Segment Reporting (Schedule Of Underwriting Results For Operating Segments) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Premiums earned	$ 2,205,811	$ 1,905,619	$ 6,285,030	$ 5,455,615
Incurred losses and LAE	1,736,210	1,371,924	4,574,066	3,515,104
Commission and brokerage	445,332	443,076	1,360,170	1,253,500
Other underwriting expenses	138,875	118,158	385,865	321,976
Underwriting gain (loss)	(114,606)	(27,539)	(35,071)	365,035
U.S. Reinsurance [Member]
Segment Reporting Information [Line Items]
Gross written premiums	2,086,961	1,736,672	5,403,080	4,678,310
Net written premiums	1,936,851	1,583,713	4,974,034	4,212,952
Premiums earned	1,669,257	1,420,799	4,656,733	4,072,078
Incurred losses and LAE	1,335,048	1,050,621	3,361,367	2,605,901
Commission and brokerage	373,251	371,098	1,130,946	1,039,113
Other underwriting expenses	51,333	43,832	135,170	117,031
Underwriting gain (loss)	(90,375)	(44,752)	29,250	310,033
Insurance [Member]
Segment Reporting Information [Line Items]
Gross written premiums	704,643	666,602	2,328,733	2,018,727
Net written premiums	511,829	484,844	1,693,603	1,491,286
Premiums earned	536,554	484,820	1,628,297	1,383,537
Incurred losses and LAE	401,162	321,303	1,212,699	909,203
Commission and brokerage	72,081	71,978	229,224	214,387
Other underwriting expenses	87,542	74,326	250,695	204,945
Underwriting gain (loss)	$ (24,231)	$ 17,213	$ (64,321)	$ 55,002