Segment Information (Gross Premiums Written By Geographic Location) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written	$ 497,052	$ 504,420	$ 646,870	$ 680,929	$ 416,537	$ 442,698	$ 519,598	$ 560,688	$ 2,329,271	$ 1,939,521	$ 1,758,397
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									1,360,190	1,080,134	993,486
Bermuda [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									611,382	565,801	545,560
Europe [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									228,791	210,435	193,000
Singapore [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									111,694	68,382	17,059
Hong Kong [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									17,214	14,769	9,292
Switzerland [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Gross premiums written									$ 43,664	$ 37,990	$ 27,597