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Investments (Number of Securities in an Unrealized Unrecognized Loss Position) (Details) (USD $)	6 Months Ended
	Jun. 30, 2012 Securities	Dec. 31, 2011 Securities
Schedule of Available-for-sale Securities [Line Items]
Unrealized Unrecognized Losses Period Increase Decrease	$ 1,900,000
Number of securities in an unrealized/unrecognized loss position	146	141
Securities In Unrealized Unrecognized Loss Position For More Than Twelve Months Unrealized Unrecognized Loss	4,200,000	4,900,000
Unrealized/unrecognized loss positions improvement	8,739,000	10,635,000
Non-Credit OTTI Charges	2,600,000	2,100,000
Decline in fair value percentage	4.00%	6.00%
Securities which have experienced a shortfall in principle and interest payments	3
Eighty Percent to Ninety-Nine Percent [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities in an unrealized/unrecognized loss position	141	140
Unrealized/unrecognized loss positions improvement	6,817,000	10,166,000
Sixty Percent to Seventy-Nine Percent [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities in an unrealized/unrecognized loss position	4	0
Unrealized/unrecognized loss positions improvement	1,580,000	0
Forty Percent to Fifty-Nine Percent [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities in an unrealized/unrecognized loss position	1	1
Unrealized/unrecognized loss positions improvement	342,000	469,000
Twenty Percent to Thirty-Nine Percent [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities in an unrealized/unrecognized loss position	0	0
Unrealized/unrecognized loss positions improvement	0	0
Zero Percent to Nineteen Percent [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities in an unrealized/unrecognized loss position	0	0
Unrealized/unrecognized loss positions improvement	$ 0	$ 0

X

- Definition

Non-credit portion of other-than-temporary impairment losses on fixed maturity securities recognized in other comprehensive income that have been in a loss position for more than twelve months. This item includes subsequent changes to these securities such as amortization on HTM securities or subsequent sales. This item does not include subsequent changes in appreciation or loss in value.

+ References

No definition available.

+ Details

Name:	sigi_NonCreditPortionOfOtherThanTemporaryImpairmentLossesRecognizedInOtherComprehensiveIncomeLossPositionMoreThanTwelveMonths
Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

This item contains disclosure of the number of available-for-sale and held-to-maturity securities determined to be in an unrealized/unrecognized loss position.

+ References

No definition available.

+ Details

Name:	sigi_NumberOfSecuritiesInUnrealizedUnrecognizedLossPosition
Namespace Prefix:	sigi_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

X

- Definition

This item represents the percentage decline in fair value of fixed maturity securities compared to amortized cost that have been in a loss position for more than twelve months. This impairment has been determined to be temporary.

+ References

No definition available.

+ Details

Name:	sigi_PercentageDeclineInFairValuePercentage
Namespace Prefix:	sigi_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Represents total unrealized and unrecognized losses of all available-for-sale and held-to-maturity securities in an unrealized and unrecognized loss position for more than 12 months as of the end of the period.

+ References

No definition available.

+ Details

Name:	sigi_SecuritiesInUnrealizedUnrecognizedLossPositionForMoreThanTwelveMonthsUnrealizedUnrecognizedLoss
Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

This item represents securities that have experienced a shortfall in principle and interest payments.

+ References

No definition available.

+ Details

Name:	sigi_Securitieswhichhaveexperiencedashortfallinprincipleandinterestpayments
Namespace Prefix:	sigi_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

X

- Definition

This item represents the sum of (i) the excess of amortized cost over fair value for available-for-sale securities that are in a loss position and (ii) the market fluctuations on held-to-maturity securities from the later of (a) the date a security is designated as held-to-maturity; or (b) the date that an other-than-temporary impairment charge is recognized on a held-to-maturity security for securities that are in an unrecognized loss position.

+ References

No definition available.

+ Details

Name:	sigi_UnrealizedUnrecognizedLoss
Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Represents the increase or decrease in unrealized/unrecognized losses for available-for-sale and held-to-maturities securities.

+ References

No definition available.

+ Details

Name:	sigi_UnrealizedUnrecognizedLossesPeriodIncreaseDecrease
Namespace Prefix:	sigi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration