Segment Information (Gross Premiums Written By Geographic Location) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$ 565,743	$ 707,884	$ 760,405	$ 901,393	$ 555,490	$ 580,893	$ 765,200	$ 837,081	$ 2,935,425	$ 2,738,664	$ 2,329,271
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									1,795,564	1,636,010	1,360,190
Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									640,948	676,174	611,382
Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									318,626	264,867	228,791
Asia Pacific
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									167,334	161,613	128,908
Canada
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									12,953	0	0
Switzerland
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									$ 67,564	$ 61,042	$ 43,664