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v3.23.1

Business Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2023 | Mar. 31, 2022
Revenue from External Customer [Line Items] |  |
Net premiums earned | $ 2,491,432 | $ 2,249,087
Insurance |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned |  | 1,962,835
Insurance | Operating Segments |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | 2,181,876 | 1,962,835
Insurance | Other liability |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | 857,149 | 749,391
Insurance | Short-tail lines |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | 430,503 | 375,212
Insurance | Commercial automobile |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | 311,422 | 282,234
Insurance | Workers' compensation |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | 305,561 | 285,423
Insurance | Professional liability |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | 277,241 | 270,575
Reinsurance & Monoline Excess |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned |  | 286,252
Reinsurance & Monoline Excess | Operating Segments |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | 309,556 | 286,252
Reinsurance & Monoline Excess | Casualty reinsurance |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | 193,730 | 184,122
Reinsurance & Monoline Excess | Monoline excess |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | 58,644 | 51,896
Reinsurance & Monoline Excess | Property reinsurance |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | $ 57,182 | $ 50,234

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- Definition Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(1)) -URI http://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 235 -Section S99 -Paragraph 2 -Subparagraph (SX 210.12-17(Column E)) -URI http://asc.fasb.org/extlink&oid=120401096&loc=d3e574992-122915 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 220 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04(13)(c)) -URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
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