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Indebtedness (Covenants in the Line of Credit) (Details) $ in Thousands	Dec. 31, 2019 USD ($)	[1]
Debt Disclosure [Abstract]
Consolidated Net Worth Requirement	$ 1,400,000
Consolidated Net Worth, Actual	$ 2,100,000
Debt-to-capitalization ratio, required	35.00%
Debt-to-capitalization ratio, actual	20.70%
[1] Calculated in accordance with Line of Credit agreement.

X

- 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:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- 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

X

- 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:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- References

No definition available.

+ Details

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