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Goodwill And Intangible Assets (Narrative) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Goodwill And Intangible Assets [Line Items]      
Impairment of goodwill $ 0 $ 80,000 $ 0
Impairment of intangible assets (excluding goodwill) 0 0 0
Goodwill, carrying value 2,624,749 [1] 2,638,838 [1] $ 2,899,140
Estimated amortization of intangible assets for 2024 175,700    
Estimated amortization of intangible assets for 2025 169,000    
Estimated amortization of intangible assets for 2026 160,600    
Estimated amortization of intangible assets for 2027 150,700    
Estimated amortization of intangible assets for 2028 140,800    
Indefinite-lived intangible assets $ 90,400 92,400  
Nephila Holdings Limited [Member]      
Goodwill And Intangible Assets [Line Items]      
Impairment of goodwill   80,000  
Goodwill, carrying value   $ 221,800  
[1] As of December 31, 2023 and 2022, goodwill was net of accumulated impairment losses of $190.6 million, of which $171.9 million was in Other and $18.7 million was in Markel Ventures.