Reserve for Losses and Loss Expenses (Reserve Loss Development) (Details) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	$ 592,062,000	$ 572,434,000	$ 632,742,000
Favorable or adverse loss development	(59,269,000)	(25,525,000)	(25,601,000)
Property Development		11,000,000sigi_PropertyDevelopment
Favorable or unfavorable loss development [Member]
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	59,300,000	25,500,000	25,500,000
Favorable or adverse loss development	(59,300,000)	(25,500,000)	(25,500,000)
Property Development	11,100,000		7,500,000
Casualty Development	48,200,000	14,500,000	18,000,000
Favorable or unfavorable loss development [Member] \| General Liability [Member]
Favorable or adverse loss development	(43,900,000)	(20,000,000)	2,500,000
Favorable or unfavorable loss development [Member] \| Commercial Automobile [Member]
Favorable or adverse loss development	(4,100,000)	(4,500,000)	(8,500,000)
Favorable or unfavorable loss development [Member] \| Workers' Compensation [Member]
Favorable or adverse loss development	0	23,500,000	2,500,000
Favorable or unfavorable loss development [Member] \| Business Owners' Policies [Member]
Favorable or adverse loss development	1,900,000	(9,500,000)	(9,000,000)
Favorable or unfavorable loss development [Member] \| Commercial Property [Member]
Favorable or adverse loss development	(2,100,000)	(7,500,000)	(3,500,000)
Favorable or unfavorable loss development [Member] \| Homeowners [Member]
Favorable or adverse loss development	(4,000,000)	(2,500,000)	(9,000,000)
Favorable or unfavorable loss development [Member] \| Personal Automobile [Member]
Favorable or adverse loss development	(10,800,000)	(3,000,000)	500,000
Favorable or unfavorable loss development [Member] \| Excess And Surplus Operations [Member]
Favorable or adverse loss development	3,700,000	(2,000,000)	0
Favorable or unfavorable loss development [Member] \| Remaining Lines Of Business [Member]
Favorable or adverse loss development	$ 0	$ 0	$ (1,000,000)

Amount of provision in the period for casualty claims incurred in prior reporting periods and related claims settlement costs.

No definition available.

+ Details

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

- Definition

Amount of provision in the period for property claims incurred in prior reporting periods and related claims settlement.

+ References

No definition available.

+ Details

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

- Definition

Amount of payments to settle claims incurred in prior periods and related claims settlement costs.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 40

-Section 50

-Paragraph 3

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437

+ Details

Name:	us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseClaimsPaidPriorYears1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of expense for claims incurred in prior reporting periods and related claims settlement costs.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 235

-Section S99

-Paragraph 3

-Subparagraph (SX 210.12-18)

-URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e575016-122915

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 18

-Article 12

+ Details

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

- Details

Name:	us-gaap_LiabilityForCatastropheClaimsByCatastrophicEventAxis=sigi_ChangeinHistoricalClaimRateExperienceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: