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Reserve for Claims and Claim Expenses (Claims and Claim Expense Reserves by Segment) (Details) - USD ($)
$ in Thousands
Jun. 30, 2021
Dec. 31, 2020
Jun. 30, 2020
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves $ 3,110,805 $ 2,779,059  
Additional Case Reserves 1,646,590 1,750,846  
IBNR 6,187,347 5,851,233  
Gross reserve for claims and claim expenses 10,944,742 10,381,138 $ 9,365,469
Property | Operating Segments      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 1,383,754 1,127,909  
Additional Case Reserves 1,513,549 1,617,003  
IBNR 1,773,492 1,627,541  
Gross reserve for claims and claim expenses 4,670,795 4,372,453  
Casualty and Specialty | Operating Segments      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 1,727,051 1,651,150  
Additional Case Reserves 133,041 133,843  
IBNR 4,413,855 4,223,692  
Gross reserve for claims and claim expenses $ 6,273,947 $ 6,008,685