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Allowance for Uncollectible Premiums Receivable (Tables) | 9 Months Ended
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Sep. 30, 2020
Premiums Receivable [Abstract] |
Premium Receivable, Allowance for Credit Loss [Table Text Block] |
($ in thousands) |  | Quarter ended September 30, 2020 |  | Nine Months ended September 30, 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 |  | 802 |  |  | 16,369 |
Write-offs charged against the allowance for credit losses |  | (996) |  |  | (3,272) |
Recoveries |  | 194 |  |  | 445 |
Allowance for credit losses, end of period |  | $ | 21,000 |  |  | $ | 21,000 |

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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