Reserve for Claims and Claim Expenses (Claims and Claim Expense Reserves by Segment) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	$ 802,108	$ 813,594
Additional Case Reserves	310,440	291,727
IBNR	1,822,140	1,742,973
Net reserve for claims and claim expenses	2,934,688	2,848,294	$ 2,811,523
Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	2,255	6,935
Additional Case Reserves	0	0
IBNR	16,119	18,459
Net reserve for claims and claim expenses	18,374	25,394
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	212,770	214,954
Additional Case Reserves	171,257	186,308
IBNR	231,586	226,512
Net reserve for claims and claim expenses	615,613	627,774
Casualty and Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	587,083	591,705
Additional Case Reserves	139,183	105,419
IBNR	1,574,435	1,498,002
Net reserve for claims and claim expenses	$ 2,300,701	$ 2,195,126