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Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
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Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2018 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Depreciation expense |  |  |  | $ 21.5 | $ 18.7 | $ 19.5
Investment yields before tax assumed in premium deficiency assessment | 3.00% | 3.50% | 3.30% |  |  |

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- Definition This element represents the investment yields assumed in the premium deficiency assessment, which are based on the entity's actual average investment yield before tax as of the calculation date. + References No definition available. + Details
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Namespace Prefix: | sigi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585 + Details
Name: | us-gaap_Depreciation
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration