XML 93 R63.htm IDEA: XBRL DOCUMENT
v3.22.0.1

Investments (Aggregated Balance Sheet Summarized Financial Information for Partnerships in our Alternative Investment Portfolio) (Details) - USD ($) $ in Millions | Sep. 30, 2021 | Sep. 30, 2020
--- | --- | ---
Equity Method Investment, Nonconsolidated Investee or Group of Investees |  |
Assets | $ 112,232 | $ 58,819
Equity Method Investment, Nonconsolidated Investee, Other |  |
Partner's capital | 99,861 | 52,075
Investments Accounted For Under The Equity Method [Member] |  |
Investments | 107,347 | 55,145
Liabilities | $ 12,371 | $ 6,744

X
- Definition The amount of assets reported by an equity method investment of the entity. + References No definition available. + Details
Name: | sigi_EquityMethodInvestmentSummarizedFinancialInformationAssetsSIGI
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of equity, including noncontrolling interest, reported by an equity method investment of the entity. + References No definition available. + Details
Name: | sigi_EquityMethodInvestmentSummarizedFinancialInformationEquitySIGI
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The amount of liabilities reported by an equity method investment of the entity. + References No definition available. + Details
Name: | sigi_EquityMethodInvestmentSummarizedFinancialInformationLiabilitiesSIGI
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition The amount of investments reported by an equity method investment of the entity. + References No definition available. + Details
Name: | sigi_EquityMethodInvestmentsSummarizedFinancialInformationInvestments
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeAxis=us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeOrGroupOfInvesteesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeAxis=us-gaap_EquityMethodInvestmentNonconsolidatedInvesteeOtherMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_InvestmentTypeAxis=sigi_InvestmentsAccountedForUnderTheEquityMethodMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |