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Statements of Cash Flow (Cash Flow Supplemental Disclosures) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Interest	$ 22,221,000us-gaap_InterestPaid	$ 21,465,000us-gaap_InterestPaid	$ 18,779,000us-gaap_InterestPaid
Federal income tax	22,699,000sigi_FederalIncomeTaxesPaidNet	20,000,000sigi_FederalIncomeTaxesPaidNet	6,421,000sigi_FederalIncomeTaxesPaidNet
Stock Split Related To Equity Securities, AFS	334,000sigi_StockSplitRelatedToEquitySecuritiesAFS	0sigi_StockSplitRelatedToEquitySecuritiesAFS	0sigi_StockSplitRelatedToEquitySecuritiesAFS
Assets acquired under capital lease arrangement	5,642,000us-gaap_CapitalLeaseObligationsIncurred	2,583,000us-gaap_CapitalLeaseObligationsIncurred	2,091,000us-gaap_CapitalLeaseObligationsIncurred
Non-Cash Purchase Of Property and Equipment [Line Items]	338,000sigi_NonCashPurchaseOfPropertyandEquipment	20,000sigi_NonCashPurchaseOfPropertyandEquipment	0sigi_NonCashPurchaseOfPropertyandEquipment
National Flood Insurance Program [Member]
Restricted cash	6,000,000us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue / sigi_ScheduleOfCashFlowSupplumentalDisclosuresAxis = sigi_NationalFloodInsuranceProgramMember	7,300,000us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue / sigi_ScheduleOfCashFlowSupplumentalDisclosuresAxis = sigi_NationalFloodInsuranceProgramMember
Afs Fixed Maturity Securities [Member]
Fair Value of Assets Acquired	20,781,000us-gaap_FairValueOfAssetsAcquired / sigi_ScheduleOfCashFlowSupplumentalDisclosuresAxis = sigi_AfsFixedMaturitySecuritiesMember	37,965,000us-gaap_FairValueOfAssetsAcquired / sigi_ScheduleOfCashFlowSupplumentalDisclosuresAxis = sigi_AfsFixedMaturitySecuritiesMember	18,942,000us-gaap_FairValueOfAssetsAcquired / sigi_ScheduleOfCashFlowSupplumentalDisclosuresAxis = sigi_AfsFixedMaturitySecuritiesMember
Held-to-maturity Securities [Member]
Fair Value of Assets Acquired	$ 4,289,000us-gaap_FairValueOfAssetsAcquired / sigi_ScheduleOfCashFlowSupplumentalDisclosuresAxis = us-gaap_HeldtomaturitySecuritiesMember	$ 15,820,000us-gaap_FairValueOfAssetsAcquired / sigi_ScheduleOfCashFlowSupplumentalDisclosuresAxis = us-gaap_HeldtomaturitySecuritiesMember	$ 25,168,000us-gaap_FairValueOfAssetsAcquired / sigi_ScheduleOfCashFlowSupplumentalDisclosuresAxis = us-gaap_HeldtomaturitySecuritiesMember

X

- Definition

The amount of cash paid during the current period to federal authorities as taxes on income, net of any cash received during the current period as refunds for the overpayment of federal taxes.

+ References

No definition available.

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X

- Definition

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+ References

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X

- Definition

This item represents a stock split which resulted in a non-cash transaction of AFS equity securities.

+ References

No definition available.

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X

- Definition

The increase during the period in capital lease obligations due to entering into new capital leases.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

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The fair value of assets acquired in noncash investing or financing activities.

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- Definition

The amount of cash paid for interest during the period.

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- Definition

The carrying amounts of cash and cash equivalent items which are restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against short-term borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits; however, time deposits and short-term certificates of deposit are not generally included in legally restricted deposits. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. For a classified balance sheet represents the current portion only (the noncurrent portion has a separate concept); there is a separate and distinct element for unclassified presentations.

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