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Investments (Unrealized Gains on Investments) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Net unrealized gains | $ 188,197 | $ 149,612 | $ 82,874
Deferred income tax expense | (65,869) | (52,364) | (29,006)
Net unrealized gains, net of deferred income tax | 122,328 | 97,248 | 53,868
Increase in net unrealized gains in OCI, net of deferred income tax | 25,080 | 43,380 | 24,529
AFS Fixed Maturity Securities [Member] |  |  |
Net unrealized gains | 165,330 | 130,517 | 56,754
Equity Securities [Member] |  |  |
Net unrealized gains | 18,941 | 13,529 | 11,597
Available-for-sale Securities [Member] |  |  |
Net unrealized gains | 184,271 | 144,046 | 68,351
Held To Maturity Fixed Maturity Securities Member |  |  |
Net unrealized gains | 3,926 | 5,566 | 14,523
Total HTM securities [Member] |  |  |
Net unrealized gains | $ 3,926 | $ 5,566 | $ 14,523

X
- Definition Tax effect of unrealized gain (loss) on investments in accumulated other comprehensive income. + References No definition available. + Details
Name: | sigi_DeferredIncomeTaxExpenseBenefitOnInvestmentSecuritiesInOtherComprehensiveIncome
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition This item represents the net unrealized gains (losses) on investments in accumulated other comprehensive income, net of deferred income tax, prior to the impact of cumulative effect adjustment due to adoption of accounting literature. + References No definition available. + Details
Name: | sigi_NetUnrealizedGainLossesOnInvestmentSecuritiesNetOfDeferredIncomeTaxBeforeCumulativeEffectAdjustment
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition This amount is equal to the sum of: (i) the non-credit portion of other-than-temporary impairment losses on fixed maturity securities recognized in other comprehensive income, net of tax, including subsequent changes to these securities such as amortization on HTM securities or subsequent sales; and (ii) the change in net unrealized gains or losses on investment securities, after tax, excluding non-credit other-than-temporary impairment charges during the period. + References No definition available. + Details
Name: | sigi_TotalUnrealizedGainsLossesOnInvestmentSecurities
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The difference between the fair value and the carrying value, or in the comparative fair values, of investments, not including unrealized gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity, held at each balance sheet date and included in other comprehensive income + References No definition available. + Details
Name: | sigi_UnrealizedGainLossOnInvestmentSecurities
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant