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Reserve for Claims and Claim Expenses (Claims and Claim Expense Reserves by Segment) (Details) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Sep. 30, 2018
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves $ 2,648,458 $ 1,463,713  
Additional Case Reserves 1,645,155 1,425,184  
IBNR 4,308,824 3,187,374  
Gross reserve for claims and claim expenses 8,602,437 6,076,271 $ 4,952,498
Other      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 2,944 1,458  
Additional Case Reserves 0 0  
IBNR 3,166 3,166  
Gross reserve for claims and claim expenses 6,110 4,624  
Property | Operating Segments      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 1,170,009 690,718  
Additional Case Reserves 1,493,600 1,308,307  
IBNR 916,314 1,087,229  
Gross reserve for claims and claim expenses 3,579,923 3,086,254  
Casualty and Specialty | Operating Segments      
Liability for Claims and Claims Adjustment Expense [Line Items]      
Case Reserves 1,475,505 771,537  
Additional Case Reserves 151,555 116,877  
IBNR 3,389,344 2,096,979  
Gross reserve for claims and claim expenses $ 5,016,404 $ 2,985,393