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Allowance for Uncollectible Premiums Receivable (Tables) | 3 Months Ended
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Mar. 31, 2021
Premiums Receivable [Abstract] |
Premium Receivable, Allowance for Credit Loss [Table Text Block] |
|  | Quarter ended March 31, |  |
($ in thousands) |  | 2021 |  | 2020 |  |  |  |
Balance at beginning of period |  | $ | 21,000 |  |  | $ | 6,400 |  |  |  |  |
Cumulative effect adjustment 1 |  | — |  |  | 1,058 |  |  |  |  |
Balance at beginning of period, as adjusted |  | $ | 21,000 |  |  | $ | 7,458 |  |  |  |  |
Current period provision for expected credit losses |  | 808 |  |  | 11,195 |  |  |  |  |
Write-offs charged against the allowance for credit losses |  | (874) |  |  | (653) |  |  |  |  |
Recoveries |  | 66 |  |  | — |  |  |  |  |
Allowance for credit losses, end of period |  | $ | 21,000 |  |  | $ | 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=121599337&loc=SL82919249-210447 + Details
Name: | us-gaap_PremiumReceivableAllowanceForCreditLossTableTextBlock
Namespace Prefix: | us-gaap_
Data Type: | nonnum:textBlockItemType
Balance Type: | na
Period Type: | duration