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Investments (Held-To-Maturity Securities Disclosure) (Details) - USD ($) $ in Thousands | Mar. 31, 2020 | Dec. 31, 2019
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Schedule of Held-to-maturity Securities [Line Items] |  |
HTM Securities Percent of Invested Assets | 1.00% |
HTM, Carrying Value | $ 20,380 | $ 20,800
Net unrealized unrecognized gain or loss on HTMs | $ 1,600 | $ 1,200

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- Definition The percentage of the total invested asset portfolio comprised of held to maturity fixed income securities. + References No definition available. + Details
Name: | sigi_HTMSecuritiesPercentofInvestedAssets
Namespace Prefix: | sigi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Definition This item represents the sum of (i) the excess or shortfall of amortized cost compared to fair value for held-to-maturity securities and (ii) the market fluctuations on held-to-maturity securities from the later of (a) the date a security is designated as held-to-maturity; or (b) the date that an other-than-temporary impairment charge is recognized on a held-to-maturity security. + References No definition available. + Details
Name: | sigi_NetunrealizedunrecognizedgainorlossonHTMs
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Amount of investment in debt security measured at amortized cost (held-to-maturity). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118954083&loc=d3e26610-111562 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27232-111563 + Details
Name: | us-gaap_HeldToMaturitySecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- 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_ScheduleOfHeldToMaturitySecuritiesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration