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Business Segments (Net Premiums Earned By Major Line of Business) (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Revenue from External Customer [Line Items]    
Net premiums earned $ 2,764,347 $ 2,491,432
Insurance    
Revenue from External Customer [Line Items]    
Net premiums earned   2,142,924
Insurance | Operating Segments    
Revenue from External Customer [Line Items]    
Net premiums earned 2,398,768 2,142,924
Insurance | Other liability    
Revenue from External Customer [Line Items]    
Net premiums earned 967,260 853,472
Insurance | Short-tail lines    
Revenue from External Customer [Line Items]    
Net premiums earned 510,809 412,534
Insurance | Auto    
Revenue from External Customer [Line Items]    
Net premiums earned 354,013 294,116
Insurance | Workers' compensation    
Revenue from External Customer [Line Items]    
Net premiums earned 301,495 305,561
Insurance | Professional liability    
Revenue from External Customer [Line Items]    
Net premiums earned 265,191 277,241
Reinsurance & Monoline Excess    
Revenue from External Customer [Line Items]    
Net premiums earned   348,508
Reinsurance & Monoline Excess | Operating Segments    
Revenue from External Customer [Line Items]    
Net premiums earned 365,579 348,508
Reinsurance & Monoline Excess | Casualty    
Revenue from External Customer [Line Items]    
Net premiums earned 197,844 214,712
Reinsurance & Monoline Excess | Monoline excess    
Revenue from External Customer [Line Items]    
Net premiums earned 65,351 58,644
Reinsurance & Monoline Excess | Property    
Revenue from External Customer [Line Items]    
Net premiums earned $ 102,384 $ 75,152