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Reinsurance - NFIP (Details) $ in Millions	12 Months Ended
	Dec. 31, 2024 USD ($)
Effects of Reinsurance [Line Items]
Percentage of NFIP	100.00%
Total rated reinsurers
Effects of Reinsurance [Line Items]
Change in reinsurance recoverable gross	$ 164.3
Federal and state pools
Effects of Reinsurance [Line Items]
Change in reinsurance recoverable gross	$ 238.3

X

- Definition

The net change in the amount before valuation allowance recoverable under reinsurance contracts.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Percentage of unpaid NFIP flood premium, losses and loss expense claims ceded by federal government.

+ References

No definition available.

+ Details

Name:	sigi_PercentageOfUnpaidNFIPClaimsCededByFederalGovernment
Namespace Prefix:	sigi_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	duration

X

- 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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X

- Details

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Namespace Prefix:
Data Type:	na
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Period Type:

X

- Details

Name:	sigi_NonRatedReinsurerAxis=sigi_TotalfederalandstatepoolsMember
Namespace Prefix:
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Period Type: