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Reserve for Losses and Loss Expenses (Reserve Loss Development) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
Favorable or adverse loss development | $ (25,525) | $ (25,601) | $ (38,746)
Favorable or unfavorable loss development [Member] |  |  |
Favorable or adverse loss development | 25,500 | 25,500 | 38,500
Property Development | 11,000 | 7,500 | 9,000
Casualty Development | 14,500 | 18,000 | 29,500
Favorable or unfavorable loss development [Member] | General Liability [Member] |  |  |
Favorable or adverse loss development | 20,000 | (2,500) | 11,500
Favorable or unfavorable loss development [Member] | Commercial Automobile [Member] |  |  |
Favorable or adverse loss development | 4,500 | 8,500 | 13,000
Favorable or unfavorable loss development [Member] | Workers' Compensation [Member] |  |  |
Favorable or adverse loss development | (23,500) | (2,500) | (6,500)
Favorable or unfavorable loss development [Member] | Business Owners' Policies [Member] |  |  |
Favorable or adverse loss development | 9,500 | 9,000 | 11,000
Favorable or unfavorable loss development [Member] | Commercial Property [Member] |  |  |
Favorable or adverse loss development | 7,500 | 3,500 | 5,500
Favorable or unfavorable loss development [Member] | Homeowners [Member] |  |  |
Favorable or adverse loss development | 2,500 | 9,000 | 4,500
Favorable or unfavorable loss development [Member] | Personal Automobile [Member] |  |  |
Favorable or adverse loss development | 3,000 | (500) | (1,000)
Favorable or unfavorable loss development [Member] | Excess And Surplus Operations [Member] |  |  |
Favorable or adverse loss development | 2,000 | 0 | 0
Favorable or unfavorable loss development [Member] | Remaining Lines Of Business [Member] |  |  |
Favorable or adverse loss development | $ 0 | $ 1,000 | $ 500

X
- Definition Amount of provision in the period for casualty claims incurred in prior reporting periods and related claims settlement costs. + References No definition available. + Details
Name: | sigi_CasualtyDevelopment
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- 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

X
- Definition Amount of provision in the period for claims incurred in prior reporting periods and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Financial Reporting Release (FRR) -Number 403 -Paragraph 04 -Subparagraph b Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
Name: | us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration