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Summary of Significant Accounting Policies (Narrative) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended			12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Depreciation				$ 12.6us-gaap_Depreciation	$ 10.2us-gaap_Depreciation	$ 9.2us-gaap_Depreciation
Percentage of fixed maturity portfolio utilizing market approach				99.00%sigi_PercentageOfFixedMaturityPortfolioUtilizingMarketApproach
Investment yields before tax assumed in premium deficiency assessment	3.00%sigi_InvestmentYieldsAssumedInPremiumDeficiencyAssessment	3.00%sigi_InvestmentYieldsAssumedInPremiumDeficiencyAssessment	3.10%sigi_InvestmentYieldsAssumedInPremiumDeficiencyAssessment
Deferred policy acquisition costs amortized to expense				$ 364.3us-gaap_DeferredPolicyAcquisitionCostAmortizationExpense	$ 331.8us-gaap_DeferredPolicyAcquisitionCostAmortizationExpense	$ 298.5us-gaap_DeferredPolicyAcquisitionCostAmortizationExpense

X

- Definition

This element represents the investment yields assumed in the premium deficiency assessment, which are based on the entity's actual average investment yield before tax as of the calculation date.

+ References

No definition available.

+ Details

Name:	sigi_InvestmentYieldsAssumedInPremiumDeficiencyAssessment
Namespace Prefix:	sigi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

This element represents the percentage of our fixed maturity portfolio utilizing a market approach, primarily using matrix pricing models prepared by external pricing services.

+ References

No definition available.

+ Details

Name:	sigi_PercentageOfFixedMaturityPortfolioUtilizingMarketApproach
Namespace Prefix:	sigi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Amount of expense related to deferred policy acquisition costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 16

-Article 12



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.7(a))

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 7

-Article 7



Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 18

-Article 12



Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 30

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=35755530&loc=d3e11264-158415



Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 30

-Section 35

-Paragraph 11

-URI http://asc.fasb.org/extlink&oid=28247317&loc=d3e10157-158409



Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 20

-Section 45

-Paragraph 2

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=6480726&loc=d3e6691-158385

+ Details

Name:	us-gaap_DeferredPolicyAcquisitionCostAmortizationExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

+ Details

Name:	us-gaap_Depreciation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration