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Reserve of Losses and Loss Expenses (Rollforward of Reserves for Losses and Loss Expenses) (Details) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Gross reserves for losses and loss expenses, at beginning of year | $ 3,349,770,000 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense | $ 4,068,941,000 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense | $ 3,144,924,000 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense
Less: reinsurance recoverable on unpaid loss and loss expenses, at beginning of year | 540,839,000 us-gaap_ReinsuranceRecoverableForUnpaidClaimsAndClaimsAdjustments | 1,409,755,000 us-gaap_ReinsuranceRecoverableForUnpaidClaimsAndClaimsAdjustments | 549,490,000 us-gaap_ReinsuranceRecoverableForUnpaidClaimsAndClaimsAdjustments
Net reserves for losses and loss expenses, at beginning of year | 2,808,931,000 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNet | 2,659,186,000 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNet | 2,595,434,000 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNet
Incurred losses and loss expense for claims occurring in the current year | 1,216,770,000 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersCurrentYearClaimsAndClaimsAdjustmentExpense | 1,147,263,000 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersCurrentYearClaimsAndClaimsAdjustmentExpense | 1,146,591,000 us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersCurrentYearClaimsAndClaimsAdjustmentExpense
Incurred losses and loss expenses for claims occurring in the prior years | (59,269,000)us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense | (25,525,000)us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense | (25,601,000)us-gaap_SupplementalInformationForPropertyCasualtyInsuranceUnderwritersPriorYearClaimsAndClaimsAdjustmentExpense
Total incurred losses and loss expenses | 1,157,501,000 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaims1 | 1,121,738,000 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaims1 | 1,120,990,000 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaims1
Paid losses and loss expenses for claims occurring in current year | 468,478,000 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidCurrentYear1 | 399,559,000 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidCurrentYear1 | 424,496,000 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidCurrentYear1
Paid losses and loss expenses for claims occurring in prior years | 592,062,000 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidPriorYears1 | 572,434,000 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidPriorYears1 | 632,742,000 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidPriorYears1
Total paid losses and loss expenses | 1,060,540,000 us-gaap_PaymentsForLossesAndLossAdjustmentExpense | 971,993,000 us-gaap_PaymentsForLossesAndLossAdjustmentExpense | 1,057,238,000 us-gaap_PaymentsForLossesAndLossAdjustmentExpense
Net reserves for losses and loss expenses, at end of year | 2,905,892,000 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNet | 2,808,931,000 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNet | 2,659,186,000 us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseNet
Add: reinsurance recoverable on unpaid loss and loss expenses, at end of year | 571,978,000 us-gaap_ReinsuranceRecoverableForUnpaidClaimsAndClaimsAdjustments | 540,839,000 us-gaap_ReinsuranceRecoverableForUnpaidClaimsAndClaimsAdjustments | 1,409,755,000 us-gaap_ReinsuranceRecoverableForUnpaidClaimsAndClaimsAdjustments
Gross reserves for losses and loss expenses, at end of year | 3,477,870,000 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense | 3,349,770,000 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense | 4,068,941,000 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense
Increase of net loss and loss expense reserves | 97,000,000 sigi_IncreaseDecreaseOfNetLossAndLossExpenseReserves | 149,700,000 sigi_IncreaseDecreaseOfNetLossAndLossExpenseReserves | 63,800,000 sigi_IncreaseDecreaseOfNetLossAndLossExpenseReserves
Anticipated recoveries for salvage and subrogation claims | $ 65,100,000 us-gaap_SalvageAndSubrogationRecoveriesValue | $ 61,000,000 us-gaap_SalvageAndSubrogationRecoveriesValue | $ 62,200,000 us-gaap_SalvageAndSubrogationRecoveriesValue

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- Definition The increase or decrease during the period in the liability for amounts representing estimated cost of settling unpaid claims under the terms of the underlying insurance policies, less estimated reinsurance recoveries on such claims. This includes an estimate for claims which have been incurred but not reported. Claim adjustment expenses represent the costs estimated to be incurred in the settlement of unpaid claims. + References No definition available. + Details
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- Definition The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a),(b),(c) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
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- Definition Amount of payments to settle claims incurred in the current period and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
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- Definition Amount of payments to settle claims incurred in prior periods and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
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- Definition Amount of expense for claims incurred and costs incurred in the claim settlement process. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
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- Definition Liability as of the balance sheet date for amounts representing estimated cost of settling unpaid claims under the terms of the underlying insurance policies, less estimated reinsurance recoveries on such claims. This includes an estimate for claims which have been incurred but not reported. Claim adjustment expenses represent the costs estimated to be incurred in the settlement of unpaid claims. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Financial Reporting Release (FRR) -Number 403 -Paragraph 04 -Subparagraph b Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 13 -Subparagraph a -Article 7 + Details
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- Definition Amount of payments to settle insured claims and pay costs incurred in the claims settlement process. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 25 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3536-108585 + Details
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- Definition Amount after valuation allowance recoverable under reinsurance contracts for claims and claims adjustments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
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- Definition The estimated amount as of the balance sheet date to be received by an insurer from the sale of property (usually damaged) on which the insurer has paid a total claim to the insured and has obtained title to the property and the right of an insurer to pursue any course of recovery of damages, in its name or in the name of the policyholder, against a third party who is liable for costs relating to an insured event that have been paid by the insurer. Estimated salvage and subrogation is generally deducted from the liability for unpaid claims. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 30 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6482241&loc=d3e13779-158432 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 30 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6482241&loc=d3e13783-158432 + Details
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- Definition Amount of expense for claims incurred in the current reporting period and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 18 -Article 12 + Details
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- Definition Amount of expense for claims incurred in prior reporting periods and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-18) -URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 18 -Article 12 + Details
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