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Retirement Plans (Retirement Savings Plan) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Deferred Compensation Arrangement With Individual, Employer Discretionary Contributions | $ 0.0 | $ 0.0 | $ 0.0
Retirement Savings Plan [Member] |  |  |
Defined Benefit Plan, Contributions by Employer | $ 14.1 | $ 13.4 | $ 12.2

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- Definition Amount of discretionary contributions made by the employer to a deferred compensation plan. + References No definition available. + Details
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Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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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=58740216&loc=d3e1928-114920 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |