XML 46 R34.htm IDEA: XBRL DOCUMENT
v3.19.1

Investments (Held-To-Maturity Securities Disclosure) (Details) - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Schedule of Held-to-maturity Securities [Line Items] |  |
HTM Securities Percent of Invested Assets | 1.00% | 1.00%
HTM, Carrying Value | $ 36,493 | $ 37,110
Net unrealized unrecognized gains on held to maturity securities | 1,500 | 1,300
Gross unrealized unrecognized loss | $ 100 | $ 200

X
- 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

X
- Definition The net unrealized/unrecognized gains or losses on held to maturity securities. + References No definition available. + Details
Name: | sigi_NetUnrealizedUnrecognizedGainsLossesonHeldtoMaturitySecurities
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition This item represents the sum of (i) the excess of amortized cost over fair value for available-for-sale securities that are in a loss position 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 for securities that are in an unrecognized loss position. + References No definition available. + Details
Name: | sigi_UnrealizedUnrecognizedLoss
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- 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=109979856&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=82897076&loc=d3e27232-111563 + Details
Name: | us-gaap_HeldToMaturitySecurities
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_ScheduleOfHeldToMaturitySecuritiesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration