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Reserve for Claims and Claim Expenses (Schedule of Claims and Claim Expense Reserves by Segment) (Details) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves $ 2,779,059 $ 2,850,272  
Additional Case Reserves 1,750,846 1,760,943  
IBNR 5,851,233 4,773,134  
Net reserve for claims and claim expenses 10,381,138 9,384,349 $ 6,076,271
Other      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 708 440  
Additional Case Reserves 0 0  
IBNR 0 0  
Net reserve for claims and claim expenses 708 440  
Property | Operating Segments      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 1,127,201 1,253,406  
Additional Case Reserves 1,617,003 1,631,223  
IBNR 1,627,541 1,189,221  
Net reserve for claims and claim expenses 4,371,745 4,073,850  
Casualty and Specialty | Operating Segments      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 1,651,150 1,596,426  
Additional Case Reserves 133,843 129,720  
IBNR 4,223,692 3,583,913  
Net reserve for claims and claim expenses $ 6,008,685 $ 5,310,059