Segment Reporting Disclosures (Summary Of Gross Written Premiums By Country) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Gross written premiums	$ 7,864,469	$ 5,506,960	$ 4,796,645
Underwriting Operations [Member]
Segment Reporting Information [Line Items]
Gross written premiums	$ 5,798,996	$ 5,253,107	$ 4,796,645
Percentage of gross premiums written	100.00%	100.00%	100.00%
United States [Member] | Underwriting Operations [Member]
Segment Reporting Information [Line Items]
Gross written premiums	$ 4,587,486	$ 4,163,753	$ 3,691,840
Percentage of gross premiums written	79.00%	79.00%	77.00%
United States [Member] | Program Services [Member]
Segment Reporting Information [Line Items]
Gross written premiums	$ 2,065,473	$ 253,853	$ 0
United Kingdom [Member] | Underwriting Operations [Member]
Segment Reporting Information [Line Items]
Gross written premiums	$ 471,818	$ 374,941	$ 358,348
Percentage of gross premiums written	8.00%	7.00%	7.00%
Canada [Member] | Underwriting Operations [Member]
Segment Reporting Information [Line Items]
Gross written premiums	$ 127,546	$ 132,018	$ 125,444
Percentage of gross premiums written	2.00%	3.00%	3.00%
Other Countries [Member] | Underwriting Operations [Member]
Segment Reporting Information [Line Items]
Gross written premiums	$ 612,146	$ 582,395	$ 621,013
Percentage of gross premiums written	11.00%	11.00%	13.00%