Investments (Investments Pledged as Collateral) (Details) $ in Millions	Sep. 30, 2016 USD ($)
Investments Pledged As Collateral [Line Items]
Carrying Value Bonds On Deposit With Various State And Regulatory Agencies	$ 23.1
Securities Pledged As Collateral	149.0
U.S. Government and Government Agencies [Member]
Investments Pledged As Collateral [Line Items]
Carrying Value Bonds On Deposit With Various State And Regulatory Agencies	23.1
Securities Pledged As Collateral	30.5
Commercial Mortgage Backed Securities [Member]
Investments Pledged As Collateral [Line Items]
Carrying Value Bonds On Deposit With Various State And Regulatory Agencies	0.0
Securities Pledged As Collateral	1.0
Residential Mortgage Backed Securities [Member]
Investments Pledged As Collateral [Line Items]
Carrying Value Bonds On Deposit With Various State And Regulatory Agencies	0.0
Securities Pledged As Collateral	117.5
Federal Home Loan Bank of Indianapolis [Member]
Investments Pledged As Collateral [Line Items]
Fixed Income Securities Pledged For FHLB Collateral	61.5
Federal Home Loan Bank of Indianapolis [Member] \| U.S. Government and Government Agencies [Member]
Investments Pledged As Collateral [Line Items]
Fixed Income Securities Pledged For FHLB Collateral	7.4
Federal Home Loan Bank of Indianapolis [Member] \| Commercial Mortgage Backed Securities [Member]
Investments Pledged As Collateral [Line Items]
Fixed Income Securities Pledged For FHLB Collateral	1.0
Federal Home Loan Bank of Indianapolis [Member] \| Residential Mortgage Backed Securities [Member]
Investments Pledged As Collateral [Line Items]
Fixed Income Securities Pledged For FHLB Collateral	53.1
Federal Home Loan Bank of New York [Member]
Investments Pledged As Collateral [Line Items]
Fixed Income Securities Pledged For FHLB Collateral	64.4
Federal Home Loan Bank of New York [Member] \| U.S. Government and Government Agencies [Member]
Investments Pledged As Collateral [Line Items]
Fixed Income Securities Pledged For FHLB Collateral	0.0
Federal Home Loan Bank of New York [Member] \| Commercial Mortgage Backed Securities [Member]
Investments Pledged As Collateral [Line Items]
Fixed Income Securities Pledged For FHLB Collateral	0.0
Federal Home Loan Bank of New York [Member] \| Residential Mortgage Backed Securities [Member]
Investments Pledged As Collateral [Line Items]
Fixed Income Securities Pledged For FHLB Collateral	$ 64.4

[Line Items] for Disclosure of financial instruments held by the entity for its own account for trading or investment purposes that are carried at fair value and pledged to counterparties as collateral for financing transactions. Description may include equity, fixed income, debt or other securities.

No definition available.

+ Details

Name:	sigi_InvestmentsPledgedAsCollateralLineItems
Namespace Prefix:	sigi_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Carrying amount of cash and investment securities on deposit with state regulatory authorities in connection with capital requirements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 320
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=28366721&loc=d3e22002-158495

+ Details

Name:	us-gaap_AssetsHeldByInsuranceRegulators
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of the collateral pledged in support of federal home loan bank advances outstanding.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6479118&loc=d3e64650-112822

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=6479336&loc=d3e64711-112823

+ Details

Name:	us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresCollateralPledged1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of financial instrument held by the entity for its own account (proprietary securities) for trading or investment purposes that are carried at fair value and pledged to counterparties as collateral for financing transactions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
-URI http://asc.fasb.org/extlink&oid=64858031&loc=SL6224234-111729

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728

+ Details

Name:	us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	invest_InvestmentSectorAxis=us-gaap_USTreasuryAndGovernmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	invest_InvestmentSectorAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	invest_InvestmentSectorAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankAxis=us-gaap_FederalHomeLoanBankOfIndianapolisMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FederalHomeLoanBankAdvancesBranchOfFHLBBankAxis=us-gaap_FederalHomeLoanBankOfNewYorkMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: