Reserve for Claims and Claim Expenses (Claims and Claim Expense Reserves by Segment) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	$ 1,392,852	$ 813,594
Additional Case Reserves	1,021,445	291,727
IBNR	2,666,111	1,742,973
Net reserve for claims and claim expenses	5,080,408	2,848,294	$ 2,767,045
Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	6,605	6,935
Additional Case Reserves	0	0
IBNR	11,921	18,459
Net reserve for claims and claim expenses	18,526	25,394
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	696,285	214,954
Additional Case Reserves	896,522	186,308
IBNR	893,583	226,512
Net reserve for claims and claim expenses	2,486,390	627,774
Casualty and Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Case Reserves	689,962	591,705
Additional Case Reserves	124,923	105,419
IBNR	1,760,607	1,498,002
Net reserve for claims and claim expenses	$ 2,575,492	$ 2,195,126