Segment Information (Schedule Of Revenue From External Customers And Long-Lived Assets, By Geographical Areas) (Details) (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Segment Reporting Information [Line Items]
Total net premiums written	$ 350,260	$ 302,169	$ 1,226,934	$ 1,105,256
United States [Member]
Segment Reporting Information [Line Items]
Total net premiums written	201,475	184,223	680,049	622,483
Bermuda [Member]
Segment Reporting Information [Line Items]
Total net premiums written	81,868	88,386	353,651	347,287
Europe [Member]
Segment Reporting Information [Line Items]
Total net premiums written	34,403	24,293	126,942	115,003
Singapore [Member]
Segment Reporting Information [Line Items]
Total net premiums written	32,990	4,222	58,557	13,825
Hong Kong [Member]
Segment Reporting Information [Line Items]
Total net premiums written	$ (476)	$ 1,045	$ 7,735	$ 6,658