XML 167 R135.htm IDEA: XBRL DOCUMENT
v3.20.4

Schedule V - Allowance for Uncollectible Premiums and Other Receivables (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Jan. 01, 2020
Balance, January 1 |  | $ 10,800 | $ 13,900 | $ 14,600 |
Balance, December 31 |  | 22,777 | 10,800 | 13,900 |
Additions |  | 17,576 | 2,730 | 4,022 |
Deductions |  | (3,754) | (5,830) | (4,722) |
Accounting Standards Update 2016-13 [Member] |  |  |  |  |
Balance, January 1 | [1] |  | 0 | 0 |
Balance, December 31 | [1] |  |  | 0 |
Cumulative effect adjustment | [1] |  |  |  | $ (1,845)
Accounting Standards Update 2016-13 [Member] | Premiums Receivable [Member] |  |  |  |  |
Balance, January 1 |  | $ 8,955 | 13,900 | 14,600 |
Balance, December 31 |  |  | $ 8,955 | $ 13,900 |
[1] | See Note 3. "Adoption of Accounting Pronouncements" in Item 8. "Financial Statements and Supplementary Data." of this Form 10-K for additional information regarding our adoption of ASU 2016-13.

X
- Definition The impact on stockholders equity of a cumulative effect adjustment. + References No definition available. + Details
Name: | sigi_EquityImpactOnCumulativeEffectAdjustment
Namespace Prefix: | sigi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of valuation and qualifying accounts and reserves. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SX 210.12-09(Column B)) -URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e24092-122690 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SX 210.12-09(Column E)) -URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e24092-122690 + Details
Name: | us-gaap_ValuationAllowancesAndReservesBalance
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Amount of increase in valuation and qualifying accounts and reserves from charge to cost and expense. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SX 210.12-09(Column C)(1)) -URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e24092-122690 + Details
Name: | us-gaap_ValuationAllowancesAndReservesChargedToCostAndExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Amount of decrease in valuation and qualifying accounts and reserves. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SX 210.12-09(Column D)) -URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e24092-122690 + Details
Name: | us-gaap_ValuationAllowancesAndReservesDeductions
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | srt_CumulativeEffectPeriodOfAdoptionAxis=us-gaap_AccountingStandardsUpdate201613Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | sigi_AllowanceforCreditLossesAxis=us-gaap_PremiumsReceivableMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |