Reserve for Claims and Claim Expenses (Claims and Claim Expense Reserves by Segment) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	$ 1,463,713	$ 1,392,852
Additional Case Reserves	1,425,184	1,021,445
IBNR	3,187,374	2,666,111
Net reserve for claims and claim expenses	6,076,271	5,080,408	$ 2,848,294
Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	1,458	6,605
Additional Case Reserves	0	0
IBNR	3,166	11,921
Net reserve for claims and claim expenses	4,624	18,526
Property | Operating Segments
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	690,718	696,285
Additional Case Reserves	1,308,307	896,522
IBNR	1,087,229	893,583
Net reserve for claims and claim expenses	3,086,254	2,486,390
Casualty and Specialty | Operating Segments
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	771,537	689,962
Additional Case Reserves	116,877	124,923
IBNR	2,096,979	1,760,607
Net reserve for claims and claim expenses	$ 2,985,393	$ 2,575,492