Reinsurance (Effect Of Reinsurance And Retrocessional Reinsurance On Premiums Written And Earned) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Effects of Reinsurance [Line Items]
Direct premiums written	$ 3,012,582	$ 2,839,412	$ 9,164,683	$ 8,491,896
Direct premiums earned	2,928,239	2,796,788	8,791,309	8,084,663
Assumed premiums written	459,127	839,394	2,959,555	2,411,107
Assumed premiums earned	935,374	737,699	2,106,919	1,794,614
Ceded premiums written	(1,489,573)	(1,651,001)	(5,631,286)	(4,449,727)
Ceded premiums earned	(1,753,505)	(1,412,742)	(4,583,394)	(3,758,685)
Net premiums written	1,982,136	2,027,805	6,492,952	6,453,276
Net premiums earned	2,110,108	2,121,745	6,314,834	6,120,592
Underwriting [Member]
Effects of Reinsurance [Line Items]
Direct premiums written	2,249,500	2,223,988	6,713,503	6,479,090
Direct premiums earned	2,206,453	2,127,244	6,554,464	6,124,444
Assumed premiums written	282,089	282,070	1,515,639	1,419,802
Assumed premiums earned	404,686	424,620	1,212,116	1,247,743
Ceded premiums written	(549,264)	(478,035)	(1,735,668)	(1,444,626)
Ceded premiums earned	(500,842)	(429,901)	(1,451,224)	(1,250,605)
Net premiums written	1,982,325	2,028,023	6,493,474	6,454,266
Net premiums earned	2,110,297	2,121,963	6,315,356	6,121,582
Program services and other fronting [Member]
Effects of Reinsurance [Line Items]
Direct premiums written	763,082	615,424	2,451,180	2,012,806
Direct premiums earned	721,786	669,544	2,236,845	1,960,219
Assumed premiums written	177,038	557,324	1,443,916	991,305
Assumed premiums earned	530,688	313,079	894,803	546,871
Ceded premiums written	(940,309)	(1,172,966)	(3,895,618)	(3,005,101)
Ceded premiums earned	(1,252,663)	(982,841)	(3,132,170)	(2,508,080)
Net premiums written	(189)	(218)	(522)	(990)
Net premiums earned	$ (189)	$ (218)	$ (522)	$ (990)