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Commitments and Contingencies (Narrative) (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Contractual Obligation Additional Investment in Alternative or Other Investments | $ 221,000 |  |
Purchase amount of annuities | 18,500 |  |
Rental expense for operating leases for office space, equipment, and vehicles | $ 10,800 | $ 12,300 | $ 11,700
Capital Lease Agreements Expiration Period | 2019 |  |
Other Investments [Member] |  |  |
Remaining Commitment | $ 220,999 | $ 143,686 |

X
- Definition This item presents capital lease agreements expiring date. + References No definition available. + Details
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- Definition Contractual obligation to invest additional amounts in alternative and other investments. + References No definition available. + Details
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Namespace Prefix: | sigi_
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Balance Type: | debit
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- Definition The amount of annuities purchased from life insurance companies to fulfill obligations under claim settlements that provide for periodic future payments to claimants. + References No definition available. + Details
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Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- 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
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition Amount of rent expense incurred for leased assets, including but not limited to, furniture and equipment, that is not directly or indirectly associated with the manufacture, sale or creation of a product or product line. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
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- Details
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Period Type: |