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Statements of Cash Flow (Cash Flow Supplemental Disclosures) (Details) (USD $)	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Interest	$ 2,887,000us-gaap_InterestPaid		$ 2,973,000us-gaap_InterestPaid
Federal income tax	4,000,000us-gaap_IncomeTaxesPaidNet		2,000,000us-gaap_IncomeTaxesPaidNet
Corporate Actions Related To Equity Securities, AFS	389,000us-gaap_ConversionOfStockAmountConverted1	[1]	0us-gaap_ConversionOfStockAmountConverted1	[1]
Assets acquired under capital lease arrangements	2,428,000us-gaap_CapitalLeaseObligationsIncurred		1,419,000us-gaap_CapitalLeaseObligationsIncurred
AFS Fixed Maturity Securities [Member]
Tax-free exchange of fixed maturity security	8,586,000us-gaap_FairValueOfAssetsAcquired / sigi_ScheduleOfCashFlowSupplumentalDisclosuresAxis = sigi_AfsFixedMaturitySecuritiesMember		0us-gaap_FairValueOfAssetsAcquired / sigi_ScheduleOfCashFlowSupplumentalDisclosuresAxis = sigi_AfsFixedMaturitySecuritiesMember
Held-to-maturity Securities [Member]
Tax-free exchange of fixed maturity security	0us-gaap_FairValueOfAssetsAcquired / sigi_ScheduleOfCashFlowSupplumentalDisclosuresAxis = us-gaap_HeldtomaturitySecuritiesMember		15,000us-gaap_FairValueOfAssetsAcquired / sigi_ScheduleOfCashFlowSupplumentalDisclosuresAxis = us-gaap_HeldtomaturitySecuritiesMember
National Flood Insurance Program [Member]
Restricted Cash	$ 6,000,000us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue / sigi_ScheduleOfCashFlowSupplumentalDisclosuresAxis = sigi_NationalFloodInsuranceProgramMember		$ 6,100,000us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue / sigi_ScheduleOfCashFlowSupplumentalDisclosuresAxis = sigi_NationalFloodInsuranceProgramMember
[1] 1Examples of such corporate actions include non-cash acquisitions and stock splits.

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- Definition

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

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The value of the stock converted in a noncash (or part noncash) transaction. Noncash is defined as transactions during a period that do not result in cash receipts or cash payments in the period. "Part noncash" refers to that portion of the transaction not resulting in cash receipts or cash payments in the period.

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