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Investments (Other Investment Portfolio by Strategy and the Remaining Commitment Amount Associated With Each Strategy) (Details) (USD $) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2012 | Mar. 31, 2011 | Dec. 31, 2011
Carrying Value | $ 125,140,000 |  | $ 128,301,000
Decrease in other investments | (3,200,000) |  |
Net investment income earned | 32,628,000 | 43,473,000 |
Other Investments [Member] |  |  |
Carrying Value | 125,140,000 |  | 128,301,000
Remaining Commitment | 54,270,000 |  |
Distributions from alternative investments | 8,100,000 |  |
Net investment income earned | 2,000,000 |  |
Additional contributions | 3,000,000 |  |
Alternative Investments [Member] |  |  |
Carrying Value | 121,485,000 |  | 124,548,000
Remaining Commitment | 53,035,000 |  |
Net investment income earned | 2,000,000 | 11,600,000 |
Secondary Private Equity Member | Alternative Investments [Member] |  |  |
Carrying Value | 29,761,000 |  | 30,114,000
Remaining Commitment | 8,590,000 |  |
Energy/Power Generation [Member] | Alternative Investments [Member] |  |  |
Carrying Value | 24,149,000 |  | 25,913,000
Remaining Commitment | 10,420,000 |  |
Private Equity [Member] | Alternative Investments [Member] |  |  |
Carrying Value | 23,175,000 |  | 21,736,000
Remaining Commitment | 4,389,000 |  |
Distressed Debt [Member] | Alternative Investments [Member] |  |  |
Carrying Value | 15,081,000 |  | 16,953,000
Remaining Commitment | 3,074,000 |  |
Real Estate Strategy [Member] | Alternative Investments [Member] |  |  |
Carrying Value | 12,825,000 |  | 13,767,000
Remaining Commitment | 10,506,000 |  |
Mezzanine Financing [Member] | Alternative Investments [Member] |  |  |
Carrying Value | 9,167,000 |  | 8,817,000
Remaining Commitment | 15,256,000 |  |
Venture Capital [Member] | Alternative Investments [Member] |  |  |
Carrying Value | 7,327,000 |  | 7,248,000
Remaining Commitment | 800,000 |  |
Other Securities [Member] |  |  |
Carrying Value | 3,655,000 |  | 3,753,000
Remaining Commitment | $ 1,235,000 |  |

X
- Definition This item represents disclosure of the amount of distributions received from alternative investments. These investments are accounted for under the equity method of accounting. + References No definition available. + Details
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Balance Type: | credit
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X
- Definition This item represents the amount of funding that the company has provided on commitments in our other investment portfolio for the period. + References No definition available. + Details
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Balance Type: | debit
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X
- Definition This item represents the change in the carrying value of other investments. + References No definition available. + Details
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Balance Type: | debit
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X
- Definition The income earned from investments in securities and other investments including dividends, interest, and returns on other investments. This item does not include realized gains (losses). + References No definition available. + Details
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X
- Definition This item represents contractual obligations to invest additional amounts on our other investment portfolio. These additional investments may or may not be required. + References No definition available. + Details
Name: | sigi_RemainingCommitment
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Other investments not otherwise specified in the taxonomy. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.1(f,g)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 12 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 7 -Section 03 -Paragraph 1 -Subparagraph g Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.12) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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