Segment Reporting Disclosures (Summary Of Gross Written Premiums By Country) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Gross written premiums	$ 4,632,912	$ 4,805,513	$ 3,920,226
Percentage of gross premiums written	100.00%	100.00%	100.00%
United States [Member]
Segment Reporting Information [Line Items]
Gross written premiums	$ 3,519,487	$ 3,523,239	$ 2,934,868
Percentage of gross premiums written	76.00%	73.00%	75.00%
United Kingdom [Member]
Segment Reporting Information [Line Items]
Gross written premiums	$ 414,941	$ 441,669	$ 245,143
Percentage of gross premiums written	9.00%	9.00%	6.00%
Canada [Member]
Segment Reporting Information [Line Items]
Gross written premiums	$ 115,191	$ 125,617	$ 128,420
Percentage of gross premiums written	2.00%	3.00%	3.00%
Other Countries [Member]
Segment Reporting Information [Line Items]
Gross written premiums	$ 583,293	$ 714,988	$ 611,795
Percentage of gross premiums written	13.00%	15.00%	16.00%