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Investments (Unrealized Gains on Investments) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011
Net unrealized gains | $ 79,237 | $ 188,197 | $ 149,612
Deferred income tax expense | (27,733) | (65,869) | (52,364)
Net unrealized gains, net of deferred income tax | 51,504 | 122,328 | 97,248
(Decrease) increase in net unrealized gains in OCI, net of deferred income tax | (70,824) | 25,080 | 43,380
AFS Fixed Maturity Securities [Member] |  |  |
Net unrealized gains | 39,559 | 165,330 | 130,517
Equity Securities [Member] |  |  |
Net unrealized gains | 37,421 | 18,941 | 13,529
Available-for-sale Securities [Member] |  |  |
Net unrealized gains | 76,980 | 184,271 | 144,046
Held To Maturity Fixed Maturity Securities [Member] |  |  |
Net unrealized gains | 2,257 | 3,926 | 5,566
Total HTM Securities [Member] |  |  |
Net unrealized gains | $ 2,257 | $ 3,926 | $ 5,566

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- 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 other comprehensive income, net of deferred income tax. + References No definition available. + Details
Name: | sigi_Netunrealizedgainlossesoninvestmentsecuritiesinothercomprehensiveincomenetofdeferredincometax
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Total change in unrealized gains/losses on investment securities net of tax. + 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