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Reserve for Claims and Claim Expenses (Claims and Claim Expense Reserves by Segment) (Details) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Jun. 30, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves $ 860,264 $ 813,594  
Additional Case Reserves 278,531 291,727  
IBNR 1,851,011 1,742,973  
Net reserve for claims and claim expenses 2,989,806 2,848,294 $ 2,844,243
Other      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 2,532 6,935  
Additional Case Reserves 0 0  
IBNR 14,933 18,459  
Net reserve for claims and claim expenses 17,465 25,394  
Property      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 221,898 214,954  
Additional Case Reserves 167,510 186,308  
IBNR 216,163 226,512  
Net reserve for claims and claim expenses 605,571 627,774  
Casualty and Specialty      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 635,834 591,705  
Additional Case Reserves 111,021 105,419  
IBNR 1,619,915 1,498,002  
Net reserve for claims and claim expenses $ 2,366,770 $ 2,195,126