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Investments (Other Investment Portfolio by Strategy and the Remaining Commitment Amount Associated With Each Strategy) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Carrying Value	$ 109,855	$ 114,076
Other Investments [Member]
Carrying Value	109,855	114,076
Remaining Commitment	56,351
Alternative Investments [Member]
Carrying Value	107,899	109,664
Remaining Commitment	54,939
Secondary Private Equity Member | Alternative Investments [Member]
Carrying Value	27,780	28,032
Remaining Commitment	7,466
Energy Power Generation [Member] | Alternative Investments [Member]
Carrying Value	18,218	18,640
Remaining Commitment	7,825
Private Equity [Member] | Alternative Investments [Member]
Carrying Value	17,732	18,344
Remaining Commitment	4,697
Distressed Debt [Member] | Alternative Investments [Member]
Carrying Value	12,406	12,728
Remaining Commitment	2,922
Real Estate Strategy [Member] | Alternative Investments [Member]
Carrying Value	12,214	11,751
Remaining Commitment	10,292
Mezzanine Financing [Member] | Alternative Investments [Member]
Carrying Value	12,179	12,692
Remaining Commitment	21,337
Venture Capital [Member] | Alternative Investments [Member]
Carrying Value	7,370	7,477
Remaining Commitment	400
Other Securities [Member]
Carrying Value	1,956	4,412
Remaining Commitment	$ 1,412

X

- Definition

This item represents contractual obligations to invest additional amounts on our other investment portfolio. These additional investments may or may not be required.

+ References

No definition available.

+ Details

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

X

- Definition

Other investments not otherwise specified in the taxonomy.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.1(f,g))

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.12)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 12

-Article 5



Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 1

-Subparagraph g

-Article 7

+ Details

Name:	us-gaap_OtherInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant