Segment Information (Gross Premiums Written By Geographic Location) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$ 760,405	$ 765,200	$ 1,661,798	$ 1,602,281
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	442,549	440,151	963,321	918,594
Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	185,872	211,040	415,503	439,712
Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	77,985	60,234	181,321	146,743
Singapore
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	45,375	48,918	87,316	87,031
Hong Kong
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	3,756	4,857	8,639	10,201
Canada
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	3,094	0	3,924	0
Australia
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	1,774	0	1,774	0
Switzerland
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$ 10,261	$ 5,868	$ 54,554	$ 46,674