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Statements of Cash Flow (Cash Flow Supplemental Disclosures) (Details) (USD $) | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Interest | $ 2,887,000 us-gaap_InterestPaid |  | $ 2,973,000 us-gaap_InterestPaid |
Federal income tax | 4,000,000 us-gaap_IncomeTaxesPaidNet |  | 2,000,000 us-gaap_IncomeTaxesPaidNet |
Corporate Actions Related To Equity Securities, AFS | 389,000 us-gaap_ConversionOfStockAmountConverted1 | [1] | 0 us-gaap_ConversionOfStockAmountConverted1 | [1]
Assets acquired under capital lease arrangements | 2,428,000 us-gaap_CapitalLeaseObligationsIncurred |  | 1,419,000 us-gaap_CapitalLeaseObligationsIncurred |
AFS Fixed Maturity Securities [Member] |  |  |  |
Tax-free exchange of fixed maturity security | 8,586,000 us-gaap_FairValueOfAssetsAcquired / sigi_ScheduleOfCashFlowSupplumentalDisclosuresAxis = sigi_AfsFixedMaturitySecuritiesMember |  | 0 us-gaap_FairValueOfAssetsAcquired / sigi_ScheduleOfCashFlowSupplumentalDisclosuresAxis = sigi_AfsFixedMaturitySecuritiesMember |
Held-to-maturity Securities [Member] |  |  |  |
Tax-free exchange of fixed maturity security | 0 us-gaap_FairValueOfAssetsAcquired / sigi_ScheduleOfCashFlowSupplumentalDisclosuresAxis = us-gaap_HeldtomaturitySecuritiesMember |  | 15,000 us-gaap_FairValueOfAssetsAcquired / sigi_ScheduleOfCashFlowSupplumentalDisclosuresAxis = us-gaap_HeldtomaturitySecuritiesMember |
National Flood Insurance Program [Member] |  |  |  |
Restricted Cash | $ 6,000,000 us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue / sigi_ScheduleOfCashFlowSupplumentalDisclosuresAxis = sigi_NationalFloodInsuranceProgramMember |  | $ 6,100,000 us-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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4313-108586 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4304-108586 + Details
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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4332-108586 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4313-108586 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4304-108586 + Details
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