Investments (Investments Pledged as Collateral) (Details) $ in Millions	Dec. 31, 2022 USD ($)
U.S. Government and Government Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Securities pledged as collateral on deposit with various state and regulatory agencies	$ 19.3
Securities owned and pledged as collateral, fair value	19.3
Obligations of States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Securities pledged as collateral on deposit with various state and regulatory agencies	3.6
Securities owned and pledged as collateral, fair value	3.6
Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities pledged as collateral on deposit with various state and regulatory agencies	0.0
Securities owned and pledged as collateral, fair value	90.2
Commercial Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities pledged as collateral on deposit with various state and regulatory agencies	0.0
Securities owned and pledged as collateral, fair value	14.2
Available-for-sale Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities pledged as collateral on deposit with various state and regulatory agencies	22.9
Securities owned and pledged as collateral, fair value	127.3
Federal Home Loan Bank of New York [Member] \| U.S. Government and Government Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Securities pledged as collateral with FHLB	0.0
Federal Home Loan Bank of New York [Member] \| Obligations of States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Securities pledged as collateral with FHLB	0.0
Federal Home Loan Bank of New York [Member] \| Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities pledged as collateral with FHLB	28.8
Federal Home Loan Bank of New York [Member] \| Commercial Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities pledged as collateral with FHLB	9.6
Federal Home Loan Bank of New York [Member] \| Available-for-sale Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities pledged as collateral with FHLB	38.4
Federal Home Loan Bank of Indianapolis [Member] \| U.S. Government and Government Agencies [Member]
Debt Securities, Available-for-sale [Line Items]
Securities pledged as collateral with FHLB	0.0
Federal Home Loan Bank of Indianapolis [Member] \| Obligations of States and Political Subdivisions [Member]
Debt Securities, Available-for-sale [Line Items]
Securities pledged as collateral with FHLB	0.0
Federal Home Loan Bank of Indianapolis [Member] \| Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities pledged as collateral with FHLB	61.4
Federal Home Loan Bank of Indianapolis [Member] \| Commercial Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities pledged as collateral with FHLB	4.6
Federal Home Loan Bank of Indianapolis [Member] \| Available-for-sale Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities pledged as collateral with FHLB	$ 66.0

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.

No definition available.

+ Details

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Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 825
-Section 50
-Paragraph 1A
-URI https://asc.fasb.org/extlink&oid=123600598&loc=SL75763979-209797

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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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (d)
-URI https://asc.fasb.org/extlink&oid=123599511&loc=d3e64711-112823

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 45
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=6479118&loc=d3e64650-112822

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- 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

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- Details

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- Details

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- Details

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