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Subsequent Events (Details) - Subsequent Event [Member] $ in Millions	1 Months Ended
	Jan. 31, 2018 USD ($)
Subsequent Event [Line Items]
Non Catastrophe Property Losses	$ 47
Loss from Catastrophes	16
Total Property Losses	63
Total Property Loss Estimate	$ 30

X

- Definition

Property losses resulting from events other than catastrophes.

+ References

No definition available.

+ Details

Name:	sigi_NonCatastrophePropertyLosses
Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Estimate of total property losses for the period.

+ References

No definition available.

+ Details

Name:	sigi_TotalPropertyLossEstimate
Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

The total catastrophe and non catastrophe property losses occurring in the period.

+ References

No definition available.

+ Details

Name:	sigi_TotalPropertyLosses
Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

The charge against earnings in the period for the uninsured portion of a loss from a fire, explosion, or natural disaster (hurricane, earthquake).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 225
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.9)
-URI http://asc.fasb.org/extlink&oid=63488584&loc=d3e20235-122688

+ Details

Name:	us-gaap_LossFromCatastrophes
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

+ References

No definition available.

+ Details

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

X

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: