XML 109 R96.htm IDEA: XBRL DOCUMENT
v3.8.0.1

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
Name: | sigi_AnnualLimitationOfNetOperatingLossCarryforwards
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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
Name: | us-gaap_OperatingLossCarryforwards
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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
Name: | us-gaap_OperatingLossCarryforwardsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration