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Allowance for Credit Losses on Premiums Receivable (Tables) | 3 Months Ended
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Mar. 31, 2023
Premiums Receivable [Abstract] |
Allowance for Credit Losses on Premiums Receivable [Table Text Block] |
|  | Quarter ended March 31, |  |
($ in thousands) |  | 2023 |  | 2022 |  |  |  |
Balance at beginning of period |  | $ | 16,100 |  |  | $ | 13,600 |  |  |  |  |
Current period change for expected credit losses |  | 1,910 |  |  | 916 |  |  |  |  |
Write-offs charged against the allowance for credit losses |  | (1,164) |  |  | (520) |  |  |  |  |
Recoveries |  | 254 |  |  | 304 |  |  |  |  |
Allowance for credit losses, end of period |  | $ | 17,100 |  |  | $ | 14,300 |  |  |  |  |

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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 https://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447 + Details
Name: | us-gaap_PremiumReceivableAllowanceForCreditLossTableTextBlock
Namespace Prefix: | us-gaap_
Data Type: | dtr-types:textBlockItemType
Balance Type: | na
Period Type: | duration