Investments (HTM Fixed Maturity Securities by Contractual Maturity) (Details) (USD $) In Thousands	Jun. 30, 2011
Investments [Abstract]
Due in one year or less Carrying Value	$ 115,247
Due after one year through five years Carrying Value	648,116
Due after five years through 10 years Carrying Value	294,010
Due after 10 years Carrying Value	13,205
Total HTM fixed maturity securities Carrying Value	1,070,578
Due in one year or less Fair Value	117,326
Due after one year through five years Fair Value	676,796
Due after five years through 10 years Fair Value	312,695
Due after 10 years Fair Value	14,160
Total HTM fixed maturity securities Fair Value	$ 1,120,977

For an unclassified balance sheet, this item represents investments in debt securities which are categorized as held-to-maturity; such investments are measured at carrying value, which represents amortized cost adjusted for net unrealized gains (losses) resulting from: (i) an other-than-temporary impairment; or (ii) a previous available-for-sale designation. The held-to-maturity category is for those securities that the Entity has the positive intent and ability to hold until maturity.

Name:	sigi_HeldToMaturityAtCarryValue
Namespace Prefix:	sigi
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents investments in debt securities which are categorized as held-to-maturity which are expected to mature after five years and through ten years from balance sheet date; such investments are measured at carrying value, which represents amortized cost adjusted for net unrealized gains (losses) resulting from: (i) an other-than-temporary impairment; or (ii) a previous available-for-sale designation. The held-to-maturity category is for those securities that the Entity has the positive intent and ability to hold until maturity.

+ Details

Name:	sigi_HeldToMaturityDebtSecuritiesMaturitiesDueAfterFiveYearsThroughTenYearsNetCarryingAmount
Namespace Prefix:	sigi
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents investments in debt securities which are categorized as held-to-maturity which are expected to mature after one year and through five years from balance sheet date; such investments are measured at carrying value, which represents amortized cost adjusted for net unrealized gains (losses) resulting from: (i) an other-than-temporary impairment; or (ii) a previous available-for-sale designation. The held-to-maturity category is for those securities that the Entity has the positive intent and ability to hold until maturity.

+ Details

Name:	sigi_HeldToMaturityDebtSecuritiesMaturitiesDueAfterOneYearThroughFiveYearsCarryingValue
Namespace Prefix:	sigi
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents investments in debt securities which are categorized as held-to-maturity which are expected to mature after ten years from balance sheet date; such investments are measured at carrying value, which represents amortized cost adjusted for net unrealized gains (losses) resulting from: (i) an other-than-temporary impairment; or (ii) a previous available-for-sale designation. The held-to-maturity category is for those securities that the Entity has the positive intent and ability to hold until maturity.

+ Details

Name:	sigi_HeldToMaturityDebtSecuritiesMaturitiesDueAfterTenYearsNetCarryingAmount
Namespace Prefix:	sigi
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents investments in debt securities which are categorized as held-to-maturity which are expected to mature within one year from the balance sheet date; such investments are measured at carrying value, which represents amortized cost adjusted for net unrealized gains (losses) resulting from: (i) an other-than-temporary impairment; or (ii) a previous available-for-sale designation. The held-to-maturity category is for those securities that the Entity has the positive intent and ability to hold until maturity.

+ Details

Name:	sigi_HeldToMaturityDebtSecuritiesMaturitiesDueWithinOneYearNetCarryingAmount
Namespace Prefix:	sigi
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the fair value of debt securities which are expected to mature after five years and through ten years from the balance sheet date and which are categorized as held-to-maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 20
-Subparagraph c

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the fair value of debt securities which are expected to mature after one year and through five years from the balance sheet date and which are categorized as held-to-maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 20
-Subparagraph b

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the fair value of debt securities which are expected to mature after ten years from the balance sheet date which are categorized as held-to-maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 20
-Subparagraph d

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the total of all debt securities grouped by maturity dates, at fair value, which are categorized as held-to-maturity.

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesDebtMaturitiesFairValue
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the fair value of debt securities which are expected to mature within one year of the balance sheet date and which are categorized as held-to-maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 20
-Subparagraph a

+ Details

Name:	us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue
Namespace Prefix:	us-gaap
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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