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Reserve for Claims and Claim Expenses (Claims and Claim Expense Reserves by Segment) (Details) - USD ($)
$ in Thousands
Sep. 30, 2020
Dec. 31, 2019
Sep. 30, 2019
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves $ 2,663,412 $ 2,850,272  
Additional Case Reserves 1,875,261 1,760,943  
IBNR 5,361,942 4,773,134  
Gross reserve for claims and claim expenses 9,900,615 9,384,349 $ 8,602,437
Other      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 332 440  
Additional Case Reserves 0 0  
IBNR 0 0  
Gross reserve for claims and claim expenses 332 440  
Property | Operating Segments      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 1,037,526 1,253,406  
Additional Case Reserves 1,752,260 1,631,223  
IBNR 1,238,478 1,189,221  
Gross reserve for claims and claim expenses 4,028,264 4,073,850  
Casualty and Specialty | Operating Segments      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 1,625,554 1,596,426  
Additional Case Reserves 123,001 129,720  
IBNR 4,123,464 3,583,913  
Gross reserve for claims and claim expenses $ 5,872,019 $ 5,310,059