Segment Information (Net Premiums Written By Geographic Location) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Segment Reporting Information [Line Items]
Total net premiums written	$ 306,832	$ 350,260	$ 395,803	$ 480,871	$ 287,199	$ 302,169	$ 369,795	$ 433,292	$ 1,533,766	$ 1,392,455	$ 1,321,125
United States [Member]
Segment Reporting Information [Line Items]
Total net premiums written									880,073	815,284	748,111
Bermuda [Member]
Segment Reporting Information [Line Items]
Total net premiums written									418,560	411,685	433,419
Europe [Member]
Segment Reporting Information [Line Items]
Total net premiums written									155,317	139,902	134,253
Hong Kong [Member]
Segment Reporting Information [Line Items]
Total net premiums written									11,696	8,553	5,342
Singapore [Member]
Segment Reporting Information [Line Items]
Total net premiums written									$ 68,120	$ 17,031