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Commitments and Contingencies (Narrative) (Details) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Purchase amount of annuities | $ 15,000,000 sigi_PurchaseAmountOfAnnuities |  |
Rental expense for operating leases for office space and equipment | 15,600,000 us-gaap_LeaseAndRentalExpense | 13,200,000 us-gaap_LeaseAndRentalExpense | 13,100,000 us-gaap_LeaseAndRentalExpense
Capital Lease Agreements Expiration Period | 2019 |  |
Total minimum rental commitments under non-concellable leases | 44,900,000 sigi_TotalMinimumPaymentRequired |  |
Other Investments [Member] |  |  |
Remaining Commitment | $ 68,431,000 sigi_RemainingCommitment / us-gaap_InvestmentTypeAxis = us-gaap_OtherInvestmentsMember |  |

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- Definition This item presents capital lease agreements expiring date. + References No definition available. + Details
Name: | sigi_CapitalLeaseAgreementsExpirationPeriod
Namespace Prefix: | sigi_
Data Type: | xbrli:gYearItemType
Balance Type: | na
Period Type: | duration

X
- 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
Name: | sigi_PurchaseAmountOfAnnuities
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

X
- Definition Amount of minimum lease payments for all leases. + References No definition available. + Details
Name: | sigi_TotalMinimumPaymentRequired
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- 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
Name: | us-gaap_LeaseAndRentalExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_InvestmentTypeAxis=us-gaap_OtherInvestmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |