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Reserve for Claims and Claim Expenses (Claims and Claim Expense Reserves by Segment) (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves $ 1,386,224 $ 1,392,852  
Additional Case Reserves 871,516 1,021,445  
IBNR 2,444,605 2,666,111  
Net reserve for claims and claim expenses 4,702,345 5,080,408 $ 2,989,806
Other      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 4,433 6,605  
Additional Case Reserves 0 0  
IBNR 4,495 11,921  
Net reserve for claims and claim expenses 8,928 18,526  
Property      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 631,642 696,285  
Additional Case Reserves 766,450 896,522  
IBNR 568,642 893,583  
Net reserve for claims and claim expenses 1,966,734 2,486,390  
Casualty and Specialty      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 750,149 689,962  
Additional Case Reserves 105,066 124,923  
IBNR 1,871,468 1,760,607  
Net reserve for claims and claim expenses $ 2,726,683 $ 2,575,492