Segment Information (Gross Premiums Written By Geographic Location) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$ 754,062	$ 707,884	$ 2,460,646	$ 2,369,682
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	489,562	483,992	1,509,314	1,447,652
Asia Pacific
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	106,039	45,070	239,323	142,798
Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	84,486	109,503	448,988	525,005
Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	70,814	64,947	251,042	245,931
Canada
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	3,161	4,372	11,979	8,296
Switzerland
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$ 12,209	$ 11,130	$ 56,876	$ 65,684