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Reserve for Losses and Loss Expenses (Roll Forward of Gross and Net Asbestos and Net Environmental Incurred Losses and Loss Expenses and Related Reserves) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |
Reserves for losses and loss expenses at beginning of year, gross | $ 32,764 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross | $ 35,575 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross | $ 36,012 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross
Incurred losses and loss expenses, gross | 167 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossIncurredLoss | 347 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossIncurredLoss | 3,059 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossIncurredLoss
Less: losses and loss expenses paid, gross | (2,278)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossClaimsPaid | (3,158)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossClaimsPaid | (3,496)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossClaimsPaid
Reserves for losses and loss expenses at the end of year, gross | 30,653 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross | 32,764 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross | 35,575 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross
Reserves for losses and loss expenses at beginning of year, net | 25,167 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet | 27,769 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet | 27,916 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet
Incurred losses and loss expenses, net | 0 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss | 68 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss | 3,000 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss
Less: losses and loss expenses paid, net | (2,173)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetClaimsPaid | (2,670)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetClaimsPaid | (3,147)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetClaimsPaid
Reserves for losses and loss expenses at the end of year, net | 22,994 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet | 25,167 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet | 27,769 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet
Asbestos [Member] |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |
Reserves for losses and loss expenses at beginning of year, gross | 8,897 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember | 9,170 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember | 8,412 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember
Incurred losses and loss expenses, gross | 60 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossIncurredLoss / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember | 0 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossIncurredLoss / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember | 1,696 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossIncurredLoss / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember
Less: losses and loss expenses paid, gross | (206)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossClaimsPaid / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember | (273)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossClaimsPaid / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember | (938)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossClaimsPaid / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember
Reserves for losses and loss expenses at the end of year, gross | 8,751 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember | 8,897 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember | 9,170 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember
Reserves for losses and loss expenses at beginning of year, net | 7,518 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember | 7,791 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember | 6,586 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember
Incurred losses and loss expenses, net | 0 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember | 0 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember | 2,000 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember
Less: losses and loss expenses paid, net | (204)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetClaimsPaid / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember | (273)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetClaimsPaid / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember | (795)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetClaimsPaid / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember
Reserves for losses and loss expenses at the end of year, net | 7,314 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember | 7,518 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember | 7,791 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember
Enviromental [Member] |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |
Reserves for losses and loss expenses at beginning of year, gross | 23,867 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember | 26,405 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember | 27,600 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember
Incurred losses and loss expenses, gross | 107 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossIncurredLoss / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember | 347 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossIncurredLoss / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember | 1,363 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossIncurredLoss / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember
Less: losses and loss expenses paid, gross | (2,072)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossClaimsPaid / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember | (2,885)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossClaimsPaid / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember | (2,558)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossClaimsPaid / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember
Reserves for losses and loss expenses at the end of year, gross | 21,902 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember | 23,867 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember | 26,405 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember
Reserves for losses and loss expenses at beginning of year, net | 17,649 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember | 19,978 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember | 21,330 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember
Incurred losses and loss expenses, net | 0 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember | 68 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember | 1,000 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember
Less: losses and loss expenses paid, net | (1,969)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetClaimsPaid / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember | (2,397)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetClaimsPaid / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember | (2,352)us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetClaimsPaid / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember
Reserves for losses and loss expenses at the end of year, net | $ 15,680 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember | $ 17,649 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember | $ 19,978 us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember

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- Definition Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to asbestos and environmental claims, before estimated recoveries from reinsurers. + 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 Amount of asbestos claims paid in the current period, before estimated recoveries from reinsurers. + 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 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14784-158437 + Details
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- Definition Increase in asbestos and environmental reserve based on known and estimated costs of settling insured claims and costs incurred in the claims settlement process, before estimated recoveries from reinsurers. + 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 Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to asbestos and environmental claims, net of estimated reinsurance recoveries. + 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 Amount of asbestos claims paid in the current period, net of reinsurance recoveries. + 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 Increase in asbestos and environmental reserve based on known and estimated costs of settling insured claims and costs incurred in the claims settlement process, net of estimated reinsurance recoveries. + 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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