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Investments (OTTI by Asset Type) (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2011 | Jun. 30, 2010 securities | Jun. 30, 2011 | Jun. 30, 2010 securities
OTTI Losses Gross | $ (163) | $ 6,162 | $ 369 | $ 12,235
Other-than-temporary impairments on fixed maturity securities recognized in other comprehensive income | (332) | (22) | (430) | (2,189)
OTTI Losses Recognized in Earnings | 169 | 6,184 | 799 | 14,424
Number of impairments due to significant decline in share price |  | 1 |  | 1
Equity Securities [Member] |  |  |  |
OTTI Losses Gross |  | 672 |  | 672
Other-than-temporary impairments on fixed maturity securities recognized in other comprehensive income |  | 0 |  | 0
OTTI Losses Recognized in Earnings |  | 672 |  | 672
Obligations of States and Political Subdivisions [Member] | Fixed Maturity Securities [Member] |  |  |  |
OTTI Losses Gross |  |  | 17 |
Other-than-temporary impairments on fixed maturity securities recognized in other comprehensive income |  |  | 0 |
OTTI Losses Recognized in Earnings |  |  | 17 |
Corporate Securities [Member] | Fixed Maturity Securities [Member] |  |  |  |
OTTI Losses Gross |  |  | 244 |
Other-than-temporary impairments on fixed maturity securities recognized in other comprehensive income |  |  | 0 |
OTTI Losses Recognized in Earnings |  |  | 244 |
Asset-backed Securities ("ABS") [Member] | Fixed Maturity Securities [Member] |  |  |  |
OTTI Losses Gross |  |  |  | 158
Other-than-temporary impairments on fixed maturity securities recognized in other comprehensive income |  |  |  | 127
OTTI Losses Recognized in Earnings |  |  |  | 31
Commercial Mortgage-backed Securities ("CMBS") [Member] | Fixed Maturity Securities [Member] |  |  |  |
OTTI Losses Gross | (260) | 3,405 | (186) | 3,445
Other-than-temporary impairments on fixed maturity securities recognized in other comprehensive income | (402) | 569 | (658) | (2,052)
OTTI Losses Recognized in Earnings | 142 | 2,836 | 472 | 5,497
Average unrealized unrecognized loss as compared to amortized cost percentage |  | 87.00% |  | 87.00%
Residential Mortgage Backed Securities ("RMBS") [Member] | Fixed Maturity Securities [Member] |  |  |  |
OTTI Losses Gross | 97 | 2,085 | 294 | 7,960
Other-than-temporary impairments on fixed maturity securities recognized in other comprehensive income | 70 | (591) | 228 | (264)
OTTI Losses Recognized in Earnings | 27 | 2,676 | 66 | 8,224
Number of Impairments due to intent to sell |  |  |  | 2
Fixed Maturity Securities [Member] |  |  |  |
OTTI Losses Gross |  | 5,490 |  | 11,563
Other-than-temporary impairments on fixed maturity securities recognized in other comprehensive income |  | (22) |  | (2,189)
OTTI Losses Recognized in Earnings |  | $ 5,512 |  | $ 13,752

X
- Definition The average unrealized/unrecognized loss in comparison to the securities' amortized cost. + Details
Name: | sigi_AverageUnrealizedUnrecognizedLossAsComparedToAmortizedCostPercentage
Namespace Prefix: | sigi
Data Type: | us-types:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition This item represents the number of investments which made up the impairment losses for debt and equity securities recognized in earnings for the reporting periods due to the Company's intent to sell. + Details
Name: | sigi_NumberOfImpairmentsDueToIntentToSell
Namespace Prefix: | sigi
Data Type: | xbrli:decimalItemType
Balance Type: | na
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X
- Definition This item represents the number of investments which made up the impairment losses for debt and equity securities recognized in earnings for the reporting periods due to a significant decline in share price. + Details
Name: | sigi_NumberOfImpairmentsDueToSignificantDeclineInSharePrice
Namespace Prefix: | sigi
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Balance Type: | na
Period Type: | instant

X
- Definition Total other than temporary impairment losses on investments. + Details
Name: | sigi_OtherThanTemporaryImpairments
Namespace Prefix: | sigi
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Non-credit portion of other-than-temporary impairment losses on investments recognized in other comprehensive income. + Details
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Namespace Prefix: | sigi
Data Type: | xbrli:monetaryItemType
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X
- Definition This element represents the amount of loss recognized for other than temporary impairments of investments in debt and equity securities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 115 -Paragraph 16 + Details
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