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Investments (Gross Credit Loss Impairments of Fixed Maturity Securities for Which a Portion of the OTTI Charge was Recognized in OCI) (Details) (USD $) In Thousands | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2011 | Jun. 30, 2010 | Jun. 30, 2011 | Jun. 30, 2010
Investments [Abstract] |  |  |  |
Balance, beginning of period | $ 14,368 | $ 24,737 | $ 17,723 | $ 22,189
Addition for the amount related to credit loss for which an OTTI was not previously recognized | 0 | 2,004 | 0 | 2,134
Reductions for securities sold during the period | 0 | (2,990) | 0 | (2,990)
Reductions for securities for which the amount previously recognized in OCI was recognized in earnings because of intention or potential requirement to sell before recovery of amortized cost | 0 | 0 | 0 | 0
Reductions for securities for which the entire amount previously recognized in OCI was recognized in earnings due to a decrease in cash flows expected | (372) | (4,358) | (3,954) | (4,652)
Additional increases to the amount related to credit loss for which an OTTI was previously recognized | 28 | 950 | 255 | 3,662
Accretion of credit loss impairments previously recognized due to an increase in cash flows expected to be collected | 0 | 0 | 0 | 0
Balance, end of period | $ 14,024 | $ 20,343 | $ 14,024 | $ 20,343

X
- Definition This item represents accretion of credit loss impairments previously recognized due to an increase in cash flow expected to be collected on securities for which an other-than-temporary impairment charge was recognized in other comprehensive income. + Details
Name: | sigi_AccretionOfCreditLossImpairmentsPreviouslyRecognizedDueToIncreaseInCashFlowsExpectedToBeCollected
Namespace Prefix: | sigi
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition This item represents additional credit losses recognized in earnings on securities for which an other-than-temporary impairment was previously recognized in other comprehensive income. + Details
Name: | sigi_AdditionalIncreasesToAmountRelatedToCreditLossForWhichOttiWasPreviouslyRecognized
Namespace Prefix: | sigi
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition This item represents credit losses during the period on securities for which an other-than-temporary impairment was not previously recognized in other comprehensive income. + Details
Name: | sigi_AdditionForAmountRelatedToCreditLossForWhichOttiWasNotPreviouslyRecognized
Namespace Prefix: | sigi
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The cumulative amount of credit loss impairments on fixed maturity securities for which a portion of the other-than-temporary impairment charge was recognized in other comprehensive income. + Details
Name: | sigi_CreditLossImpairmentsOnFixedMaturitySecuritiesForWhichPortionOfOttiChargeWasRecognizedInOci
Namespace Prefix: | sigi
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition This item represents the removal of a credit loss from the table due to the fact that the other-than-temporary impairment charge that was previously recognized in other comprehensive income was recognized in earnings during the period due to the company's intention or potential requirement to sell before recovery of amortized cost. + Details
Name: | sigi_ReductionsForSecuritiesForWhichAmountPreviouslyRecognizedOciWasRecognizedInEarningsBecauseOfIntentionOrPotentialRequirementToSellBeforeRecoveryOfAmortizedCost
Namespace Prefix: | sigi
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition This item represents the removal of a credit loss from the table due to the fact that the other-than-temporary charge that was previously recognized in other comprehensive income was recognized in earnings during the period due to a decrease in the securities expected cash flows. + Details
Name: | sigi_ReductionsForSecuritiesForWhichEntireAmountPreviouslyRecognizedInOciWasRecognizedInEarningsDueToDecreaseInCashFlowsExpected
Namespace Prefix: | sigi
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition This item represents a decrease to the cumulative amount of credit losses recognized in earnings on fixed maturity securities for which a portion of the other-than-temporary impairment charge was recognized in other comprehensive income due to the sale of the securities during the reporting period. + Details
Name: | sigi_ReductionsForSecuritiesSoldDuringPeriod
Namespace Prefix: | sigi
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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