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Retirement Plans (Retirement Savings Plan) (Details) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Minimum Employee Contribution Of Defined Compensation To Retirement Savings Plan Percentage	2.00%
Maximum Employee Contribution Of Defined Compensation To Retirement Savings Plan Percentage	50.00%
Retirement Savings Plan [Member]
Employer contribution	$ 7.0	$ 6.3	$ 6.0
Parent's common stock issued under this plan			13,983
Employer Match [Member] | Retirement Savings Plan [Member]
Maximum Employer Match Of Employee Contribution To Retirement Savings Plan Percentage		65.00%
Maximum Employer Match Of Employee Contribution Of Defined Compensation To Retirement Savings Plan Percentage		7.00%
Vesting Period Of Employer Contribution		6
Enhanced Employer Match [Member] | Retirement Savings Plan [Member]
Maximum Employer Match Of Employee Contribution To Retirement Savings Plan Percentage		100.00%
Maximum Employer Match Of Employee Contribution Of Defined Compensation To Retirement Savings Plan Percentage		2.00%
Non Elective Employer Contribution [Member] | Retirement Savings Plan [Member]
Maximum Employer Match Of Employee Contribution Of Defined Compensation To Retirement Savings Plan Percentage	4.00%	2.00%
Vesting Period Of Employer Contribution		3
First Tier Of Employee Contributions [Member] | Employer Match [Member] | Retirement Savings Plan [Member]
Maximum Employer Match Of Employee Contribution To Retirement Savings Plan Percentage	100.00%
Maximum Employer Match Of Employee Contribution Of Defined Compensation To Retirement Savings Plan Percentage	3.00%
Second Tier Of Employee Contributions [Member] | Employer Match [Member] | Retirement Savings Plan [Member]
Maximum Employer Match Of Employee Contribution To Retirement Savings Plan Percentage	50.00%
Maximum Employer Match Of Employee Contribution Of Defined Compensation To Retirement Savings Plan Percentage	3.00%

X

- Definition

Maximum percentage of defined compensation that an employee can contribute to the Retirement Savings Plan.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	instant

X

- Definition

Maximum percentage of an employee's contribution to the Retirement Savings Plan that the employer will match.

+ References

No definition available.

+ Details

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Namespace Prefix:	sigi_
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X

- Definition

Percentage of an employee's contribution to the Retirement Savings Plan that the employer will match.

+ References

No definition available.

+ Details

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Namespace Prefix:	sigi_
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Balance Type:	na
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X

- Definition

Minimum percentage of defined compensation that an employee can contribute to the Retirement Savings Plan.

+ References

No definition available.

+ Details

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

X

- Definition

The period of time over which an employee is required to provide service in exchange for the employer match of the Retirement Savings Plan.

+ References

No definition available.

+ Details

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

X

- Definition

The increase in the fair value of plan assets from contributions made by the employer.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (b)(3)

-URI http://asc.fasb.org/extlink&oid=6962843&loc=d3e1928-114920

+ Details

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

X

- Definition

Number of shares issued during the period to an employee benefit plan, such as a defined contribution or defined benefit plan.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration