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Retirement Plans (Retirement Savings Plan) (Details) (USD $) In Millions, unless otherwise specified |  | 12 Months Ended |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 31, 2013 | Dec. 31, 2013 Retirement Savings Plan [Member] | Dec. 31, 2012 Retirement Savings Plan [Member] | Dec. 31, 2011 Retirement Savings Plan [Member] | Apr. 05, 2013 Non Elective Employer Contribution [Member] Retirement Savings Plan [Member] | Dec. 31, 2011 Non Elective Employer Contribution [Member] Retirement Savings Plan [Member] | Apr. 05, 2013 First Tier Of Employee Contributions [Member] Employer Match [Member] Retirement Savings Plan [Member] | Dec. 31, 2011 First Tier Of Employee Contributions [Member] Employer Match [Member] Retirement Savings Plan [Member] | Apr. 05, 2013 Second Tier Of Employee Contributions [Member] Employer Match [Member] Retirement Savings Plan [Member] | Dec. 31, 2011 Second Tier Of Employee Contributions [Member] Employer Match [Member] Retirement Savings Plan [Member]
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% |  |  |  |  |  |  |  |  |
Maximum employer match of employee contribution of defined compensation to retirement savings plan percentage |  |  |  |  | 4.00% | 4.00% | 3.00% | 3.00% | 3.00% | 3.00%
Defined contribution plan, employer matching contribution, percentage |  |  |  |  |  |  | 100.00% | 100.00% | 50.00% | 50.00%
Employer contribution |  | $ 12.2 | $ 8.2 | $ 7.0 |  |  |  |  |  |

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- Definition Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan. + References No definition available. + Details
Name: | sigi_DefinedContributionPlanEmployerMatchingContributionPercentage
Namespace Prefix: | sigi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Maximum percentage of defined compensation that an employee can contribute to the Retirement Savings Plan. + References No definition available. + Details
Name: | sigi_MaximumEmployeeContributionOfDefinedCompensationToRetirementSavingsPlanPercentage
Namespace Prefix: | sigi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Maximum percentage of an employee's contribution to the Retirement Savings Plan that the employer will match. + References No definition available. + Details
Name: | sigi_MaximumEmployerMatchOfEmployeeContributionOfDefinedCompensationToRetirementSavingsPlanPercentage
Namespace Prefix: | sigi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition Minimum percentage of defined compensation that an employee can contribute to the Retirement Savings Plan. + References No definition available. + Details
Name: | sigi_MinimumEmployeeContributionOfDefinedCompensationToRetirementSavingsPlanPercentage
Namespace Prefix: | sigi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- 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 Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(3) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
Name: | us-gaap_DefinedBenefitPlanContributionsByEmployer
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration