Segment Information (Gross Premiums Written By Geographic Location) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$ 707,884	$ 580,893	$ 2,369,682	$ 2,183,174
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	483,992	377,618	1,447,652	1,296,212
Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	109,503	111,103	525,005	550,815
Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	64,947	52,004	245,931	198,747
Asia Pacific
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	45,070	40,168	142,798	137,400
Canada
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	4,372	0	8,296	0
Switzerland
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$ 11,130	$ 10,509	$ 65,684	$ 57,183