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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, 2023 | Sep. 30, 2022 | Sep. 30, 2023 | Sep. 30, 2022
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | $ 2,641,914 | $ 2,441,961 | $ 7,686,073 | $ 7,048,205
Insurance |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned |  | 2,129,014 |  | 6,162,005
Insurance | Operating Segments |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | 2,319,435 | 2,129,014 | 6,747,704 | 6,162,005
Insurance | Other liability |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | 918,054 | 824,595 | 2,668,831 | 2,363,349
Insurance | Short-tail lines |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | 490,598 | 407,211 | 1,383,654 | 1,184,191
Insurance | Commercial automobile |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | 335,572 | 307,754 | 965,416 | 891,729
Insurance | Workers' compensation |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | 300,694 | 307,323 | 905,405 | 892,390
Insurance | Professional liability |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | 274,517 | 282,131 | 824,398 | 830,346
Reinsurance & Monoline Excess |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned |  | 312,947 |  | 886,200
Reinsurance & Monoline Excess | Operating Segments |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | 322,479 | 312,947 | 938,369 | 886,200
Reinsurance & Monoline Excess | Casualty reinsurance |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | 192,071 | 193,726 | 574,660 | 568,507
Reinsurance & Monoline Excess | Monoline excess |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | 60,363 | 54,675 | 177,853 | 161,181
Reinsurance & Monoline Excess | Property reinsurance |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | $ 70,045 | $ 64,546 | $ 185,856 | $ 156,512

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