Segment Information (Gross Premiums Written By Geographic Location) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$ 555,490	$ 580,893	$ 765,200	$ 837,081	$ 497,052	$ 504,420	$ 646,870	$ 680,929	$ 2,738,664	$ 2,329,271	$ 1,939,521
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									1,636,010	1,360,190	1,080,134
Bermuda
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									676,174	611,382	565,801
Europe
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									264,867	228,791	210,435
Singapore
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									142,393	111,694	68,382
Hong Kong
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									19,220	17,214	14,769
Switzerland
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									$ 61,042	$ 43,664	$ 37,990