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Retirement Plans (Retirement Savings Plan) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
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	$ 8.2	$ 7.0	$ 6.3
Employer Match [Member] | Retirement Savings Plan [Member]
Maximum Employer Match Of Employee Contribution Of Defined Compensation To Retirement Savings Plan Percentage			7.00%
Vesting Period Of Employer Contribution		6 years
Defined Contribution Plan, Employer Matching Contribution, Percent			65.00%
Enhanced Employer Match [Member] | Retirement Savings Plan [Member]
Maximum Employer Match Of Employee Contribution Of Defined Compensation To Retirement Savings Plan Percentage			2.00%
Defined Contribution Plan, Employer Matching Contribution, Percent			100.00%
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 years
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 Of Defined Compensation To Retirement Savings Plan Percentage		3.00%
Defined Contribution Plan, Employer Matching Contribution, Percent		50.00%

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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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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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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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
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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

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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=21915506&loc=d3e1928-114920

+ Details

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X

- Definition

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

+ References

No definition available.

+ Details

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