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Indebtedness - Schedule of Line of Credit Covenant Requirements (Details) $ in Billions	Sep. 30, 2025 USD ($)
Debt Disclosure [Abstract]
Consolidated net worth, required	$ 2.4
Debt to total capitalization ratio, required (as a percent)	35.00%
Consolidated net worth, actual	$ 3.7
Debt to total capitalization ratio, actual (as a percent)	19.80%

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- Definition

This element represents the consolidated net worth of the entity that is calculated based on the line of credit requirements.

+ References

No definition available.

+ Details

Name:	sigi_ConsolidatedNetWorthActual
Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

This element represents the debt-to-capitalization ratio which is calculated by taking the entity's debt and dividing by the sum of the entity's debt and stockholders' equity as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	sigi_DebtToCapitalizationRatio
Namespace Prefix:	sigi_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

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- Definition

The covenant amount required for the line of credit for consolidated net worth as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	sigi_LineOfCreditConsolidatedNetWorthRequirement
Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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- Definition

The maximum covenant percentage required for the line of credit for the debt-to-capitalization ratio as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	sigi_LineOfCreditDebtToCapitalizationRatioRequirement
Namespace Prefix:	sigi_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

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- References

No definition available.

+ Details

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