Segment Reporting Disclosures (Summary Of Gross Written Premiums By Country) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Gross written premiums	$ 8,779,971	$ 7,864,469	$ 5,506,960
Underwriting Operations [Member]
Segment Reporting Information [Line Items]
Gross written premiums	$ 6,434,327	$ 5,798,996	$ 5,253,107
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	$ 5,172,074	$ 4,587,486	$ 4,163,753
Percentage of gross premiums written	81.00%	79.00%	79.00%
United States [Member] | Program Services and Other [Member]
Segment Reporting Information [Line Items]
Gross written premiums	$ 2,345,644	$ 2,065,473	$ 253,853
United Kingdom [Member] | Underwriting Operations [Member]
Segment Reporting Information [Line Items]
Gross written premiums	$ 458,370	$ 471,818	$ 374,941
Percentage of gross premiums written	7.00%	8.00%	7.00%
Canada [Member] | Underwriting Operations [Member]
Segment Reporting Information [Line Items]
Gross written premiums	$ 151,606	$ 127,546	$ 132,018
Percentage of gross premiums written	2.00%	2.00%	3.00%
Other [Member] | Underwriting Operations [Member]
Segment Reporting Information [Line Items]
Gross written premiums	$ 652,277	$ 612,146	$ 582,395
Percentage of gross premiums written	10.00%	11.00%	11.00%