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v3.25.1

Business Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2025 | Mar. 31, 2024
Revenue from External Customer [Line Items] |  |
Net premiums earned | $ 3,012,381 | $ 2,764,347
Insurance |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned |  | 2,398,768
Insurance | Operating Segments |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | 2,642,507 | 2,398,768
Insurance | Other liability |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | 1,072,728 | 967,260
Insurance | Short-tail lines |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | 596,109 | 510,809
Insurance | Auto |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | 389,949 | 354,013
Insurance | Workers' compensation |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | 311,028 | 301,495
Insurance | Professional liability |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | 272,693 | 265,191
Reinsurance and Monoline Excess Segment |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned |  | 365,579
Reinsurance and Monoline Excess Segment | Operating Segments |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | 369,874 | 365,579
Reinsurance and Monoline Excess Segment | Casualty |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | 181,767 | 197,844
Reinsurance and Monoline Excess Segment | Monoline excess |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | 67,264 | 65,351
Reinsurance and Monoline Excess Segment | Property |  |
Revenue from External Customer [Line Items] |  |
Net premiums earned | $ 120,843 | $ 102,384

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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/1943274/2147477965/944-235-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/1943274/2147479032/944-605-50-1 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/1943274/2147478524/942-220-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/1943274/2147477250/944-220-S99-1 + Details
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