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Federal Income Tax (Federal Tax Net Operating Loss Carryforwards) (Details) $ in Millions	Dec. 31, 2017 USD ($)
Operating Loss Carryforwards [Line Items]
Annual Limitation Of Net Operating Loss Carryforwards	$ 1.9
Operating Loss Carryforwards	$ 0.3

X

- Definition

Amount of the annual limitation of operating loss carryforwards.

+ References

No definition available.

+ Details

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Namespace Prefix:	sigi_
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Period Type:	instant

X

- Definition

Amount of operating loss carryforward, before tax effects, available to reduce future taxable income under enacted tax laws.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32559-109319

+ Details

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Period Type:	instant

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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