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Reserve for Losses and Loss Expenses (Reserve Loss Development) (Details) (USD $) | 12 Months Ended
--- | ---
Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Favorable or adverse loss development | $ (25,601,000) | $ (38,746,000) | $ (43,589,000)
Favorable or unfavorable loss development [Member] |  |  |
Favorable or adverse loss development | 26,000,000 | 39,000,000 | 44,000,000
Property Development | 8,000,000 | 9,000,000 | 5,000,000
Casualty Development | 18,000,000 | 30,000,000 | 39,000,000
Favorable or unfavorable loss development [Member] | General Liability [Member] |  |  |
Favorable or adverse loss development | (3,000,000) | 12,000,000 | 26,000,000
Favorable or unfavorable loss development [Member] | Commercial Automobile [Member] |  |  |
Favorable or adverse loss development | 9,000,000 | 13,000,000 | 28,000,000
Favorable or unfavorable loss development [Member] | Workers' Compensation [Member] |  |  |
Favorable or adverse loss development | (2,000,000) | (7,000,000) | (22,000,000)
Favorable or unfavorable loss development [Member] | Business Owners' Policies [Member] |  |  |
Favorable or adverse loss development | 9,000,000 | 11,000,000 | 3,000,000
Favorable or unfavorable loss development [Member] | Commercial Property [Member] |  |  |
Favorable or adverse loss development | 3,000,000 | 6,000,000 | 3,000,000
Favorable or unfavorable loss development [Member] | Homeowners [Member] |  |  |
Favorable or adverse loss development | 9,000,000 | 4,000,000 | 6,000,000
Favorable or unfavorable loss development [Member] | Personal Automobile [Member] |  |  |
Favorable or adverse loss development | 0 | (1,000,000) | (2,000,000)
Favorable or unfavorable loss development [Member] | Remaining Lines Of Business [Member] |  |  |
Favorable or adverse loss development | $ 1,000,000 | $ 1,000,000 | $ 2,000,000

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

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- 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 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Statement of Position (SOP) -Number 94-5 -Paragraph 11 -Subparagraph b -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
Name: | us-gaap_LiabilityForUnpaidClaimsAndClaimsAdjustmentExpenseIncurredClaimsPriorYears
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration