XML 124 R111.htm IDEA: XBRL DOCUMENT v3.8.0.1

Share-Based Payments (Weighted-Average Fair Value of Options and Stock Per Share) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restricted Stock Units (RSUs) [Member]
Stock, weighted average grant date fair value	$ 42.66	$ 32.53	$ 25.22
Employee Stock Purchase Plan [Member]
Options, weighted average grant date fair value	2.73	2.63	1.26
15% of grant date market value	7.06	5.23	4.16
Total ESPP	9.79	7.86	5.42
Agent Stock Purchase Plan [Member]
Stock, weighted average grant date fair value	$ 5.04	$ 3.79	$ 2.94

X

- Definition

The weighted average fair value discount at grant date, for employee stock purchase plan shares issued during the period.

+ References

No definition available.

+ Details

Name:	sigi_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsEmployeeStockPurchasePlanGrantsInPeriodWeightedAverageGrantDateFairValue
Namespace Prefix:	sigi_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The total weighted average fair value at grant date for employee stock purchase plan shares issued during the period.

+ References

No definition available.

+ Details

Name:	sigi_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsEmployeeStockPurchasePlanGrantsInPeriodWeightedAverageGrantDateFairValueTotal
Namespace Prefix:	sigi_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(iii)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)(1)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_RestrictedStockUnitsRSUMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_EmployeeStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_AwardTypeAxis=sigi_AgentStockPurchasePlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: