Amount of Federal Home Loan Bank (FHLBank) advances to member financial institutions.

No definition available.

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

- Definition

Interest rate of advances made and reported by Federal Home Loan Bank (FHLBank).

+ References

No definition available.

+ Details

Name:	us-gaap_FederalHomeLoanBankAdvancesInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The effective interest rate at the end of the reporting period.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

The carrying value as of the balance sheet date of the current portion of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_LinesOfCreditCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Reflects the total carrying amount as of the balance sheet date of debt having initial terms less than one year or the normal operating cycle, if longer.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(a))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16)(a)(1))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

+ Details

Name:	us-gaap_ShortTermBorrowings
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_ShortTermDebtLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=us-gaap_ShortTermDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=sigi_ZeroPointFiftyEightPercentBorrowingfromFHLBIDueDecember142020.Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=sigi_ZeroPointThirtySixPercentBorrowingFromFHLBNYDueNovember162020Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=sigi_SelectiveInsuranceGroupMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=sigi_SelectiveInsuranceCompanyOfSouthCarolinaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=sigi_SelectiveInsuranceCompanyOfSoutheastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=sigi_SelectiveInsuranceCompanyOfAmericaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=sigi_ZeroPointSixtyEightPercentBorrowingfromFHLBNYDueSeptember182020.Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=sigi_ZeroPointSeventyEightPercentBorrowingfromFHLBNYDueSeptember142020.Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=sigi_OnePointEightyOnePercentBorrowingfromFHLBNYDueMarch182020Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Indebtedness (Short Term Debt Activity) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Short-term Debt [Line Items]
Short-term Debt	$ 167,000		$ 0
Short-term Debt [Member]
Short-term Debt [Line Items]
Short-term Debt	167,000	$ 302,000	0
Zero Point Fifty Eight Percent Borrowing from FHLBI Due December 14, 2020. [Member]
Short-term Debt [Line Items]
Short-term Debt	67,000		0
Zero Point Thirty Six Percent Borrowing from FHLBNY Due November 16, 2020 [Member]
Short-term Debt [Line Items]
Short-term Debt	$ 100,000		$ 0
Federal Home Loan Bank, Advances, Interest Rate	0.36%
Selective Insurance Group [Member]
Short-term Debt [Line Items]
Line of Credit	$ 50,000
Line of Credit Interest Rate	2.244%
Selective Insurance Company of South Carolina [Member] \| Zero Point Fifty Eight Percent Borrowing from FHLBI Due December 14, 2020. [Member]
Short-term Debt [Line Items]
Federal Home Loan Bank Advances	$ 39,000
Federal Home Loan Bank, Advances, Interest Rate	0.58%
Selective Insurance Company of the Southeast [Member] \| Zero Point Fifty Eight Percent Borrowing from FHLBI Due December 14, 2020. [Member]
Short-term Debt [Line Items]
Federal Home Loan Bank Advances	$ 28,000
Federal Home Loan Bank, Advances, Interest Rate	0.58%
Selective Insurance Company of America [Member] \| Zero Point Sixty Eight Percent Borrowing from FHLBNY Due September 18, 2020. [Member]
Short-term Debt [Line Items]
Federal Home Loan Bank Advances	$ 85,000
Federal Home Loan Bank, Advances, Interest Rate	0.68%
Selective Insurance Company of America [Member] \| Zero Point Thirty Six Percent Borrowing from FHLBNY Due November 16, 2020. [Member]
Short-term Debt [Line Items]
Federal Home Loan Bank Advances	$ 100,000
Federal Home Loan Bank, Advances, Interest Rate	0.78%
Selective Insurance Company of America [Member] \| One Point Eighty One Percent Borrowing from FHLBNY Due March 18, 2020 [Member]
Short-term Debt [Line Items]
Federal Home Loan Bank, Advances, Interest Rate	1.81%