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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