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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, 2024 | Sep. 30, 2023 | Sep. 30, 2024 | Sep. 30, 2023
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | $ 2,926,823 | $ 2,641,914 | $ 8,537,585 | $ 7,686,073
Insurance |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned |  | 2,291,917 |  | 6,650,027
Insurance | Operating Segments |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | 2,564,490 | 2,291,917 | 7,447,828 | 6,650,027
Insurance | Other liability |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | 1,031,087 | 916,394 | 3,000,335 | 2,660,850
Insurance | Short-tail lines |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | 563,736 | 472,805 | 1,613,342 | 1,330,173
Insurance | Auto |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | 376,762 | 327,507 | 1,092,354 | 929,201
Insurance | Workers' compensation |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | 313,154 | 300,694 | 925,214 | 905,405
Insurance | Professional liability |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | 279,751 | 274,517 | 816,583 | 824,398
Reinsurance & Monoline Excess |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned |  | 349,997 |  | 1,036,046
Reinsurance & Monoline Excess | Operating Segments |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | 362,333 | 349,997 | 1,089,757 | 1,036,046
Reinsurance & Monoline Excess | Casualty |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | 188,284 | 201,795 | 585,208 | 618,855
Reinsurance & Monoline Excess | Property |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | 108,386 | 87,838 | 308,024 | 239,338
Reinsurance & Monoline Excess | Monoline excess |  |  |  |
Revenue from External Customer [Line Items] |  |  |  |
Net premiums earned | $ 65,663 | $ 60,364 | $ 196,525 | $ 177,853

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