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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
	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	$ 32,764us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross	$ 35,575us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross	$ 36,012us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross
Incurred losses and loss expenses, gross	167us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossIncurredLoss	347us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossIncurredLoss	3,059us-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	30,653us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross	32,764us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross	35,575us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross
Reserves for losses and loss expenses at beginning of year, net	25,167us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet	27,769us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet	27,916us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet
Incurred losses and loss expenses, net	0us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss	68us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss	3,000us-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	22,994us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet	25,167us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet	27,769us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet
Asbestos [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Reserves for losses and loss expenses at beginning of year, gross	8,897us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember	9,170us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember	8,412us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember
Incurred losses and loss expenses, gross	60us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossIncurredLoss / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember	0us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossIncurredLoss / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember	1,696us-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	8,751us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember	8,897us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember	9,170us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember
Reserves for losses and loss expenses at beginning of year, net	7,518us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember	7,791us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember	6,586us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember
Incurred losses and loss expenses, net	0us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember	0us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember	2,000us-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	7,314us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember	7,518us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = us-gaap_AsbestosIssueMember	7,791us-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	23,867us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember	26,405us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember	27,600us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember
Incurred losses and loss expenses, gross	107us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossIncurredLoss / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember	347us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGrossIncurredLoss / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember	1,363us-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	21,902us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember	23,867us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember	26,405us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsGross / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember
Reserves for losses and loss expenses at beginning of year, net	17,649us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember	19,978us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember	21,330us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember
Incurred losses and loss expenses, net	0us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember	68us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNetIncurredLoss / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember	1,000us-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	$ 15,680us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember	$ 17,649us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember	$ 19,978us-gaap_LiabilityForAsbestosAndEnvironmentalClaimsNet / sigi_ScheduleOfLiabilityForUnpaidEnvironmentalClaimsAndClaimsAdjustmentExpenseAxis = sigi_EnvironmentalclaimsMember

X

- 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

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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)

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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

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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

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Amount of asbestos claims paid in the current period, net of reinsurance recoveries.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

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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

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-Section 50

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