Reserve for Claims and Claim Expenses (Schedule of Claims and Claim Expense Reserves by Segment) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	$ 2,850,272	$ 1,463,713
Additional Case Reserves	1,760,943	1,425,184
IBNR	4,773,134	3,187,374
Net reserve for claims and claim expenses	9,384,349	6,076,271	$ 5,080,408
Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	440	1,458
Additional Case Reserves	0	0
IBNR	0	3,166
Net reserve for claims and claim expenses	440	4,624
Property | Operating Segments
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	1,253,406	690,718
Additional Case Reserves	1,631,223	1,308,307
IBNR	1,189,221	1,087,229
Net reserve for claims and claim expenses	4,073,850	3,086,254
Casualty and Specialty | Operating Segments
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	1,596,426	771,537
Additional Case Reserves	129,720	116,877
IBNR	3,583,913	2,096,979
Net reserve for claims and claim expenses	$ 5,310,059	$ 2,985,393