Reserve for Claims and Claim Expenses (Claims and Claim Expense Reserves by Segment) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	$ 813,594	$ 854,332
Additional Case Reserves	291,727	295,704
IBNR	1,742,973	1,617,009
Net reserve for claims and claim expenses	2,848,294	2,767,045	$ 1,412,510
Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	6,935	2,071
Additional Case Reserves	0	0
IBNR	18,459	25,607
Net reserve for claims and claim expenses	25,394	27,678
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	214,954	298,687
Additional Case Reserves	186,308	165,838
IBNR	226,512	241,676
Net reserve for claims and claim expenses	627,774	706,201
Casualty and Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	591,705	553,574
Additional Case Reserves	105,419	129,866
IBNR	1,498,002	1,349,726
Net reserve for claims and claim expenses	$ 2,195,126	$ 2,033,166