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Reserve for Claims and Claim Expenses (Claims and Claim Expense Reserves by Segment) (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves $ 1,387,904 $ 1,392,852  
Additional Case Reserves 847,022 1,021,445  
IBNR 2,717,572 2,666,111  
Gross reserve for claims and claim expenses 4,952,498 5,080,408 $ 5,192,313
Other      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 3,240 6,605  
Additional Case Reserves 0 0  
IBNR 3,108 11,921  
Gross reserve for claims and claim expenses 6,348 18,526  
Property      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 610,932 696,285  
Additional Case Reserves 744,391 896,522  
IBNR 774,220 893,583  
Gross reserve for claims and claim expenses 2,129,543 2,486,390  
Casualty and Specialty      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 773,732 689,962  
Additional Case Reserves 102,631 124,923  
IBNR 1,940,244 1,760,607  
Gross reserve for claims and claim expenses $ 2,816,607 $ 2,575,492