The amount of net income (loss) reported by an equity method investment of the entity.

No definition available.

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- Definition

This item represents the disclosure of summarized financial information, specifically the aggregate change in unrealized appreciation (depreciation), for unconsolidated subsidiaries and 50 percent-or-less owned entities accounted for under the equity method of accounting.

+ References

No definition available.

+ Details

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- Definition

This item represents the disclosure of summarized financial information, specifically aggregate net investment income, for unconsolidated subsidiaries and 50 percent-or-less owned entities accounted for under the equity method accounting.

+ References

No definition available.

+ Details

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- Definition

This item represents the disclosure of summarized financial information, specifically aggregate realized gains (losses), for unconsolidated subsidiaries and 50 percent-or-less owned entities accounted for under the equity method of accounting.

+ References

No definition available.

+ Details

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Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
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- Definition

Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(2))
-URI http://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263

+ Details

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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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Investments (Aggregated Income Statement Summarized Financial Information for Partnerships in our Alternative Investment Portfolio) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Schedule of Equity Method Investments [Line Items]
Net investment income earned	$ 326,589		$ 227,107		$ 222,543
Investments Accounted For Under The Equity Method [Member]
Schedule of Equity Method Investments [Line Items]
Net investment (loss) income		$ 653,000		$ (26,000)		$ (8,000)
Realized gains		6,121,000		1,452,000		695,000
Net change in unrealized appreciation		26,877,000		4,898,000		5,543,000
Net Income before tax		33,651,000		6,324,000		6,230,000
Alternative Investments [Member]
Schedule of Equity Method Investments [Line Items]
Net investment income earned		$ 117,700		$ 26,500		$ 17,900