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Business Combinations (Narrative) (Details) (USD $) | 12 Months Ended |
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Dec. 31, 2010 | Sep. 30, 2011
Business Combinations [Abstract] |  |
Gross premiums of excess and surplus renewal book of business purchased | $ 77,000,000 |
Increase in premiums written including potential impact of renewal book of business purchased | 6.00% |
Estimated value of business acquisition expected to close |  | $ 55,000,000

X
- Definition The total estimated value of an acquisition, including the cash paid to shareholders of acquired entities, fair value of debt and equity securities issued to shareholders of acquired entities, and the fair value of the liabilities assumed. + Details
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Namespace Prefix: | sigi
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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X
- Definition This item represents the gross premiums written related to the excess and surplus renewal book of business purchased + Details
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Namespace Prefix: | sigi
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition This item represents the increase in net premiums written including the potential impact of the renewal book of business purchased. + Details
Name: | sigi_IncreaseInPremiumsWrittenIncludingPotentialImpactOfRenewalBookOfBusinessPurchased
Namespace Prefix: | sigi
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_BusinessCombinationsAbstract
Namespace Prefix: | us-gaap
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration