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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.

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No definition available.

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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.

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No definition available.

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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.

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No definition available.

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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

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No definition available.

+ Details

Name:	sigi_UnrealizedGainLossOnInvestmentSecurities
Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant