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Business Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2024 | Mar. 31, 2023
Revenue from External Customer [Line Items] |  |
Net premiums earned | $ 2,764,347 | $ 2,491,432
Insurance |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned |  | 2,142,924
Insurance | Operating Segments |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | 2,398,768 | 2,142,924
Insurance | Other liability |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | 967,260 | 853,472
Insurance | Short-tail lines |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | 510,809 | 412,534
Insurance | Auto |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | 354,013 | 294,116
Insurance | Workers' compensation |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | 301,495 | 305,561
Insurance | Professional liability |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | 265,191 | 277,241
Reinsurance & Monoline Excess |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned |  | 348,508
Reinsurance & Monoline Excess | Operating Segments |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | 365,579 | 348,508
Reinsurance & Monoline Excess | Casualty |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | 197,844 | 214,712
Reinsurance & Monoline Excess | Monoline excess |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | 65,351 | 58,644
Reinsurance & Monoline Excess | Property |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | $ 102,384 | $ 75,152

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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 -Topic 944 -SubTopic 235 -Name Accounting Standards Codification -Section S99 -Paragraph 2 -Subparagraph (SX 210.12-17(Column E)) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480149/944-235-S99-2 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 605 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (c)(3) -Publisher FASB -URI https://asc.fasb.org//1943274/2147479655/944-605-50-1 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04(13)(c)) -SubTopic 220 -Topic 942 -Publisher FASB -URI https://asc.fasb.org//1943274/2147483589/942-220-S99-1 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04(1)) -SubTopic 220 -Topic 944 -Publisher FASB -URI https://asc.fasb.org//1943274/2147483586/944-220-S99-1 + Details
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