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Allowance for Uncollectible Premiums Receivable (Tables) | 3 Months Ended
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Mar. 31, 2020
Premiums Receivable [Abstract] |
Premium Receivable, Allowance for Credit Loss [Table Text Block] |
($ in thousands) | March 31, 2020
Balance, December 31, 2019 | $ | 6,400 |
Cumulative effect adjustment 1 | 1,058 |
Balance, January 1, 2020 | $ | 7,458 |
Current period provision for expected credit losses | 11,195 |
Write-offs charged against the allowance | (653 | )
Recoveries | — |
Allowance for credit losses, end of period | $ | 18,000 |

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- Definition Premiums Receivable [Abstract] + References No definition available. + Details
Name: | sigi_PremiumsReceivableAbstract
Namespace Prefix: | sigi_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Tabular disclosure of allowance for credit loss on premium receivable. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919249-210447 + Details
Name: | us-gaap_PremiumReceivableAllowanceForCreditLossTableTextBlock
Namespace Prefix: | us-gaap_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration