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

X

- 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: