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Reinsurance (Total Reinsurance Balances Segregated By Reinsurer) (Details) - USD ($)
$ in Thousands

Dec. 31, 2023

Dec. 31, 2022

Dec. 31, 2021

Reinsurance recoverable, net of allowance for credit losses

$ 656,825

$ 782,810

Total prepaid reinsurance premiums

203,320

172,371

Total reinsurance balance

860,145

955,181

Less: collateral

^[1]

(126,418)

(126,167)

Reinsurance Recoverable, Allowance for Credit Loss

(1,700)

(1,600)

$ (1,600)

National Flood Insurance Program [Member]

Total reinsurance balance

^[2]

$ 203,273

$ 276,541

% of Net Reinsurance Balance

^[2]

24.00%

29.00%

NJ Unsatisfied Claim Judgment Fund [Member]

Total reinsurance balance

^[2]

$ 46,715

$ 45,496

% of Net Reinsurance Balance

^[2]

5.00%

5.00%

Other Federal And State Pools [Member]

Total reinsurance balance

^[2]

$ 1,779

$ 3,488

% of Net Reinsurance Balance

^[2]

0.00%

0.00%

Total Federal And State Pools [Member]

Total reinsurance balance

^[2]

$ 251,767

$ 325,525

% of Net Reinsurance Balance

^[2]

29.00%

34.00%

Remaining Unsecured Reinsurance [Member]

Total reinsurance balance

$ 610,078

$ 631,256

% of Net Reinsurance Balance

71.00%

66.00%

Remaining Reinsurance, before Allowance for Credit Loss

$ 608,378

$ 629,656

Reinsurance Recoverables, net of allowance for credit losses

608,378

629,656

Munich Re Group [Member]

Total reinsurance balance

$ 135,564

$ 124,706

% of Net Reinsurance Balance

16.00%

13.00%

Hanover Ruckversicherungs AG [Member]

Total reinsurance balance

$ 132,831

$ 127,106

% of Net Reinsurance Balance

15.00%

13.00%

AXIS Reinsurance Company [Member]

Total reinsurance balance

$ 76,286

$ 70,957

% of Net Reinsurance Balance

9.00%

8.00%

Swiss Re Group [Member]

Total reinsurance balance

$ 30,222

$ 36,525

% of Net Reinsurance Balance

4.00%

4.00%

Transatlantic Reinsurance Company [Member]

Total reinsurance balance

$ 27,475

$ 32,730

% of Net Reinsurance Balance

3.00%

3.00%

All Other Reinsurers [Member]

Total reinsurance balance

$ 207,700

$ 239,232

% of Net Reinsurance Balance

24.00%

25.00%

Remaining Unsecured Reinsurance, Net of Collateral [Member]

Total reinsurance balance

$ 481,960

$ 503,489

[1]	Includes letters of credit, trust funds, and funds held against reinsurance recoverables
[2]	Considered to have minimal risk of default.

X

- Definition

This element represents the amount collateralized by the entity's reinsurers. The amount includes letters of credit, trust funds, and funds withheld.

+ References

No definition available.

+ Details

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

X

- Definition

This element represents the percentage of the total net unsecured reinsurance balance.

+ References

No definition available.

+ Details

Name:	sigi_PercentageOfNetUnsecuredReinsurance
Namespace Prefix:	sigi_
Data Type:	dtr-types1:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

This element represents the known and estimated amount of reinsurance recoverables and the unexpired portion of premiums ceded on policies in force, less all federal and state pools, as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	sigi_RemainingReinsuranceBalances
Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

This element represents the known and estimated amount of reinsurance recoverables and the unexpired portion of premiums ceded on policies in force, less all federal and state pools, as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	sigi_RemainingReinsuranceBeforeAllowanceForCreditLoss
Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

The unexpired portion of premiums ceded on policies in force as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 825
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479383/944-825-50-1B

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
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-Section 25
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479928/944-340-25-1

+ Details

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

X

- Definition

Amount, after valuation allowance, recoverable under reinsurance contracts including premium paid under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479838/944-20-50-5

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-Name Accounting Standards Codification
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-Subparagraph (SX 210.7-03(a)(6))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479440/944-210-S99-1

+ Details

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

X

- Definition

Amount of allowance for credit loss on reinsurance recoverable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479838/944-20-50-5

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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-URI https://asc.fasb.org//1943274/2147479616/944-310-45-5

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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-URI https://asc.fasb.org//1943274/2147479319/326-20-50-13

+ Details

Name:	us-gaap_ReinsuranceRecoverablesAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Amount, after valuation allowance, recoverable under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, policy benefits and policy reserves. Excludes premiums paid under reinsurance contracts.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 310
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479584/944-310-50-2

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
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-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479838/944-20-50-5

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-Topic 944
-SubTopic 310
-Name Accounting Standards Codification
-Section 45
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479616/944-310-45-5

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
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-Section 45
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479616/944-310-45-6

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-Topic 944
-SubTopic 825
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-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479383/944-825-50-1B

+ Details

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

X

- Details

Name:	us-gaap_CededCreditRiskAxis=sigi_NationalFloodInsuranceProgramMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_CededCreditRiskAxis=sigi_NjUnsatisfiedClaimJudgmentFundMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_CededCreditRiskAxis=sigi_OtherFederalAndStatePoolsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_CededCreditRiskAxis=sigi_TotalFederalAndStatePoolsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_CededCreditRiskAxis=sigi_RemainingUnsecuredReinsuranceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_CededCreditRiskAxis=sigi_MunichReGroupMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_CededCreditRiskAxis=sigi_HanoverRuckversicherungsAgMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_CededCreditRiskAxis=sigi_AxisReinsuranceCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_CededCreditRiskAxis=sigi_SwissReGroupMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_CededCreditRiskAxis=sigi_TransatlanticReinsuranceCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_CededCreditRiskAxis=sigi_AllOtherReinsurersMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_CededCreditRiskAxis=sigi_RemainingUnsecuredReinsuranceNetofCollateralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: