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Reserve for Loss and Loss Expense (Roll Forward of Gross and Net Asbestos and Net Environmental Incurred Losses and Loss Expenses and Related Reserves) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |
Reserve for loss and loss expense at beginning of year, gross | $ 28,701 | $ 30,020 | $ 28,415
Incurred loss and loss expense, Gross | (127) | 348 | 3,059
Less: loss and loss expense paid, Gross | (44) | (1,667) | (1,454)
Reserve for loss and loss expense at the end of year, gross | 28,530 | 28,701 | 30,020
Reserve for loss and loss expense at beginning of year, net | 21,589 | 22,783 | 21,212
Incurred loss and loss expense, Net | (154) | 234 | 2,877
Less: loss and loss expense paid, Net | (14) | (1,428) | (1,306)
Reserve for loss and loss expense at the end of year, net | 21,421 | 21,589 | 22,783
Asbestos [Member] |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |
Reserve for loss and loss expense at beginning of year, gross | 6,288 | 7,328 | 7,577
Incurred loss and loss expense, Gross | 320 | (375) | 0
Less: loss and loss expense paid, Gross | (354) | (665) | (249)
Reserve for loss and loss expense at the end of year, gross | 6,254 | 6,288 | 7,328
Reserve for loss and loss expense at beginning of year, net | 5,057 | 6,097 | 6,346
Incurred loss and loss expense, Net | 320 | (375) | 0
Less: loss and loss expense paid, Net | (354) | (665) | (249)
Reserve for loss and loss expense at the end of year, net | 5,023 | 5,057 | 6,097
Enviromental [Member] |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |
Reserve for loss and loss expense at beginning of year, gross | 22,413 | 22,692 | 20,838
Incurred loss and loss expense, Gross | (447) | 723 | 3,059
Less: loss and loss expense paid, Gross | 310 | (1,002) | (1,205)
Reserve for loss and loss expense at the end of year, gross | 22,276 | 22,413 | 22,692
Reserve for loss and loss expense at beginning of year, net | 16,532 | 16,686 | 14,866
Incurred loss and loss expense, Net | (474) | 609 | 2,877
Less: loss and loss expense paid, Net | 340 | (763) | (1,057)
Reserve for loss and loss expense at the end of year, net | $ 16,398 | $ 16,532 | $ 16,686

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- Definition Amount, before effects of reinsurance, of expense for claims incurred and costs incurred in the claim settlement process attributable to asbestos and environmental claims. + References No definition available. + Details
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Namespace Prefix: | sigi_
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- Definition Amount of cash outflow to settle insured claims and pay costs incurred in the claims settlement process attributable to asbestos and environmental claims. + References No definition available. + Details
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- Definition Amount, after effects of reinsurance, of expense for claims incurred and costs incurred in the claim settlement process attributable to asbestos and environmental claims. + References No definition available. + Details
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- Definition Amount, after effects of reinsurance, of payments to settle insured claims and pay costs incurred in the claims settlement process attributable to asbestos and environmental claims. + References No definition available. + Details
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- Definition Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to asbestos and environmental claims, before estimated recoveries from reinsurers. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (cc) -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 + Details
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- Definition Carrying amount as of the balance sheet date of reserves for the costs of settling insured claims and costs incurred in the claims settlement process attributable to asbestos and environmental claims, net of estimated reinsurance recoveries. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (cc) -URI http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 + Details
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