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Reserve for Claims and Claim Expenses (Claims and Claim Expense Reserves by Segment) (Details) - USD ($)
$ in Thousands
Mar. 31, 2020
Dec. 31, 2019
Mar. 31, 2019
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves $ 2,740,572 $ 2,850,272  
Additional Case Reserves 1,813,259 1,760,943  
IBNR 4,852,876 4,773,134  
Gross reserve for claims and claim expenses 9,406,707 9,384,349 $ 8,391,484
Other      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 330 440  
Additional Case Reserves 0 0  
IBNR 0 0  
Gross reserve for claims and claim expenses 330 440  
Property | Operating Segments      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 1,095,840 1,253,406  
Additional Case Reserves 1,707,754 1,631,223  
IBNR 958,827 1,189,221  
Gross reserve for claims and claim expenses 3,762,421 4,073,850  
Casualty and Specialty | Operating Segments      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 1,644,402 1,596,426  
Additional Case Reserves 105,505 129,720  
IBNR 3,894,049 3,583,913  
Gross reserve for claims and claim expenses $ 5,643,956 $ 5,310,059