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

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

No definition available.

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

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, net of allowance for Credit Loss

No definition available.

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

No definition available.

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

The annual cap limiting the amount of aggregate subject losses for participating insurers under the Terrorism Risk Insurance Program.

No definition available.

Name:	sigi_TerrorismRiskInsuranceProgramAnnualCap
Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

The Company's estimated deductible for the year following the date of this report under the Terrorism Risk Insurance Program.

No definition available.

Name:	sigi_TerrorismRiskInsuranceProgramCompanysEstimatedDeductible
Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

The percentage that the insured has to pay for losses under the Terrorism Risk Insurance Program, beginning the next calendar year.

No definition available.

Name:	sigi_TerrorismRiskInsuranceProgramInsurerRetainer
Namespace Prefix:	sigi_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

The percentage of subject losses, after an insurer deductible, subject to an annual cap, reimbursed by the Federal Government under the Terrorism Risk Insurance Program, beginning the next calendar year.

No definition available.

Name:	sigi_TerrorismRiskInsuranceProgramReimbursedPercentageMaximum
Namespace Prefix:	sigi_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 340
-Section 25
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=6485982&loc=d3e22818-158507

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 825
-Section 50
-Paragraph 1B
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=123600520&loc=SL75241803-196195

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

Amount of allowance for credit loss on reinsurance recoverable.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (f)
-URI https://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=124255206&loc=SL82895884-210446

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

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.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 310
-Section 45
-Paragraph 6
-URI https://asc.fasb.org/extlink&oid=99404803&loc=d3e21412-158489

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 310
-Section 45
-Paragraph 5
-URI https://asc.fasb.org/extlink&oid=99404803&loc=d3e21409-158489

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 20
-Section 50
-Paragraph 5
-URI https://asc.fasb.org/extlink&oid=99397103&loc=d3e6824-158387

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 825
-Section 50
-Paragraph 1B
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=123600520&loc=SL75241803-196195

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

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

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

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

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

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

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

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

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

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

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

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