Segment Information (Gross Premiums Written By Geographic Location) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$ 765,200	$ 646,870	$ 1,602,281	$ 1,327,799
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	440,151	349,577	918,594	738,548
Bermuda [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	211,040	193,195	439,712	375,358
Europe [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	60,234	60,008	146,743	135,384
Singapore [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	48,918	40,128	87,031	69,311
Hong Kong [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	4,857	3,962	10,201	9,198
Switzerland [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$ 5,868	$ 4,071	$ 46,674	$ 33,435