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Reserve for Claims and Claim Expenses (Claims and Claim Expense Reserves by Segment) (Details) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Sep. 30, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves $ 3,033,689 $ 2,779,059  
Additional Case Reserves 1,470,285 1,750,846  
IBNR 8,729,270 5,851,233  
Gross reserve for claims and claim expenses 13,233,244 10,381,138 $ 9,900,615
Property | Operating Segments      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 1,265,040 1,127,909  
Additional Case Reserves 1,319,022 1,617,003  
IBNR 4,005,279 1,627,541  
Gross reserve for claims and claim expenses 6,589,341 4,372,453  
Casualty and Specialty | Operating Segments      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 1,768,649 1,651,150  
Additional Case Reserves 151,263 133,843  
IBNR 4,723,991 4,223,692  
Gross reserve for claims and claim expenses $ 6,643,903 $ 6,008,685