Segment Information (Gross Premiums Written By Geographic Location) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$ 825,970	$ 760,405	$ 1,706,584	$ 1,661,798
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	483,115	442,549	1,019,747	963,321
Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	167,874	185,872	364,502	415,503
Asia Pacific
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	93,226	50,905	133,283	97,729
Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	76,260	77,985	180,230	181,321
Canada
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	5,495	3,094	8,822	3,924
Switzerland
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$ 4,284	$ 10,261	$ 44,668	$ 54,554