Reinsurance (Effect Of Reinsurance And Retrocessional Reinsurance On Premiums Written And Earned) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effects of Reinsurance [Line Items]
Direct premiums written	$ 7,341,388	$ 6,584,804	$ 4,172,467
Direct premiums earned	6,962,507	6,235,218	4,068,622
Assumed premiums written	1,438,583	1,279,665	1,334,493
Assumed premiums earned	1,368,153	1,319,613	1,287,395
Ceded premiums written	(3,367,900)	(3,076,891)	(1,089,173)
Ceded premiums earned	(3,280,867)	(2,842,771)	(1,108,039)
Net premiums written	5,412,071	4,787,578	4,417,787
Net premiums earned	5,049,793	4,712,060	4,247,978
Underwriting [Member]
Effects of Reinsurance [Line Items]
Direct premiums written	5,084,641	4,562,256	3,919,602
Direct premiums earned	4,767,836	4,384,562	3,777,335
Assumed premiums written	1,349,686	1,236,740	1,333,505
Assumed premiums earned	1,289,375	1,291,032	1,286,043
Ceded premiums written	(1,024,097)	(1,013,406)	(835,320)
Ceded premiums earned	(1,008,970)	(964,549)	(815,400)
Net premiums written	5,410,230	4,785,590	4,417,787
Net premiums earned	5,048,241	4,711,045	4,247,978
Program Services and Other [Member]
Effects of Reinsurance [Line Items]
Direct premiums written	2,256,747	2,022,548	252,865
Direct premiums earned	2,194,671	1,850,656	291,287
Assumed premiums written	88,897	42,925	988
Assumed premiums earned	78,778	28,581	1,352
Ceded premiums written	(2,343,803)	(2,063,485)	(253,853)
Ceded premiums earned	(2,271,897)	(1,878,222)	(292,639)
Net premiums written	1,841	1,988	0
Net premiums earned	$ 1,552	$ 1,015	$ 0