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Investments (Aggregated Balance Sheet Summarized Financial Information for Partnerhips in our Alternative Investment Portfolio) (Details) (Details) (Investments Accounted For Under The Equity Method [Member], USD $) In Millions, unless otherwise specified | Sep. 30, 2014 | Sep. 30, 2013
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Investments Accounted For Under The Equity Method [Member] |  |
Investments | $ 10,096 sigi_EquityMethodInvestmentsSummarizedFinancialInformationInvestments / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember | $ 11,020 sigi_EquityMethodInvestmentsSummarizedFinancialInformationInvestments / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember
Total assets | 10,695 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationAssets / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember | 11,727 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationAssets / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember
Total liabilities | 545 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationLiabilities / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember | 573 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationLiabilities / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember
Partners' capital | $ 10,150 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationEquity / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember | $ 11,154 us-gaap_EquityMethodInvestmentSummarizedFinancialInformationEquity / us-gaap_InvestmentTypeAxis = sigi_InvestmentsAccountedForUnderTheEquityMethodMember

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- Definition The amount of assets reported by an equity method investment of the entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(g)) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33912-111571 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 28 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=6957238&loc=d3e14064-108612 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph bb -Article 1 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph g -Subparagraph 1, 2 -Article 4 Reference 7: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph w -Article 1 + Details
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- Definition Amount of equity, including noncontrolling interest, reported by an equity method investment of the entity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=6957238&loc=d3e14064-108612 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 323 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6382943&loc=d3e33918-111571 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 28 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=6957238&loc=d3e14064-108612 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.g) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690 + Details
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