XML 135 R105.htm IDEA: XBRL DOCUMENT v3.22.0.1

Retirement Plans (Quantitative Disclosures of Retirement Income Plan's Invested Assets that are Measured at Fair Value on a Recurring Basis) (Details) - Retirement Income Plan [Member] - USD ($) $ in Thousands		Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Total Assets		$ 450,305	$ 432,716	$ 385,087
Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		456,502	435,761
Total Equities [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		300,389	277,667
Return Seeking Assets [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		300,849	278,169
Liability Hedging Assets [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		151,487	152,246
Cash And Short Term Investments [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		4,166	5,346
Alternative Investments [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	[1]	460	502
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		453,620	433,186
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Total Equities [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		300,389	277,667
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Return Seeking Assets [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		300,389	277,667
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Liability Hedging Assets [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		151,487	152,246
Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Cash And Short Term Investments [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		1,744	3,273
Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		2,422	2,073
Significant Other Observable Inputs (Level 2) [Member] | Total Equities [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		0	0
Significant Other Observable Inputs (Level 2) [Member] | Return Seeking Assets [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		0	0
Significant Other Observable Inputs (Level 2) [Member] | Liability Hedging Assets [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		0	0
Significant Other Observable Inputs (Level 2) [Member] | Cash And Short Term Investments [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		2,422	2,073
Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		0	0
Significant Unobservable Inputs (Level 3) [Member] | Total Equities [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		0	0
Significant Unobservable Inputs (Level 3) [Member] | Return Seeking Assets [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		0	0
Significant Unobservable Inputs (Level 3) [Member] | Liability Hedging Assets [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		0	0
Significant Unobservable Inputs (Level 3) [Member] | Cash And Short Term Investments [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		0	0
Global Equity [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		144,634	142,320
Global Equity [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		144,634	142,320
Global Equity [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		0	0
Global Equity [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		0	0
Diversified Credit [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		66,165	73,762
Diversified Credit [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		66,165	73,762
Diversified Credit [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		0	0
Diversified Credit [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		0	0
Real Assets Equity [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		89,590	61,585
Real Assets Equity [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		89,590	61,585
Real Assets Equity [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		0	0
Real Assets Equity [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		0	0
Real Assets [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	[1]	47	73
Real Assets [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	[1]	0	0
Real Assets [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	[1]	0	0
Real Assets [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	[1]	0	0
Private Equity Funds [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	[1]	413	400
Private Equity Funds [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	[1]	0	0
Private Equity Funds [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	[1]	0	0
Private Equity Funds [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	[1]	0	0
Private Credit Funds [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	[1]		29
Private Credit Funds [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	[1]		0
Private Credit Funds [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	[1]		0
Private Credit Funds [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	[1]		0
Alternative Investments [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	[1]	0	0
Alternative Investments [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	[1]	0	0
Alternative Investments [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets	[1]	0	0
Fixed Income Securities [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		86,183	99,490
Fixed Income Securities [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		86,183	99,490
Fixed Income Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		0	0
Fixed Income Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		0	0
US Treasury Securities [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		65,304	52,756
US Treasury Securities [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		65,304	52,756
US Treasury Securities [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		0	0
US Treasury Securities [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		0	0
Short-term Investments [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		1,744	3,273
Short-term Investments [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		1,744	3,273
Short-term Investments [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		0	0
Short-term Investments [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		0	0
Deposit Administration Contracts [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		2,422	2,073
Deposit Administration Contracts [Member] | Quoted Prices in Active Markets for Identical Assets/Liabilities (Level 1) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		0	0
Deposit Administration Contracts [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		2,422	2,073
Deposit Administration Contracts [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fair value, recurring [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets		$ 0	$ 0
[1] In accordance with the FASB issued ASU 2015-07, Disclosures for Investments in Certain Entities That Calculate Net Asset Value per Share (or Its Equivalent), certain investments that are measured at fair value using the net asset value per share (or its practical expedient) have not been classified in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to total Pension Plan invested assets.

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

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

X

- Definition

Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(iv)(01)
-URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(iv)(01)
-URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(ii)
-URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920

Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 55
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=123450688&loc=d3e4179-114921

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(iv)(02)
-URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920

Reference 7: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(ii)
-URI http://asc.fasb.org/extlink&oid=123447040&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsRecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_InvestmentTypeAxis=sigi_TotalEquitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_InvestmentTypeAxis=sigi_ReturnSeekingAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_InvestmentTypeAxis=sigi_LiabilityHedgingAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_InvestmentTypeAxis=sigi_CashAndShortTermInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_InvestmentTypeAxis=srt_PartnershipInterestMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel2Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=sigi_GlobalEquityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=sigi_DiversifiedCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=sigi_RealAssetsEquityMemberMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=sigi_RealAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_PrivateEquityFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=sigi_PrivateCreditFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=srt_PartnershipInterestMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_FixedIncomeSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=sigi_USTreasuryFundMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=us-gaap_ShortTermInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DefinedBenefitPlanByPlanAssetCategoriesAxis=sigi_DepositAdministrationContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: