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

This represents the drivers of the decrease in reinsurance recoverable balance.

No definition available.

Name:	sigi_DecreaseofReinsuranceRecoverables
Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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 the collateral related to reinsurance recoverables, as of the balance sheet date.

No definition available.

Name:	sigi_NetUnsecuredReinsuranceBalances
Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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:	num:percentItemType
Balance Type:	na
Period Type:	instant

The known and estimated amount recoverable as of the balance sheet date from reinsurers for claims paid or incurred by the ceding insurer and associated claims settlement expenses, including estimated amounts for claims incurred but not reported, and policy benefits, net of any related valuation allowance.

No definition available.

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

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Prepaid Reinsurance Premiums

-URI http://asc.fasb.org/extlink&oid=6521719

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 340

-Section 25

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6485982&loc=d3e22818-158507

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

Total of allowances and reserves, the valuation and qualifying accounts that are either netted against the cost of an asset (in order to value it at its carrying value) or that reflect a liability established to represent expected future costs.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 09

-Article 12