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Reserve for Claims and Claim Expenses (Claims and Claim Expense Reserves by Segment) (Details) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Sep. 30, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves $ 976,103 $ 813,594  
Additional Case Reserves 549,513 291,727  
IBNR 3,666,697 1,742,973  
Net reserve for claims and claim expenses 5,192,313 2,848,294 $ 2,861,099
Other      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 9,904 6,935  
Additional Case Reserves 0 0  
IBNR 14,649 18,459  
Net reserve for claims and claim expenses 24,553 25,394  
Property      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 310,871 214,954  
Additional Case Reserves 416,758 186,308  
IBNR 1,917,573 226,512  
Net reserve for claims and claim expenses 2,645,202 627,774  
Casualty and Specialty      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 655,328 591,705  
Additional Case Reserves 132,755 105,419  
IBNR 1,734,475 1,498,002  
Net reserve for claims and claim expenses $ 2,522,558 $ 2,195,126