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Business Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2022 | Sep. 30, 2021 | Sep. 30, 2022 | Sep. 30, 2021
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | $ 2,441,961 | $ 2,081,018 | $ 7,048,205 | $ 5,902,598
Insurance |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned |  | 1,819,071 |  | 5,151,253
Insurance | Operating Segments |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | 2,129,014 | 1,819,071 | 6,162,005 | 5,151,253
Insurance | Other liability |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | 824,595 | 691,014 | 2,363,349 | 1,946,598
Insurance | Short-tail lines |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | 407,211 | 347,835 | 1,184,191 | 1,012,465
Insurance | Workers' compensation |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | 307,323 | 286,474 | 892,390 | 845,394
Insurance | Commercial automobile |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | 307,754 | 257,314 | 891,729 | 715,519
Insurance | Professional liability |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | 282,131 | 236,434 | 830,346 | 631,277
Reinsurance & Monoline Excess |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned |  | 261,947 |  | 751,345
Reinsurance & Monoline Excess | Operating Segments |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | 312,947 | 261,947 | 886,200 | 751,345
Reinsurance & Monoline Excess | Casualty reinsurance |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | 193,726 | 164,095 | 568,507 | 466,264
Reinsurance & Monoline Excess | Monoline excess |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | 54,675 | 52,361 | 161,181 | 146,481
Reinsurance & Monoline Excess | Property reinsurance |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | $ 64,546 | $ 45,491 | $ 156,512 | $ 138,600

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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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