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Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended			12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Percent of Portfolio Level 3 Securities				1.00%
Percent of Investment Portfolio Comprised of Fixed Income Securities				89.00%
Percent of Fixed Income Securities Comprised of AFS				99.00%
Percent of Fixed Income Securities Comprised of HTM				1.00%
Percent of Loan Principal				102.00%
Depreciation expense				$ 19.5	$ 17.8	$ 17.4
Investment yields before tax assumed in premium deficiency assessment	3.30%	2.90%	2.40%

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

The percent of the fixed income portfolio comprised of available-for-sale securities.

+ References

No definition available.

+ Details

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

X

- Definition

The percent of the fixed income portfolio that is comprised of held-to-maturity securities.

+ References

No definition available.

+ Details

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

X

- Definition

The percentage of the investment portfolio that is comprised of fixed income securities.

+ References

No definition available.

+ Details

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

X

- Definition

Percent of loan principal collateralized.

+ References

No definition available.

+ Details

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

X

- Definition

Percent of investment portfolio in level 3 securities.

+ References

No definition available.

+ Details

Name:	sigi_PercentofPortfolioLevel3Securities
Namespace Prefix:	sigi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

+ Details

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