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Industry Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | $ 11,548,485 | $ 10,400,687 | $ 9,561,429
Insurance |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 10,086,308 | 9,007,376 | 8,171,828
Insurance | Other liability |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 4,068,662 | 3,605,298 | 3,188,399
Insurance | Short-tail lines |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 2,201,661 | 1,825,027 | 1,562,122
Insurance | Workers' compensation |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 1,237,888 | 1,212,294 | 1,197,810
Insurance | Auto |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 1,481,569 | 1,270,907 | 1,097,704
Insurance | Professional liability |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 1,096,528 | 1,093,850 | 1,125,793
Reinsurance & Monoline Excess |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 1,462,177 | 1,393,311 | 1,389,601
Reinsurance & Monoline Excess | Casualty |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 771,329 | 821,826 | 893,777
Reinsurance & Monoline Excess | Monoline Excess (3) |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 266,552 | 241,126 | 217,017
Reinsurance & Monoline Excess | Property (2) |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | $ 424,296 | $ 330,359 | $ 278,807

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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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