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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, 2022 | Dec. 31, 2021 | Dec. 31, 2020
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | $ 9,561,429 | $ 8,106,031 | $ 6,930,843
Insurance |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 8,369,062 | 7,077,708 | 6,067,669
Insurance | Other liability |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 3,206,846 | 2,673,098 | 2,269,458
Insurance | Short-tail lines |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 1,630,371 | 1,389,068 | 1,247,908
Insurance | Workers' compensation |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 1,197,811 | 1,131,283 | 1,127,487
Insurance | Commercial automobile |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 1,208,241 | 990,945 | 794,171
Insurance | Professional liability |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 1,125,793 | 893,314 | 628,645
Reinsurance & Monoline Excess |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 1,192,367 | 1,028,323 | 863,174
Reinsurance & Monoline Excess | Casualty |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 764,793 | 643,193 | 521,559
Reinsurance & Monoline Excess | Monoline Excess |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | 217,017 | 201,187 | 171,522
Reinsurance & Monoline Excess | Property |  |  |
Segment Reporting Information [Line Items] |  |  |
Net premiums earned | $ 210,557 | $ 183,943 | $ 170,093

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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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