Reserve for Claims and Claim Expenses (Claims and Claim Expense Reserves by Segment) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	$ 1,447,010	$ 1,392,852
Additional Case Reserves	968,878	1,021,445
IBNR	2,496,839	2,666,111
Net reserve for claims and claim expenses	4,912,727	5,080,408	$ 2,934,688
Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	4,999	6,605
Additional Case Reserves	0	0
IBNR	8,269	11,921
Net reserve for claims and claim expenses	13,268	18,526
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	701,202	696,285
Additional Case Reserves	850,498	896,522
IBNR	687,209	893,583
Net reserve for claims and claim expenses	2,238,909	2,486,390
Casualty and Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	740,809	689,962
Additional Case Reserves	118,380	124,923
IBNR	1,801,361	1,760,607
Net reserve for claims and claim expenses	$ 2,660,550	$ 2,575,492