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Reinsurance - Narrative (Details) $ in Millions	9 Months Ended
	Sep. 30, 2025 USD ($)
Effects of Reinsurance [Line Items]
Percentage of NFIP	100.00%
Federal and state pools
Effects of Reinsurance [Line Items]
Change in reinsurance recoverable gross	$ (216.9)

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- Definition

Change In Reinsurance Recoverable, Gross

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No definition available.

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- Definition

Percentage Of Unpaid NFIP Claims Ceded By Federal Government

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No definition available.

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Data Type:	dtr-types:percentItemType
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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.

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No definition available.

+ Details

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Balance Type:	na
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- Details

Name:	sigi_NonRatedReinsurerAxis=sigi_TotalfederalandstatepoolsMember
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Data Type:	na
Balance Type:
Period Type: