar. 31, 2012
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Other Investments [Member] |
Remaining Commitment | $ 54,270

X
- Definition This item represents contractual obligations to invest additional amounts on our other investment portfolio. These additional investments may or may not be required. + References No definition available. + Details
Name: | sigi_RemainingCommitment
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant