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Industry Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | $ 10,400,687 | $ 9,561,429 | $ 8,106,031
Insurance |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 9,130,324 | 8,369,062 | 7,077,708
Insurance | Other liability |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 3,613,900 | 3,206,846 | 2,673,098
Insurance | Short-tail lines |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 1,897,070 | 1,630,371 | 1,389,068
Insurance | Workers' compensation |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 1,212,294 | 1,197,811 | 1,131,283
Insurance | Auto |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 1,313,210 | 1,208,241 | 990,945
Insurance | Professional liability |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 1,093,850 | 1,125,793 | 893,314
Reinsurance & Monoline Excess |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 1,270,363 | 1,192,367 | 1,028,323
Reinsurance & Monoline Excess | Casualty |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 770,920 | 764,793 | 643,193
Reinsurance & Monoline Excess | Monoline Excess |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 241,126 | 217,017 | 201,187
Reinsurance & Monoline Excess | Property |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | $ 258,317 | $ 210,557 | $ 183,943

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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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