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Retirement Plans (Schedule of Alternative Investment Portfolio by Strategy and Remaining Commitment) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Other investments	$ 128,301	$ 137,865
Alternative Investments [Member]
Other investments	124,548	135,110
Remaining Commitment	55,344
Alternative Investments [Member] | Private Equity [Member]
Other investments	21,736	21,601
Remaining Commitment	5,368
Alternative Investments [Member] | Real Estate Strategy [Member]
Other investments	13,767	14,192
Remaining Commitment	10,569
Retirement Income Plan [Member] | Alternative Investments [Member]
Other investments	17,000	19,000
Remaining Commitment	4,800
Retirement Income Plan [Member] | Alternative Investments [Member] | Private Equity [Member]
Other investments	12,600	14,500
Remaining Commitment	4,200
Retirement Income Plan [Member] | Alternative Investments [Member] | Equity Long Short Hedge [Member]
Other investments	1,800	1,800
Remaining Commitment	0
Retirement Income Plan [Member] | Alternative Investments [Member] | Real Estate Strategy [Member]
Other investments	2,600	2,700
Remaining Commitment	$ 600

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 SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 12

-Article 5



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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 7

-Section 03

-Paragraph 1

-Subparagraph g



Reference 4: 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

+ Details

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